U.S. patent application number 11/947525 was filed with the patent office on 2008-07-31 for liquid crystal display device and driving method thereof.
This patent application is currently assigned to Semiconductor Energy Laboratory Co., Ltd.. Invention is credited to Hajime Kimura, Yasunori Yoshida.
Application Number | 20080180385 11/947525 |
Document ID | / |
Family ID | 39167636 |
Filed Date | 2008-07-31 |
United States Patent
Application |
20080180385 |
Kind Code |
A1 |
Yoshida; Yasunori ; et
al. |
July 31, 2008 |
Liquid Crystal Display Device and Driving Method Thereof
Abstract
To provide a hold-type display device without a problem of
motion blur and a driving method thereof. The length of a period
for displaying a blanking image in one frame period is controlled
in accordance with a control parameter showing the degree of motion
blur, and the level of a signal supplied to a display element is
changed in accordance with the length of the period for displaying
the blanking image. Accordingly, the hold-type display device
without a problem of motion blur and the driving method thereof can
be provided.
Inventors: |
Yoshida; Yasunori; (Atsugi,
JP) ; Kimura; Hajime; (Atsugi, JP) |
Correspondence
Address: |
COOK, ALEX, McFARRON, MANZO,;CUMMINGS & MEHLER, LTD.
SUITE 2850, 200 WEST ADAMS STREET
CHICAGO
IL
60606
US
|
Assignee: |
Semiconductor Energy Laboratory
Co., Ltd.
|
Family ID: |
39167636 |
Appl. No.: |
11/947525 |
Filed: |
November 29, 2007 |
Current U.S.
Class: |
345/102 ;
345/87 |
Current CPC
Class: |
G09G 3/20 20130101; G09G
2300/0876 20130101; G09G 2310/08 20130101; G09G 2330/021 20130101;
G09G 2360/16 20130101; G09G 2300/0852 20130101; G09G 3/3659
20130101; G09G 2300/0842 20130101; G09G 2310/0237 20130101; G09G
3/3258 20130101; G09G 2320/043 20130101; G09G 2310/0235 20130101;
G09G 2320/0613 20130101; G09G 3/22 20130101; H04M 1/0214 20130101;
H04W 52/0251 20130101; G09G 2320/0261 20130101; G09G 2360/144
20130101; G09G 2320/106 20130101; G09G 3/3607 20130101; G09G 3/3677
20130101; G09G 2300/0465 20130101; G09G 2320/0247 20130101; G09G
3/2074 20130101; H04M 1/0266 20130101; G09G 2300/0819 20130101;
G09G 2320/0252 20130101; G09G 3/3413 20130101; G09G 2300/0809
20130101; G09G 2320/041 20130101; G02F 1/13306 20130101; G09G 3/342
20130101; G09G 2310/024 20130101; G09G 2320/0257 20130101; G09G
2320/0233 20130101; G09G 3/2081 20130101; G09G 3/3688 20130101;
G09G 3/2018 20130101; G09G 2310/0251 20130101; H04M 2250/16
20130101; G09G 2300/088 20130101; G09G 2320/0646 20130101; G09G
2310/061 20130101; G09G 2340/16 20130101; G09G 3/2022 20130101;
Y02D 30/70 20200801; G09G 3/3406 20130101; G09G 2320/028 20130101;
G09G 3/3648 20130101 |
Class at
Publication: |
345/102 ;
345/87 |
International
Class: |
G09G 3/36 20060101
G09G003/36 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 5, 2006 |
JP |
2006-328670 |
Claims
1. A driving method of a liquid crystal display device in which an
image is displayed by applying a signal voltage in accordance with
an image signal to a liquid crystal element, the driving method
comprising: applying a first voltage to the liquid crystal element
in a first subframe period of one frame period; and applying a
second voltage at which the liquid crystal element performs black
display in a second subframe period of the one frame period,
wherein a level of the first voltage is determined in accordance
with a difference between the second voltage and the signal
voltage, and a length of the first subframe period.
2. A driving method of a liquid crystal display device according to
claim 1, wherein the larger the difference between the first
voltage and the signal voltage is, the shorter the length of the
subframe period is.
3. A driving method of a liquid crystal display device comprising a
backlight, in which an image is displayed by applying a signal
voltage in accordance with an image signal to a liquid crystal
element, the driving method comprising: applying an initialization
voltage to the liquid crystal element; and applying a first voltage
to the liquid crystal element in one frame period right after
applying the initialization voltage, wherein a level of the first
voltage is determined in accordance with a difference between the
initialization voltage and the signal voltage, and a length of a
backlight lighting period in the one frame period.
4. A driving method of a liquid crystal display device according to
claim 3, wherein the larger the difference between the
initialization voltage and the signal voltage is, the shorter the
length of the backlight lighting period is.
5. A driving method of a liquid crystal display device comprising a
backlight, in which an image is displayed by applying a signal
voltage in accordance with an image signal to a liquid crystal
element, wherein the backlight is divided into the plurality of
light-emitting regions, the driving method comprising: while
sequentially scanning the plurality of light-emitting regions,
applying an initialization voltage to the liquid crystal element;
and applying a first voltage to the liquid crystal element in one
frame period right after applying the initialization voltage,
wherein a level of the first voltage is determined in accordance
with a difference between the initialization voltage and the signal
voltage, and a length of a lighting period of each of the plurality
of light-emitting regions in the one frame period.
6. A driving method of a liquid crystal display device according to
claim 5, wherein the larger the difference between the
initialization voltage and the signal voltage is, the shorter the
length of the backlight lighting period is.
7. A driving method of a liquid crystal display device comprising a
backlight, in which an image is displayed by applying a signal
voltage in accordance with an image signal to a liquid crystal
element, the driving method comprising: applying an initialization
voltage to the liquid crystal element; applying a first voltage to
the liquid crystal element in one frame period right after applying
the initialization voltage; and applying a second voltage at which
the liquid crystal element performs black display in the second
subframe period, wherein a level of the first voltage is determined
in accordance with a difference between the second voltage and the
signal voltage, a length of the backlight lighting period in the
one frame period, and a length of the first subframe period.
8. A driving method of a liquid crystal display device comprising a
backlight, in which an image is displayed by applying a signal
voltage in accordance with an image signal to a liquid crystal
element, wherein the backlight is divided into a plurality of
light-emitting regions, the driving method comprising: while
sequentially scanning a plurality of light-emitting regions,
applying an initialization voltage to the liquid crystal element;
applying a first voltage to the liquid crystal element in one frame
period right after applying the initialization voltage; and
applying a second voltage at which the liquid crystal element
performs black display in the second subframe period, wherein a
level of the first voltage is determined in accordance with a
difference between the second voltage and the signal voltage, a
length of a lighting period of each of the plurality of
light-emitting regions, and a length of the first subframe
period.
9. A liquid crystal display device comprising: a liquid crystal
element, wherein an image is configured to be displayed by applying
a signal voltage in accordance with an image signal to the liquid
crystal element, wherein a level of a first voltage which is
applied to the liquid crystal element in a first subframe period of
one frame period is determined in accordance with a difference
between a second voltage which is applied to the liquid crystal
element in a second subframe period of the one frame period and the
signal voltage, and a length of the first subframe period, and
wherein the second voltage is voltage at which the liquid crystal
element performs black display.
10. A liquid crystal display device according to claim 9, wherein
the larger the difference between the first voltage and the signal
voltage is, the shorter the length of the subframe period is.
11. A liquid crystal display device, comprising: a liquid crystal
element; and a backlight, wherein an image is configured to be
displayed by applying a signal voltage in accordance with an image
signal to the liquid crystal element, and wherein a level of a
first voltage which is applied to the liquid crystal element in one
frame period is determined in accordance with a difference between
an initialization voltage which is applied to the liquid crystal
element right before the one frame period and the signal voltage,
and a length of a backlight lighting period in the one frame
period.
12. A liquid crystal display device according to claim 11, wherein
the larger the difference between the initialization voltage and
the signal voltage is, the shorter the length of the backlight
lighting period is.
13. A liquid crystal display device, comprising: a liquid crystal
element; and a backlight, wherein an image is configured to be
displayed by applying a signal voltage in accordance with an image
signal to a liquid crystal element, wherein the backlight is
divided into a plurality of light-emitting regions and is
configured to be sequentially scanned to emit light, and wherein a
level of a first voltage which is applied to the liquid crystal
element in one frame period is determined in accordance with a
difference between an initialization voltage which is applied to
the liquid crystal element right before the one frame period and
the signal voltage, and a length of the lighting period of each of
the plurality of light-emitting regions in the one frame
period.
14. A liquid crystal display device according to claim 13, wherein
the larger the difference between the initialization voltage and
the signal voltage is, the shorter the length of the backlight
lighting period is.
15. A liquid crystal display device, comprising: a liquid crystal
element; and a backlight, wherein an image is configured to be
displayed by applying a signal voltage in accordance with an image
signal to the liquid crystal element, wherein a level of a first
voltage which is applied to the liquid crystal element in a first
subframe period of one frame period is determined in accordance
with a difference between a second voltage which is applied to the
liquid crystal element in a second subframe period of the one frame
period and the signal voltage, and a length of a backlight lighting
period in the one frame period, and wherein the second voltage
which is applied to the liquid crystal element in the second
subframe period is voltage at which the liquid crystal element
performs black display.
16. A liquid crystal display device, comprising: a liquid crystal
element; and a backlight, wherein an image is configured to
displayed by applying a signal voltage in accordance with an image
signal to the liquid crystal element, wherein the backlight is
divided into a plurality of light-emitting regions and is
configured to be sequentially scanned to emit light, wherein a
level of a first voltage which is applied to the liquid crystal
element in a first subframe period of one frame period is
determined in accordance with a difference between a second voltage
which is applied to the liquid crystal element in a second subframe
period and the signal voltage, a length of the lighting period of
each of the plurality of light-emitting regions in the one frame
period, and a length of the first subframe period; and wherein the
second voltage is voltage at which a liquid crystal element
performs black display.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a display device and an
operating method of the display device. In particular, the present
invention relates to a method for improving quality of a moving
image of a hold-type display device.
[0003] 2. Description of the Related Art
[0004] In recent years, there have been growing interests in thin
display devices. Liquid crystal displays, plasma displays,
projection displays, and the like have been developed and becoming
popular instead of CRT displays. Further, field emission displays,
inorganic electroluminescence displays, organic electroluminescence
displays, electronic paper, and the like have been developed as
next-generation display devices.
[0005] In a display portion which is provided in the
above-described display device, pixels which are minimum units for
forming an image are arranged. Each of the pixels emits light with
desired luminance by being supplied with a signal generated by
image data. Accordingly, an image is displayed in the display
portion.
[0006] In addition, the signal supplied to the pixel is updated
(refreshed) at a constant period. An inverse number of this period
is referred to as a frame rate. Further, time after the signal is
updated once and before the signal is updated next is referred to
as one frame period. Display of a moving image in the display
portion is realized by supplying a signal which is different from
the signal supplied before to the pixel when the signal is updated.
On the other hand, display of a still image in the display portion
is realized by supplying a signal which is the same as the signal
supplied before to the pixel when the signal is updated.
[0007] Further, driving methods of display devices can be
classified by temporal distribution of luminance of a pixel in one
frame period. In hold-type display devices which are used for
active matrix display devices, a pixel continuously emits light in
one frame period. On the other hand, in impulsive-type display
devices typified by CRTs, a pixel immediately attenuates and does
not emit light after the pixel strongly emits light once in one
frame period. In impulsive-type display devices, most one frame
period is a non-light emitting state.
[0008] It has become obvious that hold-type display devices have a
problem such that a moving object seems to leave traces when a
moving image is displayed and part of an image is moved or the
entire image blurs when the entire image is moved (motion blur).
This is characteristics of hold-type display devices and a problem
of motion blur does not occur in impulsive-type display
devices.
[0009] As a method for solving a problem of motion blur in
hold-type display devices, the following two methods have been
mainly proposed (see Reference 1). A first method is a method of
providing a period during which the original image is displayed and
a period during which a black image is displayed in one frame
period. Thus, display can be made closer to that of impulsive-type
display devices, so that quality of a moving image can be improved
(see References 2 and 3). A second method is a method in which
display is performed by shortening the length of one frame period
(increasing a frame rate) and generating a temporally compensated
image with respect to an increased frame at the same time. Thus,
quality of a moving image can be improved (see Reference 4). In
addition, as an improvement technology of the first method, it is
disclosed that quality of a moving image can be improved by
displaying a darker image than the original image instead of a
black image (see References 5, 6, 9, 10, and 11). Further, a method
of changing driving methods in accordance with conditions is also
disclosed (see Reference 7 and 8). [0010] [Reference 1] Japanese
Published Patent Application No. H04-302289 [0011] [Reference 2]
Japanese Published Patent Application No. H09-325715 [0012]
[Reference 3] Japanese Published Patent Application No. 2000-200063
[0013] [Reference 4] Japanese Published Patent Application No.
2005-268912 [0014] [Reference 5] Japanese Published Patent
Application No. 2002-23707 [0015] [Reference 6] Japanese Published
Patent Application No. 2004-240317 [0016] [Reference 7] Japanese
Published Patent Application No. 2002-91400 [0017] [Reference 8]
Japanese Published Patent Application No. 2004-177575 [0018]
[Reference 9] Society For Information Display '05 DIGEST, 60.2, pp.
1734 to 1737, (2005) [0019] [Reference 10] Society For Information
Display '06 DIGEST, 69.4, pp. 1950 to 1953, (2006) [0020]
[Reference 11] Society For Information Display '06 DIGEST, 69.5,
pp. 1954 to 1957, (2006)
SUMMARY OF THE INVENTION
[0021] Although various methods for solving a problem of motion
blur in hold-type display devices have been considered,
advantageous effects thereof are not sufficient in some cases. In
addition, other troubles are increased by using the methods in some
cases. For example, a flicker increases in a method in which
display is made closer to that of impulsive-type display devices by
displaying a black image. In addition, by displaying the black
image, luminance of an image decreases compared with the case of
not inserting a black image. In that case, in order to obtain
luminance which is equal to that of the case of not inserting a
black image, it is necessary to increase luminance instantaneously.
Accordingly, loads on display devices are increased to decrease
reliability or increase power consumption, which becomes
problematic.
[0022] In a method of increasing a frame rate, a driver circuit
which can process data at high speed is necessary because data
processing becomes complicated, so that manufacturing cost
increases, heat is generated in accordance with data processing,
and power consumption increases, which become problematic. In
addition, in a method in which a new image is generated by
compensating image data, it is difficult to obtain high-quality
compensated image, and on the contrary, quality of a moving image
decreases in some cases by displaying an image by insertion of
compensated data.
[0023] Further, when the above-described method for solving a
problem of motion blur is applied to a liquid crystal display
device, there is a problem in that speed during which transmittance
of a liquid crystal is change is slow and it is difficult to
sufficiently follow change in signals. Furthermore, there is a
problem in that degree of light emission of a pixel is changed
depending on viewing angles.
[0024] The present invention has been made in view of the foregoing
problems. It is an object of the present invention to provide a
hold-type display device without a problem of motion blur and a
driving method of thereof. It is another object of the present
invention to provide a display device with low power consumption
and a driving method of thereof. In addition, it is another object
of the present invention to provide a display device with improved
quality for still images and moving images and a driving method of
thereof. Further, it is another object of the present invention to
provide a display device with a wider viewing angle and a driving
method of thereof. Furthermore, it is an object of the present
invention to provide a display device with improved response speed
of a liquid crystal and a driving method of thereof.
[0025] One aspect of the present invention is a driving method of a
liquid crystal display device in which an image is displayed by
applying signal voltage V.sub.i in accordance with an image signal
to a liquid crystal element. One frame period is divided into a
first subframe period and a second subframe period. When the length
of the first subframe period is denoted by .tau..sub.a, first
voltage which is applied to the liquid crystal element in the first
subframe period is denoted by V.sub.a, and second voltage which is
applied to the liquid crystal element in the second subframe period
is denoted by V.sub.b, the first voltage V.sub.1 is determined in
accordance with a difference between the second voltage V.sub.b and
the signal voltage V.sub.i, and the length of the first subframe
period .tau..sub.a. The second voltage V.sub.b is voltage at which
the liquid crystal element performs black display.
[0026] Another aspect of the present invention is a driving method
of a liquid crystal display device in which an image is displayed
by applying signal voltage V.sub.i in accordance with an image
signal to a liquid crystal element. The liquid crystal display
device includes a backlight. When the length of a backlight
lighting period in one frame period is denoted by .tau..sub.a,
first voltage which is applied to the liquid crystal element in one
frame period is denoted by V.sub.a, and initialization voltage
which is applied to the liquid crystal element right before one
frame period is denoted by V.sub.0, the first voltage V.sub.1 is
determined in accordance with a difference between the
initialization voltage V.sub.0 and the signal voltage V.sub.i, and
the length of the backlight lighting period .tau..sub.a.
[0027] Another aspect of the present invention is a driving method
of a liquid crystal display device in which an image is displayed
by applying signal voltage V.sub.i in accordance with an image
signal to a liquid crystal element. The liquid crystal display
device includes a backlight. The backlight is divided into a
plurality of light-emitting regions in a display region and is
sequentially scanned to emit light. When the length of a lighting
period of each of a plurality of light-emitting regions in one
frame period is denoted by .tau..sub.a, first voltage which is
applied to the liquid crystal element in one frame period is
denoted by V.sub.a, and initialization voltage which is applied to
the liquid crystal element right before one frame period is denoted
by V.sub.0, the first voltage V.sub.a is determined in accordance
with a difference between the initialization voltage V.sub.0 and
the signal voltage V.sub.i, and the length of the lighting period
.tau..sub.a of each of the plurality of light-emitting regions.
[0028] Another aspect of the present invention is a driving method
of a liquid crystal display device in which an image is displayed
by applying signal voltage V.sub.i in accordance with an image
signal to a liquid crystal element. The liquid crystal display
device includes a backlight. One frame period is divided into a
first subframe period and a second subframe period. When the length
of a backlight lighting period in one frame period is denoted by
.tau..sub.a1, the length of the first subframe period is denoted by
.tau..sub.a2, first voltage which is applied to the liquid crystal
element in the first subframe period is denoted by V.sub.a, and
second voltage which is applied to the liquid crystal element in
the second subframe period is denoted by V.sub.b, the first voltage
V.sub.a is determined in accordance with a difference between the
second voltage V.sub.b and the signal voltage V.sub.i, the length
of the backlight lighting period .tau..sub.a1, and the length of
the first subframe period .tau..sub.a2. The second voltage V.sub.b
is voltage at which the liquid crystal element performs black
display.
[0029] Another aspect of the present invention is a driving method
of a liquid crystal display device in which an image is displayed
by applying signal voltage V.sub.i in accordance with an image
signal to a liquid crystal element. The liquid crystal display
device includes a backlight. The backlight is divided into a
plurality of light-emitting regions in a display region and is
sequentially scanned to emit light. One frame period is divided
into a first subframe period and a second subframe period. When the
length of a lighting period of each of a plurality of
light-emitting regions in one frame period is denoted by
.tau..sub.a1, the length of the first subframe period is denoted by
.tau..sub.a2, first voltage which is applied to the liquid crystal
element in the first subframe period is denoted by V.sub.a, and
second voltage which is applied to the liquid crystal element in
the second subframe period is denoted by V.sub.b, the first voltage
V.sub.a is determined in accordance with a difference between the
second voltage V.sub.b and the signal voltage V.sub.i, the length
of the lighting period .tau..sub.a1 of each of the plurality of
light-emitting regions, and the length of the first subframe period
.tau..sub.a2. The second voltage V.sub.b is voltage at which the
liquid crystal element performs black display.
[0030] Another aspect of the present invention is a liquid crystal
display device in which an image is displayed by applying signal
voltage V.sub.i in accordance with an image signal to a liquid
crystal element. One frame period is divided into a first subframe
period and a second subframe period. When the length of the first
subframe period is denoted by .tau..sub.a, first voltage which is
applied to the liquid crystal element in the first subframe period
is denoted by V.sub.a, and second voltage which is applied to the
liquid crystal element in the second subframe period is denoted by
V.sub.b, the first voltage V.sub.a is determined in accordance with
a difference between the second voltage V.sub.b and the signal
voltage V.sub.i, and the length of the first subframe period
.tau..sub.a. The second voltage V.sub.b is voltage at which the
liquid crystal element performs black display.
[0031] Another aspect of the present invention is a liquid crystal
display device in which an image is displayed by applying signal
voltage V.sub.i in accordance with an image signal to a liquid
crystal element. The liquid crystal display device includes a
backlight. When the length of a backlight lighting period in one
frame period is denoted by .tau..sub.a, first voltage which is
applied to the liquid crystal element in one frame period is
denoted by V.sub.a, and initialization voltage which is applied to
the liquid crystal element right before one frame period is denoted
by V.sub.0, the first voltage V.sub.a is determined in accordance
with a difference between the initialization voltage V.sub.0 and
the signal voltage V.sub.i, and the length of the backlight
lighting period .tau..sub.a.
[0032] Another aspect of the present invention is a liquid crystal
display device in which an image is displayed by applying signal
voltage V.sub.i in accordance with an image signal to a liquid
crystal element. The liquid crystal display device includes a
backlight. The backlight is divided into a plurality of
light-emitting regions in a display region and is sequentially
scanned to emit light. When the length of a lighting period of each
of a plurality of light-emitting regions in one frame period is
denoted by .tau..sub.a, first voltage which is applied to the
liquid crystal element in one frame period is denoted by V.sub.a,
and initialization voltage which is applied to the liquid crystal
element right before one frame period is denoted by V.sub.0, the
first voltage V.sub.a is determined in accordance with a difference
between the initialization voltage V.sub.0 and the signal voltage
V.sub.i, and the length of the lighting period .tau..sub.a of each
of the plurality of light-emitting regions.
[0033] Another aspect of the present invention is a liquid crystal
display device in which an image is displayed by applying signal
voltage V.sub.i in accordance with an image signal to a liquid
crystal element. The liquid crystal display device includes a
backlight. One frame period is divided into a first subframe period
and a second subframe period. When the length of a backlight
lighting period in one frame period is denoted by .tau..sub.a1, the
length of the first subframe period is denoted by .tau..sub.a2,
first voltage which is applied to the liquid crystal element in the
first subframe period is denoted by V.sub.a, and second voltage
which is applied to the liquid crystal element in the second
subframe period is denoted by V.sub.b, the first voltage V.sub.a is
determined in accordance with a difference between the second
voltage V.sub.b and the signal voltage V.sub.i, the length of the
backlight lighting period .tau..sub.a1, and the length of the first
subframe period .tau..sub.a2. The second voltage V.sub.b is voltage
at which the liquid crystal element performs black display.
[0034] Another aspect of the present invention is a liquid crystal
display device in which an image is displayed by applying signal
voltage V.sub.i in accordance with an image signal to a liquid
crystal element. The liquid crystal display device includes a
backlight. The backlight is divided into a plurality of
light-emitting regions in a display region and is sequentially
scanned to emit light. One frame period is divided into a first
subframe period and a second subframe period. When the length of a
lighting period of each of a plurality of light-emitting regions in
one frame period is denoted by .tau..sub.a1, the length of the
first subframe period is denoted by .tau..sub.a2, first voltage
which is applied to the liquid crystal element in the first
subframe period is denoted by V.sub.a, and second voltage which is
applied to the liquid crystal element in the second subframe period
is denoted by V.sub.b, the first voltage V.sub.a is determined in
accordance with a difference between the second voltage V.sub.b and
the signal voltage V.sub.i, the length of the lighting period
.tau..sub.a1 of each of the plurality of light-emitting regions,
and the length of the first subframe period .tau..sub.a2. The
second voltage V.sub.b is voltage at which the liquid crystal
element performs black display.
[0035] Note that in this specification, a condition where the
darkest gray scale among gray scales which can be displayed is
displayed even a slight amount of light is emitted is described
that "luminance is 0" in some cases in addition to a condition
where light is not emitted at all.
[0036] Note that various types of switches can be used as a switch
shown in this document (a specification, a claim, a drawing, and
the like). An electrical switch, a mechanical switch, and the like
are given as examples. That is, any element can be used as long as
it can control a current flow, without limiting to a certain
element. For example, a transistor (e.g., a bipolar transistor or a
MOS transistor), a diode (e.g., a PN diode, a PIN diode, a Schottky
diode, a MIM (metal insulator metal) diode, a MIS (metal insulator
semiconductor) diode, or a diode-connected transistor), a
thyristor, or the like can be used as a switch. Alternatively, a
logic circuit in which such elements are combined can be used as a
switch.
[0037] In the case of using a transistor as a switch, polarity (a
conductivity type) of the transistor is not particularly limited
because it operates just as a switch. However, a transistor of
polarity with smaller off-current is preferably used when
off-current is to be suppressed. A transistor provided with an LDD
region, a transistor with a multi-gate structure, and the like are
given as examples of a transistor with smaller off-current. In
addition, it is preferable that an N-channel transistor be used
when a potential of a source terminal of the transistor which is
operated as a switch is closer to a potential of a
low-potential-side power supply (e.g., Vss, GND, or 0 V), while a
P-channel transistor be used when the potential of the source
terminal is closer to a potential of a high-potential-side power
supply (e.g., Vdd). This is because the absolute value of
gate-source voltage can be increased when the potential of the
source terminal of the transistor which is operated as the switch
is closer to a potential of a low-potential-side power supply in an
N-channel transistor and when the potential of the source terminal
of the transistor which is operated as the switch is closer to a
potential of a high-potential-side power supply in a P-channel
transistor, so that the transistor is useful to be operated as a
switch. This is also because the transistor does not often perform
a source follower operation, so that reduction in output voltage
does not often occur.
[0038] Note that a CMOS switch using both N-channel and P-channel
transistors may be used. By using a CMOS switch, the switch can
easily operate as a switch because current can flow when the
P-channel transistor or the N-channel transistor is turned on. For
example, voltage can be appropriately output regardless of whether
voltage of an input signal of the switch is high or low. In
addition, since a voltage amplitude value of a signal for turning
on or off the switch can be made small, power consumption can be
reduced.
[0039] Note also that when a transistor is used as a switch, the
switch includes an input terminal (one of a source terminal and a
drain terminal), an output terminal (the other of the source
terminal and the drain terminal), and a terminal for controlling
electrical conduction (a gate electrode). On the other hand, when a
diode is used as a switch, the switch does not have a terminal for
controlling electrical conduction in some cases. Therefore, when a
diode is used as a switch, the number of wirings for controlling
terminals can be more reduced than the case of using a transistor
as a switch.
[0040] Note that in this document (the specification, the claim,
the drawing, and the like), when it is explicitly described that "A
and B are connected", the case where elements are electrically
connected, the case where elements are functionally connected, and
the case where elements are directly connected are included
therein. Here, each of A and B corresponds to an object (e.g., a
device, an element, a circuit, a wiring, an electrode, a terminal,
a conductive film, or a layer). Accordingly, in structures
disclosed in this document (the specification, the claim, the
drawing, and the like), another element may be interposed between
elements having a connection relation shown in drawings and texts,
without limiting to a predetermined connection relation, for
example, the connection relation shown in the drawings and the
texts.
[0041] For example, in the case where A and B are electrically
connected, one or more elements which enable electrical connection
of A and B (e.g., a switch, a transistor, a capacitor, an inductor,
a resistor, and/or a diode) may be provided between A and B. In
addition, in the case where A and B are functionally connected, one
or more circuits which enable functional connection of A and B
(e.g., a logic circuit such as an inverter, a NAND circuit, or a
NOR circuit, a signal converter circuit such as a DA converter
circuit, an AD converter circuit, or a gamma correction circuit, a
potential level converter circuit such as a power supply circuit
(e.g., a boosting circuit or a voltage lower control circuit) or a
level shifter circuit for changing a potential level of a signal, a
voltage source, a current source, a switching circuit, or an
amplifier circuit such as a circuit which can increase signal
amplitude, the amount of current, or the like (e.g., an operational
amplifier, a differential amplifier circuit, a source follower
circuit, or a buffer circuit), a signal generating circuit, a
memory circuit, and/or a control circuit) may be provided between A
and B. Alternatively, in the case where A and B are directly
connected, A and B may be directly connected without interposing
another element or another circuit therebetween.
[0042] Note that when it is explicitly described that "A and B are
directly connected", the case where A and B are directly connected
(i.e., the case where A and B are connected without interposing
another element or another circuit therebetween) and the case where
A and B are electrically connected (i.e., the case where A and B
are connected by interposing another element or another circuit
therebetween) are included therein.
[0043] Note that when it is explicitly described that "A and B are
electrically connected", the case where A and B are electrically
connected (i.e., the case where A and B are connected by
interposing another element or another circuit therebetween), the
case where A and B are functionally connected (i.e., the case where
A and B are functionally connected by interposing another circuit
therebetween), and the case where A and B are directly connected
(i.e., the case where A and B are connected without interposing
another element or another circuit therebetween) are included
therein. That is, when it is explicitly described that "A and B are
electrically connected", the description is the same as the case
where it is explicitly only described that "A and B are
connected".
[0044] Note that a display element, a display device which is a
device having a display element, a light-emitting element, and a
light-emitting device which is a device having a light-emitting
element can use various types and can include various elements. For
example, as a display element, a display device, a light-emitting
element, and a light-emitting device, whose a display medium,
contrast, luminance, reflectivity, transmittivity, or the like
changes by an electromagnetic action, such as an EL element (e.g.,
an EL element including organic and inorganic materials, an organic
EL element, or an inorganic EL element), an electron emitter, a
liquid crystal element, electronic ink, an electrophoresis element,
a grating light valve (GLV), a plasma display panel (PDP), a
digital micromirror device (DMD), a piezoelectric ceramic display,
or a carbon nanotube can be used. Note that display devices using
an EL element include an EL display; display devices using an
electron emitter include a field emission display (FED), an
SED-type flat panel display (SED: Surface-conduction
Electron-emitter Display), and the like; display devices using a
liquid crystal element include a liquid crystal display (e.g., a
transmissive liquid crystal display, a semi-transmissive liquid
crystal display, a reflective liquid crystal display, a direct-view
liquid crystal display, or a projection liquid crystal display);
and display devices using electronic ink include electronic
paper.
[0045] Note that by using a catalyst (e.g., nickel) in the case of
forming microcrystalline silicon, crystallinity can be further
improved and a transistor having excellent electric characteristics
can be formed. At this time, crystallinity can be improved by
performing heat treatment without using a laser. Accordingly, a
gate driver circuit (e.g., a scan line driver circuit) and part of
a source driver circuit (e.g., an analog switch) can be formed over
the same substrate. In addition, in the case of not using a laser
for crystallization, crystallinity unevenness of silicon can be
suppressed. Therefore, an image having high quality can be
displayed.
[0046] Note also that polycrystalline silicon and microcrystalline
silicon can be formed without using a catalyst (e.g., nickel).
[0047] In addition, a transistor can be formed by using a
semiconductor substrate, an SOI substrate, or the like. In that
case, a MOS transistor, a junction transistor, a bipolar
transistor, or the like can be used as a transistor described in
this specification. Therefore, a transistor with few variations in
characteristics, sizes, shapes, or the like, with high current
supply capacity, and with a small size can be formed. By using such
a transistor, power consumption of a circuit can be reduced or a
circuit can be highly integrated.
[0048] In addition, a transistor including a compound semiconductor
or a oxide semiconductor such as ZnO, a-InGaZnO, SiGe, GaAs, IZO,
indium tin oxide (ITO), or SnO, and a thin film transistor or the
like obtained by thinning such a compound semiconductor or a oxide
semiconductor can be used. Therefore, manufacturing temperature can
be lowered and for example, such a transistor can be formed at room
temperature. Accordingly, the transistor can be formed directly on
a substrate having low heat resistance such as a plastic substrate
or a film substrate. Note that such a compound semiconductor or an
oxide semiconductor can be used for not only a channel portion of
the transistor but also other applications. For example, such a
compound semiconductor or an oxide semiconductor can be used as a
resistor, a pixel electrode, or a light-transmitting electrode.
Further, since such an element can be formed at the same time as
the transistor, cost can be reduced.
[0049] A transistor or the like formed by using an inkjet method or
a printing method can also be used. Accordingly, a transistor can
be formed at room temperature, can be formed at a low vacuum, or
can be formed using a large substrate. In addition, since the
transistor can be formed without using a mask (a reticle), layout
of the transistor can be easily changed. Further, since it is not
necessary to use a resist, material cost is reduced and the number
of steps can be reduced. Furthermore, since a film is formed only
in a necessary portion, a material is not wasted compared with a
manufacturing method in which etching is performed after the film
is formed over the entire surface, so that cost can be reduced.
[0050] Further, a transistor or the like including an organic
semiconductor or a carbon nanotube can be used. Accordingly, such a
transistor can be formed using a substrate which can be bent.
Therefore, a device using a transistor or the like including an
organic semiconductor or a carbon nanotube can resist a shock.
[0051] Furthermore, transistors with various structures can be
used. For example, a MOS transistor, a junction transistor, a
bipolar transistor, or the like can be used as a transistor
described in this document (the specification, the claim, the
drawing, and the like). By using a MOS transistor, the size of the
transistor can be reduced. Thus, a plurality of transistors can be
mounted. By using a bipolar transistor, large current can flow.
Thus, a circuit can be operated at high speed.
[0052] A MOS transistor, a bipolar transistor, and the like may be
formed over one substrate. Thus, reduction in power consumption,
reduction in size, high speed operation, and the like can be
realized.
[0053] Furthermore, various transistors can be used.
[0054] A transistor can be formed using various types of
substrates. The type of a substrate where a transistor is formed is
not limited to a certain type. For example, a single crystalline
substrate, an SOI substrate, a glass substrate, a quartz substrate,
a plastic substrate, a paper substrate, a cellophane substrate, a
stone substrate, a wood substrate, a cloth substrate (including a
natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber
(e.g., nylon, polyurethane, or polyester), a regenerated fiber
(e.g., acetate, cupra, rayon, or regenerated polyester), or the
like), a leather substrate, a rubber substrate, a stainless steel
substrate, a substrate including a stainless steel foil, or the
like can be used as a substrate where the transistor is formed.
Alternatively, a skin (e.g., epidermis or corium) or hypodermal
tissue of an animal such as a human being can be used as a
substrate where the transistor is formed. In addition, the
transistor may be formed using one substrate, and then, the
transistor may be transferred to another substrate. A single
crystalline substrate, an SOI substrate, a glass substrate, a
quartz substrate, a plastic substrate, a paper substrate, a
cellophane substrate, a stone substrate, a wood substrate, a cloth
substrate (including a natural fiber (e.g., silk, cotton, or hemp),
a synthetic fiber (e.g., nylon, polyurethane, or polyester), a
regenerated fiber (e.g., acetate, cupra, rayon, or regenerated
polyester), or the like), a leather substrate, a rubber substrate,
a stainless steel substrate, a substrate including a stainless
steel foil, or the like can be used as a substrate to which the
transistor is transferred. Alternatively, a skin (e.g., epidermis
or corium) or hypodermal tissue of an animal such as a human being
can be used as a substrate to which the transistor is transferred.
Further alternatively, the transistor may be formed using one
substrate and the substrate may be thinned by polishing. A single
crystalline substrate, an SOI substrate, a glass substrate, a
quartz substrate, a plastic substrate, a paper substrate, a
cellophane substrate, a stone substrate, a wood substrate, a cloth
substrate (including a natural fiber (e.g., silk, cotton, or hemp),
a synthetic fiber (e.g., nylon, polyurethane, or polyester), a
regenerated fiber (e.g., acetate, cupra, rayon, or regenerated
polyester), or the like), a leather substrate, a rubber substrate,
a stainless steel substrate, a substrate including a stainless
steel foil, or the like can be used as a substrate to be polished.
Alternatively, a skin (e.g., epidermis or corium) or hypodermal
tissue of an animal such as a human being can be used as a
substrate to be polished. By using such a substrate, a transistor
with excellent properties or a transistor with low power
consumption can be formed, a device with high durability or high
heat resistance can be formed, or reduction in weight or thickness
can be achieved.
[0055] A structure of a transistor can be various modes without
limiting to a certain structure. For example, a multi-gate
structure having two or more gate electrodes may be used. When the
multi-gate structure is used, a structure where a plurality of
transistors are connected in series is provided because a structure
where channel regions are connected in series is provided. By using
the multi-gate structure, off-current can be reduced or the
withstand voltage of the transistor can be increased to improve
reliability. Alternatively, by using the multi-gate structure,
drain-source current does not fluctuate very much even if
drain-source voltage fluctuates when the transistor operates in a
saturation region, so that a flat slope of voltage-current
characteristics can be obtained. By utilizing the flat slope of the
voltage-current characteristics, an ideal current source circuit or
an active load having a high resistance value can be realized.
Accordingly, a differential circuit or a current mirror circuit
having excellent properties can be realized. In addition, a
structure where gate electrodes are formed above and below a
channel may be used. By using the structure where gate electrodes
are formed above and below the channel, a channel region is
enlarged, so that the amount of current flowing therethrough can be
increased or a depletion layer can be easily formed to decrease an
S value. When the gate electrodes are formed above and below the
channel, a structure where a plurality of transistors are connected
in parallel is provided.
[0056] Further, a structure where a gate electrode is formed above
a channel, a structure where a gate electrode is formed below a
channel, a staggered structure, an inversely staggered structure, a
structure where a channel region is divided into a plurality of
regions, or a structure where channel regions are connected in
parallel or in series can be used. In addition, a source electrode
or a drain electrode may overlap with a channel region (or part of
it). By using the structure where the source electrode or the drain
electrode may overlap with the channel region (or part of it), the
case can be prevented in which electric charges are accumulated in
part of the channel region, which would result in an unstable
operation. Further, an LDD region may be provided. By providing the
LDD region, off-current can be reduced or the withstand voltage of
the transistor can be increased to improve reliability.
Alternatively, drain-source current does not fluctuate very much
even if drain-source voltage fluctuates when the transistor
operates in the saturation region, so that a flat slope of
voltage-current characteristics can be obtained.
[0057] Note that various types of transistors can be used for a
transistor in this document (the specification, the claim, the
drawing, and the like) and the transistor can be formed using
various types of substrates. Accordingly, all of circuits which are
necessary to realize a predetermined function may be formed using
the same substrate. For example, all of the circuits which are
necessary to realize the predetermined function may be formed using
a glass substrate, a plastic substrate, a single crystalline
substrate, an SOI substrate, or any other substrate. When all of
the circuits which are necessary to realize the predetermined
function are formed using the same substrate, cost can be reduced
by reduction in the number of component parts or reliability can be
improved by reduction in the number of connections to circuit
components. Alternatively, part of the circuits which are necessary
to realize the predetermined function may be formed using one
substrate and another part of the circuits which are necessary to
realize the predetermined function may be formed using another
substrate. That is, not all of the circuits which are necessary to
realize the predetermined function are required to be formed using
the same substrate. For example, part of the circuits which are
necessary to realize the predetermined function may be formed with
transistors using a glass substrate and another part of the
circuits which are necessary to realize the predetermined function
may be formed using a single crystalline substrate, so that an IC
chip formed by a transistor using the single crystalline substrate
may be connected to the glass substrate by COG (chip on glass) and
the IC chip may be provided over the glass substrate.
Alternatively, the IC chip may be connected to the glass substrate
by TAB (tape automated bonding) or a printed wiring board. When
part of the circuits are formed using the same substrate in this
manner, cost can be reduced by reduction in the number of component
parts or reliability can be improved by reduction in the number of
connections to circuit components. In addition, for example, by
forming a portion with high driving voltage or a portion with high
driving frequency, which consumes large power, using a single
crystalline substrate and using an IC chip formed by the circuit
instead of forming such a portion using the same substrate,
increase in power consumption can be prevented.
[0058] Note also that one pixel corresponds to one element whose
brightness can be controlled in this document (the specification,
the claim, the drawing, and the like). For example, one pixel
corresponds to one color element which expresses brightness.
Therefore, in the case of a color display device having color
elements of R (Red), G (Green), and B (Blue), a minimum unit of an
image is formed of three pixels of an R pixel, a G pixel, and a B
pixel. Note that the color elements are not limited to three
colors, and color elements of more than three colors may be used or
a color other than RGB may be added. For example, RGBW may be used
by adding W (white). In addition, RGB plus one or more colors of
yellow, cyan, magenta emerald green, vermilion, and the like may be
used. Further, a color similar to at least one of R, G, and B may
be added to RGB. For example, R, G, B1, and B2 may be used.
Although both B1 and B2 are blue, they have slightly different
frequency. Similarly, R1, R2, G, and B may be used, for example. By
using such color elements, display which is closer to the real
object can be performed. Alternatively, by using such color
elements, power consumption can be reduced. Furthermore, as another
example, in the case of controlling brightness of one color element
by using a plurality of regions, one region may correspond to one
pixel. For example, in the case of performing area ratio gray scale
display or in the case of including a subpixel, a plurality of
regions which control brightness are provided in each color element
and gray scales are expressed with the whole regions. In this case,
one region which controls brightness may correspond to one pixel.
Thus, in that case, one color element includes a plurality of
pixels. Alternatively, even when the plurality of regions which
control brightness are provided in one color element, these regions
may be collected as one pixel. Thus, in that case, one color
element includes one pixel. In that case, one color element
includes one pixel. In the case where brightness is controlled in a
plurality of regions in each color element, regions which
contribute to display have different area dimensions depending on
pixels in some cases. In addition, in the plurality of regions
which control brightness in each color element, signals supplied to
each of the plurality of regions may be slightly varied to widen a
viewing angle. That is, potentials of pixel electrodes included in
the plurality of regions provided in each color element may be
different from each other. Accordingly, voltage applied to liquid
crystal molecules are varied depending on the pixel electrodes.
Therefore, the viewing angle can be widened.
[0059] Note that when it is explicitly described that "one pixel
(for three colors)", it corresponds to the case where three pixels
of R, G, and B are considered as one pixel. Meanwhile, when it is
explicitly described that "one pixel (for one color)", it
corresponds to the case where the plurality of regions are provided
in each color element and collectively considered as one pixel.
[0060] Note also that in this document (the specification, the
claim, the drawing, and the like), pixels are provided (arranged)
in matrix in some cases. Here, description that pixels are provided
(arranged) in matrix includes the case where the pixels are
arranged in a straight line and the case where the pixels are
arranged in a jagged line, in a longitudinal direction or a lateral
direction. Therefore, in the case of performing full color display
with three color elements (e.g., RGB), the following cases are
included therein: the case where the pixels are arranged in stripes
and the case where dots of the three color elements are arranged in
a delta pattern. In addition, the case is also included therein in
which dots of the three color elements are provided in Bayer
arrangement. Note that the color elements are not limited to three
colors, and color elements of more than three colors may be used.
RGBW, RGB plus one or more of yellow, cyan, magenta, and the like,
or the like is given as an example. Further, the sizes of display
regions may be different between respective dots of color elements.
Thus, power consumption can be reduced or the life of a display
element can be prolonged.
[0061] Note also that in this document (the specification, the
claim, the drawing, and the like), an active matrix method in which
an active element is included in a pixel or a passive matrix method
in which an active element is not included in a pixel can be
used.
[0062] In the active matrix method, as an active element (a
non-linear element), not only a transistor but also various active
elements (non-linear elements) can be used. For example, a MIM
(metal insulator metal), a TFD (thin film diode), or the like can
also be used. Since such an element has few number of manufacturing
steps, manufacturing cost can be reduced or yield can be improved.
Further, since the size of the element is small, an aperture ratio
can be improved, so that power consumption can be reduced or high
luminance can be achieved.
[0063] As a method other than the active matrix method, the passive
matrix method in which an active element (a non-linear element) is
not used can also be used. Since an active element (a non-linear
element) is not used, manufacturing steps is few, so that
manufacturing cost can be reduced or the yield can be improved.
Further, since an active element (a non-linear element) is not
used, the aperture ratio can be improved, so that power consumption
can be reduced or high luminance can be achieved.
[0064] Note that a transistor is an element having at least three
terminals of a gate, a drain, and a source. The transistor has a
channel region between a drain region and a source region, and
current can flow through the drain region, the channel region, and
the source region. Here, since the source and the drain of the
transistor may change depending on the structure, the operating
condition, and the like of the transistor, it is difficult to
define which is a source or a drain. Therefore, in this document
(the specification, the claim, the drawing, and the like), a region
functioning as a source and a drain may not be called the source or
the drain. In such a case, for example, one of the source and the
drain may be referred to as a first terminal and the other thereof
may be referred to as a second terminal. Alternatively, one of the
source and the drain may be referred to as a first electrode and
the other thereof may be referred to as a second electrode. Further
alternatively, one of the source and the drain may be referred to
as a source region and the other thereof may be called a drain
region.
[0065] Note also that a transistor may be an element having at
least three terminals of a base, an emitter, and a collector. In
this case also, one of the emitter and the collector may be
similarly called a first terminal and the other terminal may be
called a second terminal.
[0066] A gate corresponds to all or part of a gate electrode and a
gate wiring (also referred to as a gate line, a gate signal line, a
scan line, a scan signal line, or the like). A gate electrode
corresponds to a conductive film which overlaps with a
semiconductor which forms a channel region with a gate insulating
film interposed therebetween. Note that part of the gate electrode
overlaps with an LDD (lightly doped drain) region, the source
region, or the drain region with the gate insulating film
interposed therebetween in some cases. A gate wiring corresponds to
a wiring for connecting a gate electrode of each transistor to each
other, a wiring for connecting a gate electrode of each pixel to
each other, or a wiring for connecting a gate electrode to another
wiring.
[0067] However, there is a portion (a region, a conductive film, a
wiring, or the like) which functions as both a gate electrode and a
gate wiring. Such a portion (a region, a conductive film, a wiring,
or the like) may be called either a gate electrode or a gate
wiring. That is, there is a region where a gate electrode and a
gate wiring cannot be clearly distinguished from each other. For
example, in the case where a channel region overlaps with part of
an extended gate wiring, the overlapped portion (region, conductive
film, wiring, or the like) functions as both a gate wiring and a
gate electrode. Accordingly, such a portion (a region, a conductive
film, a wiring, or the like) may be called either a gate electrode
or a gate wiring.
[0068] In addition, a portion (a region, a conductive film, a
wiring, or the like) which is formed of the same material as a gate
electrode, forms the same island as the gate electrode, and is
connected to the gate electrode may also be called a gate
electrode. Similarly, a portion (a region, a conductive film, a
wiring, or the like) which is formed of the same material as a gate
wiring, forms the same island as the gate wiring, and is connected
to the gate wiring may also be called a gate wiring. In a strict
sense, such a portion (a region, a conductive film, a wiring, or
the like) does not overlap with a channel region or does not have a
function of connecting the gate electrode to another gate electrode
in some cases. However, there is a portion (a region, a conductive
film, a wiring, or the like) which is formed of the same material
as a gate electrode or a gate wiring, forms the same island as the
gate electrode or the gate wiring, and is connected to the gate
electrode or the gate wiring because of conditions in a
manufacturing step. Thus, such a portion (a region, a conductive
film, a wiring, or the like) may also be called either a gate
electrode or a gate wiring.
[0069] In a multi-gate transistor, for example, a gate electrode is
often connected to another gate electrode by using a conductive
film which is formed of the same material as the gate electrode.
Since such a portion (a region, a conductive film, a wiring, or the
like) is a portion (a region, a conductive film, a wiring, or the
like) for connecting the gate electrode to another gate electrode,
it may be called a gate wiring, and it may also be called a gate
electrode because a multi-gate transistor can be considered as one
transistor. That is, a portion (a region, a conductive film, a
wiring, or the like) which is formed of the same material as a gate
electrode or a gate wiring, forms the same island as the gate
electrode or the gate wiring, and is connected to the gate
electrode or the gate wiring may be called either a gate electrode
or a gate wiring. In addition, for example, part of a conductive
film which connects the gate electrode and the gate wiring and is
formed of a material which is different from that of the gate
electrode or the gate wiring may also be called either a gate
electrode or a gate wiring.
[0070] Note that a gate terminal corresponds to part of a portion
(a region, a conductive film, a wiring, or the like) of a gate
electrode or a portion (a region, a conductive film, a wiring, or
the like) which is electrically connected to the gate
electrode.
[0071] Note that when a wiring is called a gate wiring, a gate
line, a gate signal line, a scan line, a scan signal line, there is
the case in which a gate of a transistor is not connected to a
wiring. In this case, the gate wiring, the gate line, the gate
signal line, the scan line, or the scan signal line corresponds to
a wiring formed in the same layer as the gate of the transistor, a
wiring formed of the same material of the gate of the transistor,
or a wiring formed at the same time as the gate of the transistor
in some cases. As examples, a wiring for storage capacitance, a
power supply line, a reference potential supply line, and the like
can be given.
[0072] Note also that a source corresponds to all or part of a
source region, a source electrode, and a source wiring (also
referred to as a source line, a source signal line, a data line, a
data signal line, or the like). A source region corresponds to a
semiconductor region including a large amount of p-type impurities
(e.g., boron or gallium) or n-type impurities (e.g., phosphorus or
arsenic). Therefore, a region including a small amount of p-type
impurities or n-type impurities, namely, an LDD (lightly doped
drain) region is not included in the source region. A source
electrode is part of a conductive layer formed of a material
different from that of a source region, and electrically connected
to the source region. However, there is the case where a source
electrode and a source region are collectively called a source
electrode. A source wiring is a wiring for connecting a source
electrode of each transistor to each other, a wiring for connecting
a source electrode of each pixel to each other, or a wiring for
connecting a source electrode to another wiring.
[0073] However, there is a portion (a region, a conductive film, a
wiring, or the like) functioning as both a source electrode and a
source wiring. Such a portion (a region, a conductive film, a
wiring, or the like) may be called either a source electrode or a
source wiring. That is, there is a region where a source electrode
and a source wiring cannot be clearly distinguished from each
other. For example, in the case where a source region overlaps with
part of an extended source wiring, the overlapped portion (region,
conductive film, wiring, or the like) functions as both a source
wiring and a source electrode. Accordingly, such a portion (a
region, a conductive film, a wiring, or the like) may be called
either a source electrode or a source wiring.
[0074] In addition, a portion (a region, a conductive film, a
wiring, or the like) which is formed of the same material as a
source electrode, forms the same island as the source electrode,
and is connected to the source electrode, or a portion (a region, a
conductive film, a wiring, or the like) which connects a source
electrode and another source electrode may also be called a source
electrode. Further, a portion which overlaps with a source region
may be called a source electrode. Similarly, a portion (a region, a
conductive film, a wiring, or the like) which is formed of the same
material as a source wiring, forms the same island as the source
wiring, and is connected to the source wiring may also be called a
source wiring. In a strict sense, such a portion (a region, a
conductive film, a wiring, or the like) does not have a function of
connecting the source electrode to another source electrode in some
cases. However, there is a portion (a region, a conductive film, a
wiring, or the like) which is formed of the same material as a
source electrode or a source wiring, forms the same island as the
source electrode or the source wiring, and is connected to the
source electrode or the source wiring because of conditions in a
manufacturing step. Thus, such a portion (a region, a conductive
film, a wiring, or the like) may also be called either a source
electrode or a source wiring.
[0075] In addition, for example, part of a conductive film which
connects a source electrode and a source wiring and is formed of a
material which is different from that of the source electrode or
the source wiring may be called either a source electrode or a
source wiring.
[0076] Note that a source terminal corresponds to part of a source
region, a source electrode, or a portion (a region, a conductive
film, a wiring, or the like) which is electrically connected to the
source electrode.
[0077] Note that when a wiring is called a source wiring, a source
line, a source signal line, a data line, a data signal line, there
is the case in which a source (a drain) of a transistor is not
connected to a wiring. In this case, the source wiring, the source
line, the source signal line, the data line, or the data signal
line corresponds to a wiring formed in the same layer as the source
(the drain) of the transistor, a wiring formed of the same material
of the source (the drain) of the transistor, or a wiring formed at
the same time as the source (the drain) of the transistor in some
cases. As examples, a wiring for storage capacitance, a power
supply line, a reference potential supply line, and the like can be
given.
[0078] Note also that the same can be said for a drain.
[0079] Note also that a semiconductor device corresponds to a
device having a circuit including a semiconductor element (e.g., a
transistor, a diode, or thyristor). The semiconductor device may
also include all devices that can function by utilizing
semiconductor characteristics.
[0080] Note also that a display element corresponds to an optical
modulation element, a liquid crystal element, a light-emitting
element, an EL element (an organic EL element, an inorganic EL
element, or an EL element including organic and inorganic
materials), an electron emitter, an electrophoresis element, a
discharging element, a light-reflective element, a light
diffraction element, a digital micro device (DMD), or the like.
Note that the present invention is not limited to this.
[0081] In addition, a display device corresponds to a device having
a display element. Note that the display device may also
corresponds to a display panel itself where a plurality of pixels
including display elements are formed over the same substrate as a
peripheral driver circuit for driving the pixels. In addition, the
display device may also include a peripheral driver circuit
provided over a substrate by wire bonding or bump bonding, namely,
an IC chip connected by chip on glass (COG) or an IC chip connected
by TAB or the like. Further, the display device may also include a
flexible printed circuit (FPC) to which an IC chip, a resistor, a
capacitor, an inductor, a transistor, or the like is attached. Note
also that the display device includes a printed wiring board (PWB)
which is connected through a flexible printed circuit (FPC) and to
which an IC chip, a resistor, a capacitor, an inductor, a
transistor, or the like is attached. The display device may also
include an optical sheet such as a polarizing plate or a
retardation plate. The display device may also include a lighting
device, a housing, an audio input and output device, a light
sensor, or the like. Here, a lighting device such as a backlight
unit may include a light guide plate, a prism sheet, a diffusion
sheet, a reflective sheet, a light source (e.g., an LED or a cold
cathode fluorescent lamp), a cooling device (e.g., a water cooling
device or an air cooling device), or the like.
[0082] Moreover, a lighting device corresponds to a device having a
backlight unit, a light guide plate, a prism sheet, a diffusion
sheet, a reflective sheet, or a light source (e.g., an LED, a cold
cathode fluorescent lamp, or a hot cathode fluorescent lamp), a
cooling device, or the like.
[0083] In addition, a light-emitting device corresponds to a device
having a light-emitting element and the like. In the case of
including a light-emitting element as a display element, the
light-emitting device is one of specific examples of a display
device.
[0084] Note that a reflective device corresponds to a device having
a light-reflective element, a light diffraction element,
light-reflective electrode, or the like.
[0085] A liquid crystal display device corresponds to a display
device including a liquid crystal element. Liquid crystal display
devices include a direct-view liquid crystal display, a projection
liquid crystal display, a transmissive liquid crystal display, a
reflective liquid crystal display, a semi-transmissive liquid
crystal display, and the like.
[0086] Note also that a driving device corresponds to a device
having a semiconductor element, an electric circuit, or an
electronic circuit. For example, a transistor which controls input
of a signal from a source signal line to a pixel (also referred to
as a selection transistor, a switching transistor, or the like), a
transistor which supplies voltage or current to a pixel electrode,
a transistor which supplies voltage or current to a light-emitting
element, and the like are examples of the driving device. A circuit
which supplies a signal to a gate signal line (also referred to as
a gate driver, a gate line driver circuit, or the like), a circuit
which supplies a signal to a source signal line (also referred to
as a source driver, a source line driver circuit, or the like) are
also examples of the driving device.
[0087] Note also that a display device, a semiconductor device, a
lighting device, a cooling device, a light-emitting device, a
reflective device, a driving device, and the like overlap with each
other in some cases. For example, a display device includes a
semiconductor device and a light-emitting device in some cases.
Alternatively, a semiconductor device includes a display device and
a driving device in some cases.
[0088] In this document (the specification, the claim, the drawing,
and the like), when it is explicitly described that "B is formed on
A" or "B is formed over A", it does not necessarily mean that B is
formed in direct contact with A. The description includes the case
where A and B are not in direct contact with each other, i.e., the
case where another object is interposed between A and B. Here, each
of A and B corresponds to an object (e.g., a device, an element, a
circuit, a wiring, an electrode, a terminal, a conductive film, or
a layer).
[0089] Accordingly, for example, when it is explicitly described
that a layer B is formed on (or over) a layer A, it includes both
the case where the layer B is formed in direct contact with the
layer A, and the case where another layer (e.g., a layer C or a
layer D) is formed in direct contact with the layer A and the layer
B is formed in direct contact with the layer C or D. Note that
another layer (e.g., a layer C or a layer D) may be a single layer
or a plurality of layers.
[0090] Similarly, when it is explicitly described that B is formed
above A, it does not necessarily mean that B is formed in direct
contact with A, and another object may be interposed therebetween.
Accordingly, for example, when it is explicitly described that a
layer B is formed above a layer A, it includes both the case where
the layer B is formed in direct contact with the layer A, and the
case where another layer (e.g., a layer C or a layer D) is formed
in direct contact with the layer A and the layer B is formed in
direct contact with the layer C or D. Note that another layer
(e.g., a layer C or a layer D) may be a single layer or a plurality
of layers.
[0091] Note that when it is explicitly described that B is formed
in direct contact with A, it includes not the case where another
object is interposed between A and B but the case where B is formed
in direct contact with A.
[0092] Note that the same can be said when it is explicitly
described that B is formed below or under A.
[0093] By using the present invention, a hold-type display device
without a problem of motion blur and a driving method thereof can
be provided. In addition, by using the present invention, a display
device with low power consumption and a driving method thereof can
be provided. Further, by using the present invention, a display
device with improved quality for still images and moving images and
a driving method thereof can be provided. Furthermore, by using the
present invention, a display device with a wider viewing angle and
a driving method thereof can be provided. Moreover, by using the
present invention, a display device with improved response speed of
a liquid crystal and a driving method thereof can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0094] In the accompanying drawings:
[0095] FIGS. 1A to 1C are diagrams each illustrating definitions of
words and signs in accordance with the present invention;
[0096] FIGS. 2A to 2C are diagrams each illustrating definitions of
words and signs in accordance with the present invention;
[0097] FIGS. 3A and 3B are diagrams each illustrating an example of
a condition of integrated luminance with respect to control
parameters in accordance with the present invention;
[0098] FIGS. 4A, 4C, 4E, and 4G are diagrams each illustrating an
example of a condition of a lighting ratio with respect to control
parameters in accordance with the present invention, and FIGS. 4B,
4D, 4F, and 4H are diagrams each illustrating an example of a
condition of average luminance with respect to control parameters
in accordance with the present invention;
[0099] FIGS. 5A to 5C are diagrams each illustrating an example of
conditions of a lighting ratio and average luminance with respect
to control parameters in accordance with the present invention;
[0100] FIGS. 6A to 6P are diagrams each illustrating an example of
a condition of a lighting ratio with respect to control parameters
in accordance with the present invention;
[0101] FIGS. 7A to 7E are diagrams each illustrating an example of
a condition of a lighting ratio with respect to control parameters
in accordance with the present invention;
[0102] FIGS. 8A to 8G are diagrams each illustrating an example of
a condition of a lighting ratio with respect to control parameters
in accordance with the present invention;
[0103] FIGS. 9A to 9F are diagrams each illustrating an example of
a timing chart of a semiconductor device in accordance with the
present invention, and FIGS. 9G and 9H are diagrams each
illustrating an example of a pixel circuit of a semiconductor
device in accordance with the present invention;
[0104] FIGS. 10A and 10B are diagrams each illustrating examples of
a timing chart and a display condition of a semiconductor device in
accordance with the present invention;
[0105] FIGS. 11A to 11J are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0106] FIGS. 12A and 12B are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0107] FIGS. 13A to 13C are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0108] FIGS. 14A, 14B, 14E, and 14F are diagrams each showing an
example of luminance with respect to a gray scale of a
semiconductor device in accordance with the present invention, and
FIGS. 14C, 14D, 14G, and 14H are diagrams each showing an example
of the number of data with respect to a gray scale of a
semiconductor device in accordance with the present invention;
[0109] FIGS. 15A and 15B are diagrams each showing an example of
luminance with respect to a gray scale of a semiconductor device in
accordance with the present invention, and FIGS. 15C and 15D are
diagrams each showing an example of the number of data with respect
to a gray scale of a semiconductor device in accordance with the
present invention;
[0110] FIGS. 16A to 16D are diagrams each showing an example of
luminance with respect to a gray scale of a semiconductor device in
accordance with the present invention;
[0111] FIGS. 17A to 17L are views each illustrating an example of
control parameters in accordance with the present invention;
[0112] FIGS. 18A to 18I are views each illustrating an example of
control parameters in accordance with the present invention, and
FIGS. 18J to 18L are diagrams in which histograms of images shown
in FIGS. 18A to 18I are compared with each other;
[0113] FIGS. 19A to 19C are views each illustrating an example of
control parameters in accordance with the present invention;
[0114] FIGS. 20A to 20C are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0115] FIGS. 21A to 21D are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention, and FIGS. 21E and 21F are diagrams each
illustrating an example of a driver circuit of a semiconductor
device in accordance with the present invention;
[0116] FIGS. 22A to 22D are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0117] FIGS. 23A to 23D are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0118] FIGS. 24A and 24B are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0119] FIGS. 25A to 25C are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0120] FIGS. 26A to 26C are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0121] FIGS. 27A to 27C are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0122] FIGS. 28A to 28C are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0123] FIGS. 29A to 29C are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0124] FIGS. 30A to 30C are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0125] FIGS. 31A to 31C are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0126] FIGS. 32A to 32C are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0127] FIG. 33A is a diagram illustrating an example of a circuit
structure of a semiconductor device in accordance with the present
invention, and FIG. 33B is a diagram of an example of a timing
chart of a semiconductor device in accordance with the present
invention;
[0128] FIGS. 34A to 34BII are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0129] FIGS. 35A to 35BII are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0130] FIGS. 36A to 36BII are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0131] FIG. 37 is a view illustrating an example of a
cross-sectional view of a semiconductor device in accordance with
the present invention;
[0132] FIGS. 38A and 38B are views each illustrating an example of
a cross-sectional view of a semiconductor device in accordance with
the present invention;
[0133] FIGS. 39A and 39B are views each illustrating an example of
a cross-sectional view of a semiconductor device in accordance with
the present invention;
[0134] FIG. 40 is a view illustrating an example of a pixel layout
of a semiconductor device in accordance with the present
invention;
[0135] FIGS. 41A and 41B are views each illustrating an example of
a pixel layout of a semiconductor device in accordance with the
present invention;
[0136] FIGS. 42A and 42B are views each illustrating an example of
a pixel layout of a semiconductor device in accordance with the
present invention;
[0137] FIG. 43 is a view illustrating an example of a
cross-sectional view of a semiconductor device in accordance with
the present invention;
[0138] FIGS. 44A to 44D are views each illustrating an example of a
peripheral component of a semiconductor device in accordance with
the present invention;
[0139] FIG. 45 is a view illustrating an example of a peripheral
component of a semiconductor device in accordance with the present
invention;
[0140] FIGS. 46A to 46C are diagrams each showing an example of a
circuit structure of a panel of a semiconductor device in
accordance with the present invention;
[0141] FIGS. 47A and 47B are views each illustrating an example of
a cross-sectional view of a semiconductor device in accordance with
the present invention;
[0142] FIGS. 48A to 48C are diagrams each illustrating an example
of a driving method of a semiconductor device in accordance with
the present invention;
[0143] FIGS. 49A and 49B are diagrams each illustrating an example
of a circuit structure of a semiconductor device in accordance with
the present invention;
[0144] FIGS. 50A to 50C are diagrams each illustrating an example
of a peripheral component of a semiconductor device in accordance
with the present invention;
[0145] FIGS. 51A and 51B are diagrams each illustrating an example
of a circuit structure of a semiconductor device in accordance with
the present invention;
[0146] FIG. 52 is a diagram illustrating an example of a circuit
structure of a semiconductor device in accordance with the present
invention;
[0147] FIG. 53 is a diagram illustrating an example of a circuit
structure of a semiconductor device in accordance with the present
invention;
[0148] FIGS. 54A and 54B are views each illustrating an example of
a cross-sectional view of a semiconductor device in accordance with
the present invention;
[0149] FIGS. 55A to 55D are views each illustrating an example of a
cross-sectional view of a semiconductor device in accordance with
the present invention;
[0150] FIGS. 56A to 56D are views each illustrating an example of a
cross-sectional view of a semiconductor device in accordance with
the present invention;
[0151] FIGS. 57A to 57D are views each illustrating an example of a
cross-sectional view of a semiconductor device in accordance with
the present invention;
[0152] FIG. 58 is a view illustrating an example of a top plan view
of a semiconductor device in accordance with the present
invention;
[0153] FIGS. 59A to 59D are views each illustrating an example of a
top plan view of a semiconductor device in accordance with the
present invention;
[0154] FIGS. 60A to 60D are views each illustrating an example of a
top plan view of a semiconductor device in accordance with the
present invention;
[0155] FIG. 61A is a view illustrating an example of a pixel layout
of a semiconductor device in accordance with the present invention,
and FIG. 61B is a view illustrating an example of a cross-sectional
view thereof;
[0156] FIG. 62A is a view illustrating an example of a pixel layout
of a semiconductor device in accordance with the present invention,
and FIG. 62B is a view illustrating an example of a cross-sectional
view thereof;
[0157] FIG. 63A is a view illustrating an example of a pixel layout
of a semiconductor device in accordance with the present invention,
and FIG. 63B is a view illustrating an example of a cross-sectional
view thereof;
[0158] FIGS. 64A and 64B are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0159] FIGS. 65A and 65B are diagrams each illustrating an example
of a timing chart of a semiconductor device in accordance with the
present invention;
[0160] FIG. 66 is a diagram illustrating an example of a circuit
structure of a semiconductor device in accordance with the present
invention;
[0161] FIG. 67 is a diagram illustrating an example of a circuit
structure of a semiconductor device in accordance with the present
invention;
[0162] FIG. 68 is a diagram illustrating an example of a circuit
structure of a semiconductor device in accordance with the present
invention;
[0163] FIG. 69 is a diagram illustrating an example of a circuit
structure of a semiconductor device in accordance with the present
invention;
[0164] FIG. 70 is a diagram illustrating an example of a circuit
structure of a semiconductor device in accordance with the present
invention;
[0165] FIGS. 71A to 71G are cross-sectional views each illustrating
a manufacturing process of a semiconductor device in accordance
with the present invention;
[0166] FIG. 72 is a view illustrating an example of a
cross-sectional view of a semiconductor device in accordance with
the present invention;
[0167] FIG. 73 is a view illustrating an example of a
cross-sectional view of a semiconductor device in accordance with
the present invention;
[0168] FIG. 74 is a view illustrating an example of a
cross-sectional view of a semiconductor device in accordance with
the present invention;
[0169] FIG. 75 is a view illustrating an example of a
cross-sectional view of a semiconductor device in accordance with
the present invention;
[0170] FIGS. 76A to 76C are cross-sectional views each illustrating
an example of a display element of a semiconductor device in
accordance with the present invention;
[0171] FIGS. 77A to 77C are cross-sectional views each illustrating
an example of a display element of a semiconductor device in
accordance with the present invention;
[0172] FIGS. 78A and 78B are views each illustrating an example of
a structure of a semiconductor device in accordance with the
present invention;
[0173] FIG. 79 is a view illustrating an example of a structure of
a semiconductor device in accordance with the present
invention;
[0174] FIG. 80 is a view illustrating an example of a structure of
a semiconductor device in accordance with the present
invention;
[0175] FIG. 81 is a view illustrating an example of a structure of
a semiconductor device in accordance with the present
invention;
[0176] FIGS. 82A to 82C are views each illustrating an example of a
structure of a semiconductor device in accordance with the present
invention;
[0177] FIG. 83 is a diagram illustrating an example of a circuit
structure of a semiconductor device in accordance with the present
invention;
[0178] FIG. 84 is a diagram illustrating an example of a timing
chart of a semiconductor device in accordance with the present
invention;
[0179] FIG. 85 is a diagram illustrating an example of a timing
chart of a semiconductor device in accordance with the present
invention;
[0180] FIGS. 86A and 86B are views each illustrating an example of
a driving method of a semiconductor device in accordance with the
present invention;
[0181] FIGS. 87A to 87E are cross-sectional views each illustrating
an example of a display element of a semiconductor device in
accordance with the present invention;
[0182] FIG. 88 is a view illustrating an example of a manufacturing
apparatus of a semiconductor device in accordance with the present
invention;
[0183] FIG. 89 is a view illustrating an example of a manufacturing
device of a semiconductor device in accordance with the present
invention;
[0184] FIG. 90 is a view illustrating an example of a structure of
a semiconductor device in accordance with the present
invention;
[0185] FIG. 91 is a view illustrating an example of a structure of
a semiconductor device in accordance with the present
invention;
[0186] FIGS. 92A and 92B are views each illustrating an example of
a structure of a semiconductor device in accordance with the
present invention;
[0187] FIGS. 93A and 93B are views each illustrating an example of
a structure of a semiconductor device in accordance with the
present invention;
[0188] FIG. 94 is a view illustrating an example of a structure of
a semiconductor device in accordance with the present
invention;
[0189] FIG. 95 is a view illustrating an example of a structure of
a semiconductor device in accordance with the present
invention;
[0190] FIGS. 96A to 96H are views each illustrating an electronic
device using a semiconductor device in accordance with the present
invention;
[0191] FIG. 97 is a view illustrating an electronic device using a
semiconductor device in accordance with the present invention;
[0192] FIG. 98 is a view illustrating an electronic device using a
semiconductor device in accordance with the present invention;
[0193] FIG. 99 is a view illustrating an electronic device using a
semiconductor device in accordance with the present invention;
[0194] FIG. 100 is a view illustrating an electronic device using a
semiconductor device in accordance with the present invention;
[0195] FIGS. 101A and 101B are views each illustrating an
electronic device using a semiconductor device in accordance with
the present invention; and
[0196] FIGS. 102A and 102B are views each illustrating an
electronic device using a semiconductor device in accordance with
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0197] Hereinafter, the present invention will be described by way
of embodiment modes with reference to the drawings. However, the
present invention can be implemented in various different ways and
it will be easily understood by those skilled in the art that
various changes and modifications are possible. Unless such changes
and modifications depart from the spirit and the scope of the
present invention, they should be construed as being included
therein. Therefore, the present invention should not be construed
as being limited to the description of the embodiment modes.
Embodiment Mode 1
[0198] In this embodiment mode, words relating to a driving method
of a display device, such as instantaneous luminance, integrated
luminance, a lighting ratio, and average luminance used in this
document (the specification, the claim, the drawing, and the like),
and control modes thereof are described.
[0199] First, meanings of words and signs used in this document are
described. First, words about time and signs thereof, i.e., t, F,
.tau..sub.a, .tau..sub.b, and R are described. The sign t expresses
time. The sign F expresses one frame period and the length thereof.
One frame period F is divided into a plurality of subframe periods,
and each of the subframe periods are classified into an image
display period or a blanking interval. Here, the image display
period is a period during which original luminance of an image is
mainly displayed. The blanking interval is a period during which an
image displayed in the image display period can be reset by human
eyes. Note that the subframe period may be a period other than the
image display period and the blanking interval. The sign
.tau..sub.a expresses the image display period and the length
thereof. The sign .tau..sub.b expresses the blanking interval and
the length thereof. Note also that periods other than the image
display period .tau..sub.a in the one frame period F each
correspond to the blanking interval .tau..sub.b. The sign R
expresses a lighting ratio. Here, the lighting ratio is a value
obtained by dividing the image display period .tau..sub.a by the
one frame period F. That is, the lighting ratio R is a ratio of the
image display period .tau..sub.a in the one frame period F.
[0200] Next, words relating to luminance and signs thereof, i.e., I
(t), L, and B are described. The sign I (t) shows instantaneous
luminance. Here, instantaneous luminance is instantaneous emission
intensity of a pixel. The sign L expresses integrated luminance.
Here, the integrated luminance is a value obtained by integrating
the instantaneous luminance I (t) by time in a range of the one
frame period F. The sign B expresses average luminance. Here, the
average luminance is a value obtained by dividing the integrated
luminance L by the image display period .tau..sub.a. That is, the
average luminance B expresses luminance of a pixel when it is
assumed that the luminance of the pixel is constant in the image
display period .tau..sub.a.
[0201] When the above-described relations are represented by
formulas, the relations can be represented by Formula 1 to Formula
4.
R = .tau. a F [ Formula 1 ] .tau. b = F - .tau. a [ Formula 2 ] L =
.intg. F I ( t ) t [ Formula 3 ] B = L .tau. a [ Formula 4 ]
##EQU00001##
[0202] Hereinafter, when the above-described signs are used without
notice in this document, meanings thereof may be considered that
they follow the above-described definitions.
[0203] Note that an actual relationship between the words which are
defined in this document and display conditions of a display device
are described. As for the lighting ratio R, driving becomes closer
to hold-type driving as R increases, and driving becomes closer to
impulsive driving as R decreases. As for the integrated luminance
L, luminance perceived by human eyes increases as L increases, and
luminance perceived by human eyes decreases as L decreases. As for
the instantaneous luminance I (t), either increase or decrease in
the integrated luminance I (t) does not have a direct relation that
luminance perceived by human eyes increases or decreased. This is
because human eyes cannot perceive fluctuation in light, brightness
of which fluctuates with high frequency and perceives the light as
constant light. At this time, brightness perceived by human eyes is
not fluctuation in brightness itself but light having a frequency
of a value obtained by integrating brightness by time in a certain
range. In addition, limit frequency in which human eyes perceive
fluctuation in brightness is approximately 50 Hz to 60 Hz. This
frequency is almost the same value as a frame rate of a general
display device. Further, the average luminance B is an amount used
for convenience to describe control modes in this document;
however, the average luminance B is similar to the integrated
luminance L in that luminance perceived by human eyes increases as
B increases, and luminance perceived by human eyes decreases as B
decreases.
[0204] Next, the words in this document are described in detail
with reference to FIGS. 1A to 1C. Here, one of pixels included in a
display device is focused on, and instantaneous luminance I (t) of
the pixel is schematically shown in FIGS. 1A to 1C. A horizontal
axis represents time t and a vertical axis represents
luminance.
[0205] FIG. 1A is an example of the case in which one frame period
is divided into two subframe periods, a first subframe period
corresponds to an image display period .tau..sub.a, and a second
subframe period corresponds to a blanking interval .tau..sub.b. In
FIG. 1A, each of the image display period .tau..sub.a and the
blanking interval .tau..sub.b is half of one frame period F
(.tau..sub.a=.tau..sub.b=F/2 is satisfied). In addition,
instantaneous luminance I (t) in the image display period
.tau..sub.a is constant and a value thereof is a. Instantaneous
luminance I (t) in the blanking interval .tau..sub.b is constant
and a value thereof is 0. At this time, as for lighting ratio R,
R=(.tau..sub.a/F)=1/2 is satisfied. As for integrated luminance L,
L=a.times.(F/2)=(aF/2) is satisfied. Therefore, as for average
luminance B, B=(L/.tau..sub.a)=a is satisfied.
[0206] FIG. 1B is a diagram showing the case in which there are a
plurality of image display periods .tau..sub.a and a plurality of
blanking intervals .tau..sub.b. In this manner, the image display
period .tau..sub.a and the blanking interval .tau..sub.b can be
divided into a plurality of sub-image display periods and
sub-blanking intervals. That is, when there are n (n is a positive
integer) pieces of sub-image display periods in one frame period,
the sub-image display periods are denoted by .tau..sub.a1,
.tau..sub.a2, .tau..sub.a3, . . . , and .tau..sub.an and the image
display period .tau..sub.a is the sum thereof. Similarly, when
there are n (n is a positive integer) pieces of sub-blanking
intervals in one frame period, the sub-blanking intervals are
denoted by .tau..sub.b1, .tau..sub.b2, .tau..sub.b3, . . . , and
.tau..sub.bn and the blanking interval .tau..sub.b is the sum
thereof. That is, when there are n pieces of sub-image display
periods and sub-blanking intervals in one frame period, the image
display period .tau..sub.a and the blanking interval .tau..sub.b
can be represented by Formula 5 and Formula 6 when j and k are
positive integers.
.tau. a = j = 1 n .tau. aj [ Formula 5 ] .tau. b = k = 1 n .tau. bk
[ Formula 6 ] ##EQU00002##
[0207] In FIG. 1B, as for the sub-image display period,
.tau..sub.a1=.tau..sub.a2=F/4 is satisfied, and as for the
sub-blanking interval, .tau..sub.b1=.tau..sub.b2=F/4 is satisfied.
Therefore, as for the image display period .tau..sub.a and the
blanking interval .tau..sub.b, .tau..sub.a=.tau..sub.b=F/2 is
satisfied. Instantaneous luminance I (t) in the image display
period .tau..sub.a is constant and a value thereof is a.
Instantaneous luminance I (t) in the blanking interval .tau..sub.b
is constant and a value thereof is 0. At this time, as for lighting
ratio R, R=(.tau..sub.a/F)=1/2 is satisfied. As for integrated
luminance L, L=a.times.(F/4)+a (F/4)=(aF/2) is satisfied.
Therefore, as for average luminance B, B=(L/.tau..sub.a)=a is
satisfied.
[0208] FIG. 1C shows the case in which there are a plurality of
image display periods .tau..sub.a and a plurality of blanking
intervals .tau..sub.b, and instantaneous luminance is different in
each of the sub-image display period. In FIG. 1C, as for the
sub-image display period, .tau..sub.a1=.tau..sub.a2=F/4 is
satisfied, and as for the sub-blanking interval,
.tau..sub.b1=.tau..sub.b2=F/4 is satisfied. Therefore, as for the
image display period .tau..sub.a and the blanking interval
.tau..sub.b, .tau..sub.a=.tau..sub.b=F/2 is satisfied.
Instantaneous luminance I (t) in the sub-image display period
.tau..sub.a1 is constant and a value thereof is a/2. Instantaneous
luminance I (t) in the sub-image display period .tau..sub.a2 is
constant and a value thereof is 3a/2. Instantaneous luminance I (t)
in the blanking interval .tau..sub.b is constant and a value
thereof is 0. At this time, as for lighting ratio R,
R=(.tau..sub.a/F)=1/2 is satisfied. As for integrated luminance L,
L=(a/2) (F/4)+(3a/2) (F/4)=(aF/2) is satisfied. Therefore, as for
average luminance B, B=(L/.tau..sub.a)=a is satisfied.
[0209] The lighting ratio R, the integrated luminance L, and the
average luminance B, which are values used in this document, are
the same between the examples shown in FIGS. 1A to 1C, although the
instantaneous luminance I (t) is a different condition in each of
FIGS. 1A to 1C. That is, this embodiment mode mainly describes how
the lighting ratio R, the integrated luminance L, and the average
luminance B are controlled; however, here, it is emphasized that
even when the lighting ratio R, the integrated luminance L, and the
average luminance B are the same, instantaneous luminance I (t)
with respect to them can be varied.
[0210] FIGS. 2A to 2C are diagrams each schematically showing
instantaneous luminance I (t) of the case of a display device using
an element having characteristics which change slowly in response
to a signal (e.g., a liquid crystal element). Even when a signal
controlling the element is input similarly to FIGS. 1A to 1C,
instantaneous luminance I (t) of the case of the display device
using the element having characteristics which change slowly in
response to the signal with delay.
[0211] However, in accordance with the definitions of this
document, the lighting ratio R, the integrated luminance L, and the
average luminance B can be calculated without a problem even in
such a case.
[0212] The image display period .tau..sub.a and the blanking
interval .tau..sub.b may be determined based on a period during
which a signal controlling luminance is input or may be determined
based on the instantaneous luminance I (t). When the image display
period .tau..sub.a and the blanking interval .tau..sub.b are
determined based on the period during which the signal controlling
the luminance is input, the time when the signal is updated is a
boundary between the periods. When the image display period
.tau..sub.a and the blanking interval .tau..sub.b are determined
based on the instantaneous luminance I (t), the time at which
change in the instantaneous luminance I (t) is drastic is a
boundary between the periods. More specifically, the time t at
which a primary function is discontinuous is a boundary between the
periods. For example, in the case of FIG. 2A, the image display
period .tau..sub.a and the blanking interval .tau..sub.b are
determined by setting time t.sub.1 at which increase in the
instantaneous luminance I (t) begins to decrease as a boundary
between the periods. In the case of FIG. 2B, the image display
period .tau..sub.a and the blanking interval .tau..sub.b are
determined by setting time t.sub.1 at which increase in the
instantaneous luminance I (t) begins to decrease as a first
boundary of the periods, setting time t.sub.2 at which decrease in
the instantaneous luminance I (t) begins to increase as a second
boundary of the periods, and setting time t.sub.3 at which decrease
in the instantaneous luminance I (t) begins to increase again as a
third boundary of the periods. FIG. 2C is similar to the case of
FIG. 2B. When the image display period .tau..sub.a and the blanking
interval .tau..sub.b are determined, the lighting ratio R can be
calculated by Formula 1.
[0213] The integrated luminance L can be calculated by Formula 3
from conditions of the instantaneous luminance I (t). In this
manner, the integrated luminance L can be calculated by Formula 3
even when the instantaneous luminance I (t) is a given
function.
[0214] The average luminance B can be calculated by Formula 4 from
the image display period .tau..sub.a and the instantaneous
luminance I (t) calculated by the above-described method.
[0215] The blanking interval .tau..sub.b is provided in one frame
period so that quality of a moving image displayed by a display
device is improved. Therefore, as far as quality of a moving image
displayed by a display device is improved in a period, the period
can be considered as the blanking interval .tau..sub.b regardless
of luminance of a pixel in the period.
[0216] Luminance of a pixel in the blanking interval .tau..sub.b is
preferably luminance in which luminance of the pixel in the image
display period .tau..sub.a can be reset by human eyes. Therefore,
the luminance of the pixel in the blanking interval .tau..sub.b is
preferably lower than the luminance of the pixel in the image
display period .tau..sub.a. More preferably, the luminance of the
pixel in the blanking interval .tau..sub.b is the lowest luminance
within display capability of the display device.
[0217] Next, control modes of the values used in this document are
described. In this embodiment mode, change in the integrated
luminance L, the lighting ratio R, and the average luminance B by a
control parameter P are particularly described.
[0218] Although various parameters can be given as the control
parameter P, details of the control parameter P is not described in
this embodiment mode. Details of the control parameter P is
described in another embodiment mode, and this embodiment mode
describes how the integrated luminance L, the lighting ratio R, and
the average luminance B are changed simply in accordance with
increase and decrease in the control parameter P.
[0219] Note that when change in the integrated luminance L, the
lighting ratio R, and the average luminance B with respect to
change in the control parameter P is described, it is assumed that
luminance of pixels perceived by human eyes is the same.
[0220] First, change in the integrated luminance L with respect to
change in the control parameter P is described with reference to
FIGS. 3A and 3B. Change in the integrated luminance L with respect
to change in the control parameter P can be described in detail by
a graph in which a horizontal axis represents the control parameter
P and a vertical axis represents the integrated luminance L, as in
FIGS. 3A and 3B.
[0221] The integrated luminance L is preferably almost constant
with respect to increase in the control parameter P. This is
because change in the integrated luminance L corresponds to change
in luminance, which is perceived by human eyes, and drastic change
in the integrated luminance L cannot be allowed under the
assumption that luminance of the pixels, which is perceived by
human eyes is the same. This condition can be understood with
reference to FIG. 3A. In a graph shown in FIG. 3A, L=L.sub.0 when
P=0, and L=4 is always satisfied even when P becomes larger than
0.
[0222] Here, when the integrated luminance L is considered as a
function particularly with respect to the control parameter P, the
integrated luminance L is referred to as integrated luminance L
(P). That is, when the graph shown in FIG. 3A is represented by a
formula, L (P)=L(0)=L.sub.0.
[0223] Note that actually, it is not necessary that L (P)=L.sub.0
be strictly satisfied, and there may be a certain range. This
condition can also be understood with reference to FIG. 3A. In the
graph shown in FIG. 3A, a fluctuation range of the integrated
luminance L, which can be allowed, is shown by two broken lines. A
formula of the broken like is L (P)=L+(L.sub.0/20). That is, it is
only necessary that the integrated luminance L be within a range
having a central value of L.sub.0 and a width of L.sub.0/10 with
respect to change in the control parameter P. When fluctuation of
the integrated luminance L is in the range, fluctuation of the
integrated luminance L can be allowed. This is because when
fluctuation of the integrated luminance L is small, it is not
perceived as fluctuation in luminance, and fluctuation of the
integrated luminance L is extremely small even when fluctuation of
the integrated luminance L is perceived as fluctuation in
luminance.
[0224] In addition, the integrated luminance L may be increased
slowly with respect to increase in the control parameter P. This is
because when change in the integrated luminance L is small, this
change is allowed, and display can be emphasized in accordance with
increase in the control parameter P when the integrated luminance
increases slowly with respect to increase in the control parameter
P. This condition can be understood with reference to FIG. 3B. In a
graph shown in FIG. 3B, L=L.sub.0 when P=0, and L=L.sub.0 is
gradually increased as P increases from 0.
[0225] Here, when the graph shown in FIG. 3B is represented by a
formula, L (P)=.alpha.P+L.sub.0. .alpha. is a proportional constant
and a positive number which is larger than 0. In addition, the
proportional constant .alpha. is preferably smaller than 1. This is
because change in the integrated luminance L is small when the
proportional constant .alpha. is small, and change in the
integrated luminance L can be allowed.
[0226] Note that it is not necessary that L (P)=.alpha.P+L.sub.0 be
strictly satisfied, and there may be a certain range. This
condition can also be understood with reference to FIG. 3B. In the
graph shown in FIG. 3B, a fluctuation range of the integrated
luminance L, which can be allowed, is shown by two broken lines. A
formula of the broken like is L (P)=.alpha.P+L.sub.0.+-.(L/20).
That is, it is only necessary that the integrated luminance L be
within a range having a central value of .alpha.P+L.sub.0 and a
width of L.sub.0/10 with respect to change in the control parameter
P. When fluctuation of the integrated luminance L is in the range,
fluctuation of the integrated luminance L can be allowed. This is
because when fluctuation of the integrated luminance L is small, it
is not perceived as fluctuation in luminance, and fluctuation of
the integrated luminance L is extremely small even when fluctuation
of the integrated luminance L is perceived as fluctuation in
luminance.
[0227] Next, change in the lighting ratio R and the average
luminance B with respect to the control parameter P is described
with reference to FIGS. 4A to 4H. Change in the lighting ratio R
and the average luminance B with respect to the control parameter P
can be described in detail by a graph in which a horizontal axis
represents the control parameter P and a vertical axis represents
the lighting ratio R or the average luminance B. FIGS. 4A, 4C, 4E,
and 4G are graphs each showing change in lighting ratio R with
respect to the control parameter P. FIGS. 4B, 4D, 4F, and 4H are
graphs each showing change in the average luminance B with respect
to the control parameter P.
[0228] FIG. 4A shows the case where the lighting ratio R is almost
constant with respect to increase in the control parameter P.
Change in the lighting ratio R corresponds to how a ratio of the
image display period .tau..sub.a in the one frame period F is
changed. This is because on a condition that the integrated
luminance L is constant with respect to the control parameter P,
the lighting ratio R is almost constant with respect to the control
parameter P when the average luminance B is almost constant with
respect to the control parameter P. This condition can be
understood with reference to the following description and FIG.
4A.
[0229] The fluctuation range of the integrated luminance L with
respect to the control parameter P, which can be allowed, is
extremely small, which has been already described. Future
discussions will be proceeded on a condition that the integrated
luminance L is almost constant with respect to the control
parameter P.
[0230] When Formula 1 and Formula 4 are transformed to be
organized, Formula 7 can be obtained.
BR = L F [ Formula 7 ] ##EQU00003##
[0231] Here, the integrated luminance L is almost constant with
respect to the control parameter P. In addition, when the one frame
period F is also almost constant with respect to the control
parameter P, the right side of Formula 7 is almost constant with
respect to the control parameter P. Therefore, a product of the
lighting ratio R and the average luminance B is almost constant
with respect to the control parameter P.
[0232] Thus, from the fact that the product of the lighting ratio R
and the average luminance B is almost constant with respect to the
control parameter P, a conclusion that the lighting ratio R is
almost constant with respect to increase in the control parameter P
when the average luminance B is almost constant with respect to the
control parameter P can be obtained.
[0233] Change in the lighting ratio R with respect to increase in
the control parameter P is described with reference to FIG. 4A.
When the lighting ratio R is considered as a function particularly
with respect to the control parameter P, the lighting ratio R is
referred to as lighting ratio R (P). In addition, R=R.sub.0 when
P=0. That is, when the graph shown in FIG. 4A is represented by a
formula, R (P)=R (0)=R.sub.0.
[0234] Note that actually, it is only necessary that R (P) be in a
range of approximately R.sub.0/10 setting R.sub.o as a certain
value even when R (P)=R.sub.0 is not strictly satisfied.
[0235] Change in the average luminance B with respect to the
control parameter P is described with reference to FIG. 4B. When
the average luminance B is considered as a function particularly
with respect to the control parameter P, the average luminance B is
referred to as average luminance B (P). In addition, B=B.sub.0 when
P=0. That is, when the graph shown in FIG. 4B is represented by a
formula, B (P)=B (0)=B.sub.0.
[0236] Note that actually, it is only necessary that B (P) be in a
range of approximately B.sub.0/10 setting B.sub.o as a certain
value even when B (P)=B.sub.0 is not strictly satisfied.
[0237] Next, the lighting ratio R can be simply decreased with
respect to increase in the control parameter P. This is because
when on a condition that the product of the lighting ratio R and
the average luminance B is almost constant with respect to the
control parameter P, the lighting ratio R monotonously decreases
with respect to the control parameter P when the average luminance
B monotonously increases with respect to the control parameter P.
This condition can be understood with reference to FIGS. 4C to
4H.
[0238] In each of graphs shown in FIGS. 4C, 4E, and 4G, the
lighting ratio R is simply decreased with respect to the control
parameter P. As in the graph shown in FIG. 4C, the lighting ratio R
may decrease linearly with respect to the control parameter P.
Alternatively, as in the graph shown in FIG. 4E, the lighting ratio
R may decrease as shown by an upward curving line with respect to
the control parameter P. Further alternatively, as in the graph
shown in FIG. 4G, the lighting ratio R may decrease as shown by a
downward curving line with respect to the control parameter P.
[0239] When the lighting ratio R decreases linearly with respect to
the control parameter P, the average luminance B increases linearly
with respect to the control parameter P as in a graph shown in FIG.
4D.
[0240] When the lighting ratio R decreases as shown by an upward
curving line with respect to with respect to the control parameter
P, the average luminance B increases as shown by a downward curving
line with respect to the control parameter P as in a graph shown in
FIG. 4F.
[0241] When the lighting ratio R decreases as shown by a downward
curving line with respect to with respect to the control parameter
P, the average luminance B increases as shown by an upward curving
line with respect to the control parameter P as in a graph shown in
FIG. 4H.
[0242] When a value of the lighting ratio R is constant, it is not
necessary that the control modes be changed precisely with respect
to change in the control parameter P. Accordingly, since algorithm
which determines a display method and a peripheral circuit which
makes many control modes to be selected are not needed,
manufacturing cost of the display device can be reduced. In
addition, since the size of a circuit and frequency of operation
can be reduced, power consumption can be reduced.
[0243] When a value of the lighting ratio R decreases linearly, the
control modes can be changed precisely with respect to change in
the control parameter P. Accordingly, by using algorithm which
determines a display method and a peripheral circuit which makes
many control modes to be selected, suitable control modes in
accordance with the control parameter P can be realized. Therefore,
high-quality display with little motion blur and little flicker can
be obtained.
[0244] When a value of the lighting ratio R decreases as shown by
an upward curving line, the control modes can be changed precisely
with respect to change in the control parameter P. In addition, the
amount of change in the lighting ratio R can be increased as the
control parameter P becomes larger. Accordingly, by using algorithm
which determines a display method and a peripheral circuit which
makes many control modes to be selected, more suitable control
modes in accordance with the control parameter P can be realized.
Therefore, higher-quality display with little motion blur and
little flicker can be obtained.
[0245] When a value of the lighting ratio R decreases as shown by a
downward curving line, the control modes can be changed finely with
respect to change in the control parameter P. In addition, the
amount of change in the lighting ratio R can be decreased as the
control parameter P becomes larger. Accordingly, by using algorithm
which determines a display method and a peripheral circuit which
makes many control modes to be selected, more suitable control
modes in accordance with the control parameter P can be realized.
Therefore, higher-quality display with few motion blur and flicker
can be obtained.
[0246] Here, change in the lighting ratio R and the average
luminance B with respect to the control parameter P is summarized.
When a condition that the product of the lighting ratio R and the
average luminance B is constant is satisfied, graphs shown in FIGS.
5A to 5D each describe a relationship between the lighting ratio R
and the average luminance B.
[0247] Each of FIGS. 5A to 5C is a graph in which a horizontal axis
represents the control parameter P and a vertical axis
logarithmically shows a ratio of the lighting ratio R with respect
to R.sub.0 or a ratio of the average luminance B with respect to
B.sub.0. Here, R.sub.0 and B.sub.0 are values of R (P)/R.sub.0 and
B (P)/B.sub.0 when P=0. When R (P)/R.sub.0 and B (P)/B.sub.0 are
expressed by a graph in which such axes are used, R (P)/R.sub.0 and
B (P)/B.sub.0 have symmetric shapes about a linear line
corresponding to 1 in the vertical axis. That is, a product of R
(P)/R.sub.0 and B (P)/B.sub.0 is 1 regardless of a value of the
control parameter P. This can be led from the fact that the product
of R (P)/R.sub.0 and B (P)/B.sub.0 is R.sub.0B.sub.0 and is
constant regardless of P when P=0.
[0248] The above-described characteristics are briefly described
below. For example, the case in which a value of R
(P.sub.X)/R.sub.0 is 10.sup.X is considered (X is a real number).
At this time, a value of B (P.sub.X)/B.sub.0 is
1/10.sup.X=10.sup.-X. Here, R (P.sub.X)/R.sub.0 and B
(P.sub.X)/B.sub.0 are plotted in a graph of a logarithmic axis. At
this time, when it is noted that a location in the logarithmic axis
is just a value of an exponent, as for a location at which 10.sup.X
is plotted and a location at which 10.sup.-X is plotted, a distance
of both positions from 10.sup.0=1 is the absolute value of X. That
is, a midpoint of line segments combining R (P.sub.X)/R.sub.0 and B
(P.sub.X)/B.sub.0 is 1. Since this characteristic is applied to all
P, it can be concluded that R (P)/R.sub.0 and B (P)/B.sub.0 have
symmetric shapes about the linear line corresponding to 1 in the
vertical axis.
[0249] FIG. 5A is a graph showing the case where the lighting ratio
R decreases linearly with respect to the control parameter P. At
this time, the average luminance B increases linearly with respect
to the control parameter P. In addition, R (P)/R.sub.0 and B
(P)/B.sub.0 have symmetric shapes about a linear line of R
(P)/R.sub.0=B (P)/B.sub.0=1.
[0250] FIG. 5B is a graph showing the case where the lighting ratio
R decreases as shown by an upward curving line with respect to the
control parameter P. At this time, the average luminance B
increases as shown by a downward curving line with respect to the
control parameter P. In addition, R (P)/R.sub.0 and B (P)/B.sub.0
have symmetric shapes centering on a linear line of R (P)/R.sub.0=B
(P)/B.sub.0=1.
[0251] FIG. 5C is a graph showing the case where the lighting ratio
R decreases as shown by a downward curving line with respect to the
control parameter P. At this time, the average luminance B
increases as shown by an upward curving line with respect to the
control parameter P. In addition, R (P)/R.sub.0 and B (P)/B.sub.0
have symmetric shapes about a linear line of R (P)/R.sub.0=B
(P)/B.sub.0=1.
[0252] From the condition that the product of the lighting ratio R
and the average luminance B in this embodiment mode is always
constant in this manner, the graph where change in the lighting
ratio R and the average luminance B with respect to the control
parameter P has a symmetric shape about 1 in a symmetric axis.
Thus, fluctuation in the integrated luminance L can be decreased,
so that it is not perceived as fluctuation in luminance by human
eyes even when the control parameter is greatly changed. Therefore,
a display device with little flicker can be obtained.
[0253] Next, another control modes of the lighting ratio R and the
average luminance B are described with reference to FIGS. 6A to 6P.
Here, since the control mode of the average luminance B can be
almost unambiguously determined by the control mode of the lighting
ratio R, description of the control mode of the average luminance B
is omitted hereinafter and the control mode of the lighting ratio R
is only described. Note that although the description is omitted,
it is preferable that the average luminance B also be controlled by
the above-described method.
[0254] FIGS. 6A to 6P each show a method in which the control
parameter P is divided into two regions (a region 1 and a region 2)
and the lighting ratio R is controlled by the above-described mode
in each region. Here, a region where the control parameter P is
small is referred to as the region 1 and a region where the control
parameter P is large is referred to as the region 2.
[0255] First, the case where a value of the lighting ratio R is
constant in the region 1 is described. In this case, R (P)=R.sub.0
is satisfied in the region 1. This is because P=0 and R (0)=R.sub.0
is satisfied in the region 1. In addition, in this case, at least
four control modes are conceivable in the region 2. That is, the
four control modes correspond to the case where R (P) in the region
2 is constant (see FIG. 6A), the case where R (P) in the region 2
decreases linearly (see FIG. 6B), the case where R (P) in the
region 2 decreases as shown by an upward curving line (see FIG.
6C), and the case where R (P) in the region 2 decreases as shown by
a downward curving line (see FIG. 6D).
[0256] Next, the case where a value of the lighting ratio R
decreases linearly in the region 1 is described. In this case, the
value of the lighting ratio R decreases linearly from R (0)=R.sub.0
as a starting point in the region 1. In addition, in this case, at
least four control modes are conceivable in the region 2. That is,
the four control modes correspond to the case where R (P) in the
region 2 is constant (see FIG. 6E), the case where R (P) in the
region 2 decreases linearly (see FIG. 6F), the case where R (P) in
the region 2 decreases as shown by an upward curving line (see FIG.
6G), and the case where R (P) in the region 2 decreases as shown by
a downward curving line (see FIG. 6H).
[0257] Next, the case where a value of the lighting ratio R
decreases as shown by an upward curving line in the region 1 is
described. In this case, the value of the lighting ratio R
decreases as shown by the upward curving line from R (0)=R.sub.0 as
a starting point in the region 1. In addition, in this case, at
least four control modes are conceivable in the region 2. That is,
the four control modes correspond to the case where R (P) in the
region 2 is constant (see FIG. 6I), the case where R (P) in the
region 2 decreases linearly (see FIG. 6J), the case where R (P) in
the region 2 decreases as shown by an upward curving line (see FIG.
6K), and the case where R (P) in the region 2 decreases as shown by
a downward curving line (see FIG. 6L).
[0258] Next, the case where a value of the lighting ratio R
decreases as shown by a downward curving line in the region 1 is
described. In this case, the value of the lighting ratio R
decreases as shown by the downward curving line from R (0)=R.sub.0
as a starting point in the region 1. In addition, in this case, at
least four control modes are conceivable in the region 2. That is,
the four control modes correspond to the case where R (P) in the
region 2 is constant (see FIG. 6M), the case where R (P) in the
region 2 decreases linearly (see FIG. 6N), the case where R (P) in
the region 2 decreases as shown by an upward curving line (see FIG.
60), and the case where R (P) in the region 2 decreases as shown by
a downward curving line (see FIG. 6P).
[0259] When a value of the lighting ratio R is constant in each
region, it is not necessary that the control modes be changed
precisely with respect to change in the control parameter P.
Accordingly, since algorithm which determines a display method and
a peripheral circuit which makes many control modes to be selected
are not needed, manufacturing cost of the display device can be
reduced. In addition, since the size of a circuit and frequency of
operation can be reduced, power consumption can be reduced.
[0260] When a value of the lighting ratio R decreases linearly in
each region, the control modes can be changed precisely with
respect to change in the control parameter P. Accordingly, by using
algorithm which determines a display method and a peripheral
circuit which makes many control modes to be selected, suitable
control modes in accordance with the control parameter P can be
realized. Therefore, high-quality display with little motion blur
and little flicker can be obtained.
[0261] When a value of the lighting ratio R decreases as shown by
an upward curving line in each region, the control modes can
changed finely with respect to change in the control parameter P.
In addition, the amount of change in the lighting ratio R can be
increased as the control parameter P becomes larger. Accordingly,
by using algorithm which determines a display method and a
peripheral circuit which makes many control modes to be selected,
more suitable control modes in accordance with the control
parameter P can be realized. Therefore, higher-quality display with
little motion blur and flicker can be obtained.
[0262] When a value of the lighting ratio R decreases as shown by a
downward curving line in each region, the control modes can be
changed precisely with respect to change in the control parameter
P. In addition, the amount of change in the lighting ratio R can be
decreased as the control parameter P becomes larger Accordingly, by
using algorithm which determines a display method and a peripheral
circuit which makes many control modes to be selected, more
suitable control modes in accordance with the control parameter P
can be realized. Therefore, higher-quality display with little
motion blur and little flicker can be obtained.
[0263] In the control mode where the control parameter P is divided
into the two regions (the region 1 and the region 2), it is
important that R (P) can have discontinuous values at a boundary
between different regions. When a difference in values at the
boundary between the different regions is small, the control mode
has an advantage in that a display defect (e.g., an unnatural
contour or a flicker) due to drastic change in the control mode
hardly occurs because change in R (P) with respect to change in P
at the vicinity of the boundary is small.
[0264] When a difference in values at the boundary between the
different regions is large, the control mode has an advantage in
that an emphatic effect on display due to drastic change in the
control mode is large and sharp display can be performed because
change in R (P) with respect to change in P in the vicinity of the
boundary is large.
[0265] Here, the number of regions obtained by dividing the control
parameter may be more than two. For example, the control parameter
P may be divided into three regions or may be divided into three or
more regions. By dividing the control parameter P into three or
more regions, more various control modes can be realized. In
particular, R (P) can have discontinuous values and the number of
boundaries of different regions is increased, which is important.
That is, in each region, more various control modes can be realized
in the case where R (P) decreases linearly with respect to the
control parameter P, the case where R (P) decreases as shown by an
upward curving line with respect to the control parameter P, and
the case where R (P) decreases as shown by a downward curving line
with respect to the control parameter P. In addition to this, even
in the case where R (P) is constant with respect to the control
parameter P in each region, it is particularly advantageous that a
certain number of control modes can be obtained. That is,
advantages of a simple circuit (e.g., reduction in manufacturing
cost and reduction in power consumption) and advantages of
realization of various control modes are compatible.
[0266] This mode can be understood with reference to FIGS. 7A to
7E. FIG. 7A shows the case where the control parameter P are
divided into three regions (a region 1, a region 2, and a region 3)
and R (P) is constant in each region.
[0267] FIG. 7B shows the case where the control parameter P are
divided into three regions (a region 1, a region 2, and a region 3)
and R (P) decreases linearly in each region.
[0268] FIG. 7C shows the case where the control parameter P are
divided into three regions (a region 1, a region 2, and a region 3)
and R (P) decreases as shown by an upward curving line in each
region.
[0269] FIG. 7D shows the case where the control parameter P are
divided into three regions (a region 1, a region 2, and a region 3)
and R (P) decreases as shown by a downward curving line in each
region.
[0270] Here, it is obvious that combinations of modes of R (P) in
each region are not limited to the combinations shown in FIGS. 7A
to 7E. Needles to say, these combinations are included in the
control modes in this embodiment mode; however, the combinations
are omitted here, and the case where the modes of R (P) in each
region are the same is typically described.
[0271] FIG. 7E shows the case where the control parameter P is
divided into n (n is a positive integer) pieces of regions (a
region 1, a region 2, a region 3, . . . , and a region n) and R (P)
is constant in each region. When n is a certain number
(approximately 5 to 15), advantages of a simple circuit (e.g.,
reduction in manufacturing cost and reduction in power consumption)
and advantages of realization of various control modes, which are
described above, are compatible.
[0272] Note that a mode where the lighting ratio R and the average
luminance B are changed with respect to the control parameter P may
be a mode which can be selected from a plurality of kinds. That is,
a plurality of different R (P) and B (P) may be prepared in
advance, and a second control parameter Q which is prepared
separately from the control parameter P may determine which R (P)
and B (P) to be used. At this time, the lighting ratio R and the
average luminance B are denoted by R.sub.Q (P) and B.sub.Q (P)
respectively, and the control parameter P is referred to as a first
parameter for convenience. For example, when the second parameter Q
is an integer ranging from 1 to n, the lighting ratio R and the
average luminance B are referred to as R.sub.1 (P), R.sub.2 (P), .
. . , and R.sub.n (P), and B.sub.1 (P), B.sub.2 (P), . . . , and
B.sub.n (P).
[0273] This mode can be understood with reference to FIGS. 8A to
8G. In FIGS. 8A to 8G, the second parameter Q is an integer ranging
from 1 to 3. FIG. 8A shows the case where each of R.sub.1 (P),
R.sub.2 (P), and R.sub.3 (P) is constant with respect to the first
parameter P. When the first control parameter P is 0, R.sub.1
(0)=R.sub.10, R.sub.2 (0)=R.sub.20, and R.sub.3 (0)=R.sub.30 are
satisfied. In this manner, in each mode of the lighting ratio R
with respect to the second control parameter Q, the lighting ratio
R can have different values from each other when the first control
parameter P is 0. Thus, advantages of a simple circuit (e.g.,
reduction in manufacturing cost and reduction in power consumption)
and advantages of realization of various control modes are
compatible.
[0274] Note that since the mode of the average luminance B can be
determined based on the mode of the lighting ratio R in some degree
similarly to another description in this embodiment mode,
description thereof is omitted here.
[0275] In another example in which the modes of the lighting ratio
R and the average luminance B are controlled by the first control
parameter P and the second control parameter Q, R.sub.1 (P) is
constant with respect to the first parameter P, R.sub.2 (P)
decreases linearly with respect to the first control parameter P,
and R.sub.3 (P) decreases linearly with respect to the first
control parameter P. Here, a gradient of linear decrease is
preferably changed in accordance with the second control parameter
Q. In addition, in each mode of the lighting ratio R with respect
to the second control parameter Q, the lighting ratio R can have
different values from each other when the first control parameter P
is 0.
[0276] This mode can be understood with reference to FIG. 8B. By
controlling in this manner, more various control modes can be
realized compared with the case where the number of control
parameters is 1.
[0277] In another example in which the modes of the lighting ratio
R and the average luminance B are controlled by the first control
parameter P and the second control parameter Q, R.sub.1 (P) is
constant with respect to the first parameter P, R.sub.2 (P)
decreases linearly with respect to the first control parameter P,
and R.sub.3 (P) decreases as shown by an upward curving line with
respect to the first control parameter P. Here, a ratio of decrease
is preferably changed in accordance with the second control
parameter Q. In addition, in each mode of the lighting ratio R with
respect to the second control parameter Q, the lighting ratio R can
have different values from each other when the first control
parameter P is 0.
[0278] This mode can be understood with reference to FIG. 8C. By
controlling in this manner, more various control modes can be
realized compared with the case where the number of control
parameters is 1.
[0279] In another example in which the modes of the lighting ratio
R and the average luminance B are controlled by the first control
parameter P and the second control parameter Q, R.sub.1 (P)
decreases linearly with respect to the first parameter P, R.sub.2
(P) decreases linearly with respect to the first control parameter
P, and R.sub.3 (P) decreases linearly with respect to the first
control parameter P. Here, a gradient of linear decrease is
preferably changed in accordance with the second control parameter
Q. In addition, in each mode of the lighting ratio R with respect
to the second control parameter Q, the lighting ratio R can have
different values from each other when the first control parameter P
is 0.
[0280] This mode can be understood with reference to FIG. 8D. By
controlling in this manner, more various control modes can be
realized compared with the case where the number of control
parameters is 1.
[0281] In another example in which the modes of the lighting ratio
R and the average luminance B are controlled by the first control
parameter P and the second control parameter Q, R.sub.1 (P)
decreases as shown by an upward curving line with respect to the
first control parameter P, R.sub.2 (P) decreases as shown by an
upward curving line with respect to the first control parameter P,
and R.sub.3 (P) decreases as shown by an upward curving line with
respect to the first control parameter P. Here, a ratio of decrease
is preferably changed in accordance with the second control
parameter Q. In addition, in each mode of the lighting ratio R with
respect to the second control parameter Q, the lighting ratio R can
have different values from each other when the first control
parameter P is 0.
[0282] This mode can be understood with reference to FIG. 8E. By
controlling in this manner, more various control modes can be
realized compared with the case where the number of control
parameters is 1.
[0283] In another example in which the modes of the lighting ratio
R and the average luminance B are controlled by the first control
parameter P and the second control parameter Q, R.sub.1 (P)
decreases as shown by an upward curving line with respect to the
first control parameter P, R.sub.2 (P) decreases as shown by an
upward curving line with respect to the first control parameter P,
and R.sub.3 (P) decreases linearly with respect to the first
control parameter P. Here, a ratio of decrease is preferably
changed in accordance with the second control parameter Q. In
addition, in each mode of the lighting ratio R with respect to the
second control parameter Q, the lighting ratio R can have different
values from each other when the first control parameter P is 0.
[0284] This mode can be understood with reference to FIG. 8F. By
controlling in this manner, more various control modes can be
realized compared with the case where the number of control
parameters is 1.
[0285] Note that only typical combinations are described in
description of the method in which the first control parameter P
and the second control parameter Q are used. However, various modes
described in this embodiment mode can be used for the lighting
ratio R and the average luminance B.
[0286] For example, as shown in FIG. 8G, the first control
parameter P is divided into n (n is a positive integer) pieces of
regions (a region 1, a region 2, a region 3, . . . , and a region
n), and the lighting ratio R and the average luminance B can be
combined with a method in which R (P) is constant in each region. A
value of R (P) in each region is preferably small as the second
control parameter Q becomes larger. Thus, advantages of a simple
circuit (e.g., reduction in manufacturing cost and reduction in
power consumption) and advantages of realization of various control
modes are compatible.
[0287] Although this embodiment mode is described with reference to
various drawings, the contents (or may be part of the contents)
described in each drawing can be freely applied to, combined with,
or replaced with the contents (or may be part of the contents)
described in another drawing. Further, even more drawings can be
formed by combining each part with another part in the
above-described drawings.
[0288] The contents (or may be part of the contents) described in
each drawing of this embodiment mode can be freely applied to,
combined with, or replaced with the contents (or may be part of the
contents) described in a drawing in another embodiment mode.
Further, even more drawings can be formed by combining each part
with part of another embodiment mode in the drawings of this
embodiment mode.
[0289] This embodiment mode shows an example of an embodied case of
the contents (or may be part of the contents) described in other
embodiment modes, an example of slight transformation thereof, an
example of partial modification thereof, an example of improvement
thereof, an example of detailed description thereof, an application
example thereof, an example of related part thereof, or the like.
Therefore, the contents described in other embodiment modes can be
freely applied to, combined with, or replaced with this embodiment
mode.
Embodiment Mode 2
[0290] In this embodiment mode, among methods in each of which the
lighting ratio R is changed under a condition that luminance
perceived by human eyes is constant and methods in each of which
luminance perceived by human eyes is changed, some typical examples
are described.
[0291] First, an example of a control method of the lighting ratio
R is described. As a control method of the lighting ratio R, (1) a
method of directly writing blanking data to each pixel, (2) a
method of blinking the whole backlight, and (3) a method of
sequentially blinking a backlight which is divided by areas can be
mainly given.
[0292] The method (1) can be applied to both the case where a
display element included in a display device is a self-luminous
element typified by an element included in an EL display, a PDP, or
an EFD and the case where a display element included in a display
device is a non-light emitting element typified by an element
included in a liquid crystal display. The methods (2) and (3) can
be applied to the case where a display element included in a
display device is a non-light emitting element.
[0293] Before the control method of the lighting ratio R is
described, a structure of pixels included in an active matrix
display device is described. FIG. 9G shows a structural example of
a pixel included in an active matrix display device.
[0294] The pixel included in the active matrix display device
includes a pixel region, a switching means, a display element, a
signal holding means, a signal transmitting means, and a switch
controlling means. A pixel region 900, a switching means 901, a
display element 902, a signal holding means 904, a signal
transmitting means 906, and a switch controlling means 907 are
included in the structural example of the pixel shown in FIG. 9G.
However, the invention is not limited to this, and various
structures can be used for the display device. For example, a
structure such as a passive matrix structure, an MIM (metal
insulator metal) structure, or a TFD (thin film diode) structure
may be used.
[0295] In FIG. 9G, more specifically, the switching means 901 is a
transistor. The display element 902 is a liquid crystal element
(hereinafter also referred to as the liquid crystal element 902).
The signal holding means 904 is a capacitor (hereinafter also
referred to as the capacitor 904). The signal transmitting means
906 is a data line (also referred to as a source line) (hereinafter
referred to as the data line 906). The switch controlling means 907
is a scan line (also referred to as a gate line) (hereinafter
referred to as the scan line 907). Note that a counter electrode
903 for controlling the liquid crystal element 902 and a common
line 905 for fixing a potential of one of electrodes of the
capacitor 904 may be provided as necessary. Note also that the
common line may be shared with another scan line.
[0296] In a display portion of the display device, the pixel
regions 900 are arranged in matrix. At this time, when the pixel
regions 900 arranged in a row sideways are focused, the scan lines
907 thereof are common. Similarly, when the pixel regions 900
arranged in tandem are focused, the data lines 906 thereof are
common.
[0297] That is, the number of wirings can be reduced when the data
lines 906 thereof are common. On the other hand, different signals
cannot be written to the pixel regions 900 arranged in tandem
concurrently. Here, the data lines 906 are used by being divided in
terms of time by sequentially scanning the scan lines 907 which are
common in the pixel regions 900 arranged in the row sideways, so
that a different data signal can be written to each pixel.
[0298] A mode of this sequential scanning can be understood with
reference to FIG. 9A. The graph shown in FIG. 9A shows a mode of
sequential scanning of the display device, in which a horizontal
axis represents time and a vertical axis represents a scanning
direction of the pixel. Solid lines in the graph show positions in
which a plurality of scan lines included in the display device are
selected. That is, in the graph shown in FIG. 9A, scanning is
performed sequentially from an upper scan line to a lower scan line
in the vertical axis when one frame period is started, and scanning
of all the scan lines are completed at timing at which one frame
period is completed. Note that a scanning order is not limited to
this and scanning may be performed sequentially from a lower scan
line to an upper scan line in the vertical axis; however, the case
where scanning is performed sequentially from an upper scan line to
a lower scan line is described typically in this embodiment
mode.
[0299] The mode of sequential scanning shown in FIG. 9A corresponds
to the case where a data signal is written to each pixel once in
one frame period. At this time, all the pixels continuously emit
light with luminance in accordance with the written data signals in
one frame period. That is, the image display period .tau..sub.a=F
(F is length of one frame period). Therefore, the lighting ratio R
at this time is 1 from Formula 1.
[0300] Next, a mode of sequential scanning when the lighting ratio
R is smaller than 1 is described. As for a method of directly
writing blanking data to each pixel, after a specific data signal
is written to each pixel, it is necessary that the signal written
to the pixel be rewritten into a signal in accordance with blanking
data at appropriate timing.
[0301] A mode of sequential scanning at this time can be understood
with reference to FIGS. 9B to 9F. The graph shown in FIG. 9B shows
a mode of sequential scanning of the display device when the
lighting ratio R=1/2. Solid lines in the diagram show timing of
data writing scanning for writing a specific data signal to each
pixel. In addition, broken lines in the diagram show timing of
blanking writing scanning for controlling the lighting ratio R.
When the lighting ratio R=1/2 is realized in this manner, it is
only necessary to start blanking writing scanning when time of F/2
passes from timing at which data writing scanning is started. Then,
a period after blanking writing scanning is performed and until
data writing scanning of the next frame is performed corresponds to
a blanking display period.
[0302] Similarly, when the lighting ratio R=1/3 is realized in this
manner, it is only necessary to start blanking writing scanning
when time of F/3 passes from timing at which data writing scanning
is started. At this time, since the display period .tau..sub.a=F/3,
the lighting ratio R at this time is 1/3 from Formula 1. A mode of
sequential scanning at this time can be understood with reference
to FIG. 9C.
[0303] Similarly, when the lighting ratio R=1/4 is realized in this
manner, it is only necessary to start blanking writing scanning
when time of F/4 passes from timing at which data writing scanning
is started. At this time, since the display period .tau..sub.a=F/4,
the lighting ratio R at this time is 1/4 from Formula 1. A mode of
sequential scanning at this time can be understood with reference
to FIG. 9D.
[0304] Similarly, when the lighting ratio R=2/3 is realized in this
manner, it is only necessary to start blanking writing scanning
when time of 2F/3 passes from timing at which data writing scanning
is started. At this time, since the display period
.tau..sub.a=2F/3, the lighting ratio R at this time is 2/3 from
Formula 1. A mode of sequential scanning at this time can be
understood with reference to FIG. 9E.
[0305] Similarly, when the lighting ratio R=3/4 is realized in this
manner, it is only necessary to start blanking writing scanning
when time of 3F/4 passes from timing at which data writing scanning
is started. At this time, since the display period
.tau..sub.a=3F/4, the lighting ratio R at this time is 3/4 from
Formula 1. A mode of sequential scanning at this time can be
understood with reference to FIG. 9F.
[0306] Values of the lighting ratio R can be set variously in
accordance with writing timing of blanking data in this manner.
[0307] Here, it should be noted that when the lighting ratio R is
controlled by performing blanking writing at specific timing after
data writing scanning is performed, there is a period during which
data writing scanning and blanking writing scanning are performed
at the same time. That is, when certain time of each of the graphs
shown in FIGS. 9B to 9F is focused, data writing scanning and
blanking writing scanning coincide at different positions.
[0308] Even when data writing scanning and blanking writing
scanning coincide at different positions in this manner, there are
a plurality of methods for accurately writing a signal in each
scanning. For example, there is a method in which a period during
which one scan line is selected (one gate election period) is
further divided into a plurality of periods and data writing
scanning and blanking writing scanning are assigned to each period.
The structure shown in FIG. 9G can be used for a structure of the
pixel region included in the display device at this time.
Therefore, the lighting ratio R can be controlled variously without
changing the pixel structure.
[0309] As another method, there is a method of adding a signal line
and a switching element which are dedicated for blanking writing
scanning to the pixel region. By using this method, signals can be
written accurately by each scanning without dividing one gate
selection period. FIG. 9H shows a structural example of a pixel
included in such an active matrix display device.
[0310] The pixel included in the active matrix display device to
which the signal line and the switching element which are dedicated
for blanking writing scanning are added includes a pixel region, a
first switching means, a second switching means, a display element,
a signal holding means, a first signal transmitting means, a second
signal transmitting means, a first switch controlling means, and a
second switch controlling means. A pixel region 910, a first
switching means 911, a second switching means 918, a display
element 912, a signal holding means 914, a first signal
transmitting means 916, a second signal transmitting means 920, a
first switch controlling means 917, and a second switch controlling
means 919 are included in the structural example of the pixel shown
in FIG. 9H.
[0311] In FIG. 9H, more specifically, the first switching means 911
and the second switching means 918 are transistors. The display
element 912 is a liquid crystal element (hereinafter also referred
to as the liquid crystal element 912). The signal holding means 914
is a capacitor (hereinafter also referred to as the capacitor 914).
The first signal transmitting means 916 is a data line (also
referred to as a source line). The second signal transmitting means
920 is a blanking signal line (hereinafter also referred to as the
blanking signal line 920). The first switch controlling means 917
is a writing scan line. The second switch controlling means 919 is
a blanking scan line. Note that a counter electrode 913 for
controlling the liquid crystal element 912 and a common line 915
for fixing a potential of one of electrodes of the capacitor 914
may be provided as necessary. Note also that the blanking signal
line may be shared with the common line, a writing scan line of
another pixel, and the blanking scan line.
[0312] In addition, a driving method of a display device in
accordance with this document can be used for both the case where a
liquid crystal element is normally black and the case where a
liquid crystal element is normally white. Here, normally black
corresponds to a mode where a black image is displayed when voltage
is not applied to a liquid crystal element. Normally white is a
mode where a white image is displayed when voltage is not applied
to a liquid crystal element. Note that a method in accordance with
this document can also be applied to a normally-white liquid
crystal element by inverting polarity of signal voltage even when
the signal voltage is shown as normally black.
[0313] By using the pixel structure to which the signal line and a
switching element which are dedicated for blanking writing scanning
are added in this manner, a signal can be written accurately by
each scanning without dividing one gate selection period.
Therefore, driving frequency of a peripheral circuit can be
suppressed low, so that power consumption can be reduced.
[0314] Next, specific operating methods of the method in which one
gate election period is further divided into a plurality of periods
and data writing scanning and blanking writing scanning are
assigned to each period, and the method of adding the signal line
and a switching element which are dedicated for blanking writing
scanning are described.
[0315] First, the method in which one gate election period is
divided into a plurality of regions and data writing scanning and
blanking writing scanning are assigned to each period is described
with reference to FIG. 10A.
[0316] FIG. 10A is a diagram for describing driving conditions of a
data line and a scan line in connection with a display condition of
a display portion of a display device. A display portion 1000
includes pixel regions arranged in matrix and performs various
kinds of display. The pixel regions in FIG. 10A are similar to the
structure shown in FIG. 9G A scan line 1001 is a scan line which
performs blanking writing at timing shown in FIG. 10A. A scan line
1002 is a scan line which performs data writing at timing shown in
FIG. 10A. A data line driver 1003 is a circuit which generates a
signal written to each pixel in accordance with a data signal. In
FIG. 10A, the signal written to each pixel is a voltage signal, and
a specific example of the voltage signal is shown above the data
line driver 1003. A scan line driver 1004 is a circuit for driving
a plurality of scan lines. Waveforms of voltage input to the scan
line 1001 and the scan line 1002 from the scan line driver 1004 are
shown on the left of the scan line driver 1004.
[0317] Timing for driving the scan line 1002 by the scan line
driver 1004 shown in FIG. 10A is a period from time t.sub.1 to time
t.sub.2. At this time, the data line outputs voltage V.sub.data1.
The voltage V.sub.data1 is voltage which should be written to a
pixel selected by the scan line 1002 at timing shown in FIG.
10A.
[0318] The scan line driver 1004 drives the scan line 1001 from the
time t.sub.2 to time t.sub.3. At this time, the data line outputs
voltage V.sub.blank. The voltage V.sub.blank is voltage supplying
luminance which should be displayed in a blanking interval.
[0319] A period from the time t.sub.1 to the time t.sub.3 in the
description heretofore corresponds to one gate selection period
under a driving condition of the lighting ratio R=1 without
providing a blanking interval. That is, one gate selection period
(a period from the time t.sub.1 to the time t.sub.3) is divided
into two periods (a period from the time t.sub.1 to the time
t.sub.2 and a period from the time t.sub.2 to the time t.sub.3) and
data writing scanning and blanking writing scanning are assigned to
each period.
[0320] The scan line driver 1004 drives a scan line which is next
to the scan line 1002 from the time t.sub.3 to time t.sub.4. At
this time, the data line outputs voltage V.sub.data2. The voltage
V.sub.data2 is voltage which should be written to a pixel selected
by the scan line which is next to the scan line 1002 at timing
shown in FIG. 10A.
[0321] The scan line driver 1004 drives a scan line which is next
to the scan line 1001 from the time t.sub.4 to time t.sub.5. At
this time, the data line outputs voltage V.sub.blank. The voltage
V.sub.blank is voltage supplying luminance which should be
displayed in the blanking interval.
[0322] By repeating driving which is described above, signals can
be accurately written in each scanning even when data writing
scanning and blanking writing scanning coincide at different
positions.
[0323] Note that voltage of the data line is an example for
describing the driving method, and voltage of V.sub.blank,
V.sub.data1, and V.sub.data2 is not limited to the voltage shown in
FIG. 10A and can have various values.
[0324] Next, the method of adding a signal line and a switching
element which are dedicated for blanking writing scanning to the
pixel region is described with reference to FIG. 10B.
[0325] FIG. 10B is a diagram for describing driving conditions of a
data line and a scan line in connection with a display condition of
a display portion of a display device. A display portion 1010
includes pixel regions arranged in matrix and performs various
kinds of display. The pixel regions in FIG. 10B are similar to the
structure shown in FIG. 9H. A blanking scan line 1011 is a blanking
scan line which performs blanking writing at timing shown in FIG.
10B. A writing scan line 1012 is a scan line which performs data
writing at timing shown in FIG. 10B. A data line driver 1013 is a
circuit which generates a signal written to each pixel in
accordance with a data signal. In FIG. 10B, the signal written to
each pixel is a voltage signal, and a specific example of the
voltage signal is shown above the data line driver 1013. A writing
scan line driver 1014 is a circuit for driving a plurality of
writing scan lines. Waveforms of voltage input to the writing scan
line 1012 from the writing scan line driver 1014 are shown on the
left of the writing scan line driver 1014. A blanking scan line
driver 1015 is a circuit for driving a plurality of blanking scan
lines. Waveforms of voltage input to the blanking scan line 1011
from the blanking scan line driver 1015 are shown on the right of
the blanking scan line driver 1015.
[0326] Timing for driving the writing scan line 1012 by the writing
scan line driver 1014 shown in FIG. 10B is a period from time
t.sub.1 to time t.sub.2. At this time, the data line outputs
voltage V.sub.data1. The voltage V.sub.data1 is voltage which
should be written to a pixel selected by the writing scan line 1012
at timing shown in FIG. 10B.
[0327] The blanking scan line driver 1015 operates concurrently and
drives the blanking scan line 1011 from the time t.sub.1 to the
time t.sub.3. At this time, a signal written to a pixel selected by
the blanking scan line 1011 at timing shown in FIG. 10B follows the
voltage V.sub.blank which is supplied to the blanking signal line
920 in the pixel structure shown in FIG. 9H.
[0328] The writing scan line driver 1014 drives a writing scan line
which is next to the writing scan line 1012 from the time t.sub.3
to time t.sub.5. At this time, the data line outputs voltage
V.sub.data2. The voltage V.sub.data2 is voltage which should be
written to a pixel selected by the writing scan line which is next
to the writing scan line 1012 at timing shown in FIG. 10B.
[0329] The blanking scan line driver 1015 operates concurrently and
drives a blanking scan line which is next to the blanking scan line
1011 from the time t.sub.3 to the time t.sub.5. At this time, a
signal written to a pixel selected by the blanking scan line which
is next to the blanking scan line 1011 at timing shown in FIG. 10B
follows the voltage V.sub.blank which is supplied to the blanking
signal line 920 in the pixel structure shown in FIG. 9H.
[0330] A period from the time t.sub.1 to the time t.sub.3 in the
description heretofore corresponds to one gate selection period
under a driving condition of the lighting ratio R=1 without
providing a blanking interval. That is, data writing scanning and
blanking writing scanning can be performed concurrently without
dividing one gate selection period into two periods.
[0331] By repeating driving which is described above, a signal can
be accurately written in each scanning even when data writing
scanning and blanking writing scanning coincide at different
positions.
[0332] Note that voltage of the data line is an example for
describing the driving method, and voltage of V.sub.data1 and
V.sub.data2 is not limited to the voltage shown in FIG. 10B and can
have various values.
[0333] Next, another mode of sequential driving when the lighting
ratio R is smaller than 1 is described. As for a method of directly
writing blanking data to each pixel, after a specific data signal
is written to each pixel, it is necessary that the signal written
to the pixel be rewritten to a signal in accordance with blanking
data at appropriate timing. Therefore, in the methods shown in
FIGS. 9A to 9H, and FIGS. 10A and 10B, it is necessary that writing
scanning and blanking scanning be performed concurrently by adding
a signal line and a switching element to a pixel region, or one
gate selection period be divided into a plurality of periods and
data writing and blanking writing be assigned to each period.
[0334] A method shown below is a method in which writing scanning
and blanking scanning are completed in time shorter than one frame
period F. By using this method, data writing scanning and blanking
writing scanning can be performed without either dividing one gate
selection period or adding a signal line and a switching element to
a pixel region.
[0335] There are a plurality of modes of the method in which
writing scanning and blanking scanning are completed in time
shorter than the one frame period F. One mode is a mode in which a
period during which writing scanning and blanking scanning are
completed is changed in accordance with a value of the lighting
ratio R. Here, a period during which writing scanning and blanking
scanning are completed is referred to as .tau..sub.w.
[0336] In the mode in which .tau..sub.w is changed in accordance
with a value of the lighting ratio R, .tau..sub.w is conformed to a
period having a smaller value between the image display period
.tau..sub.a and the blanking interval .tau..sub.b, which lead the
lighting ratio R. A mode of sequential scanning in this method can
be understood with reference to FIGS. 11A, 11C, 11E, 11G, 11I, and
11J. Here, each of graphs shown in FIGS. 11A to 11J shows a mode of
sequential scanning of the display device, in which a horizontal
axis represents time and a vertical axis represents a scanning
direction of a pixel. A form of the graphs is similar to those of
FIGS. 9A to 9F.
[0337] When the blanking interval .tau..sub.b is 0, blanking
scanning is not performed. A mode of sequential scanning at this
time can be understood with reference to FIG. 11A. That is,
sequential scanning is performed by setting .tau..sub.w as F. At
this time, the lighting ratio R is 1.
[0338] When .tau..sub.a=.tau..sub.b=F/2, sequential scanning is
performed by setting .tau..sub.w as F/2. A mode of sequential
scanning at this time can be understood with reference to FIG. 11C.
That is, blanking scanning is started right after writing scanning
is completed in a period of F/2, and blanking scanning is completed
when one frame period is completed. At this time, the lighting
ratio R is 1/2.
[0339] When .tau..sub.a=F/3 and .tau..sub.b=2F/3, sequential
scanning is performed by setting .tau..sub.w as F/3. A mode of
sequential scanning at this time can be understood with reference
to FIG. 11E. That is, blanking scanning is started right after
writing scanning is completed in a period of F/3, and blanking
scanning is completed at time 2F/3. At this time, the lighting
ratio R is 1/3.
[0340] When .tau..sub.a=2F/3 and .tau..sub.b=F/3, sequential
scanning is performed by setting .tau..sub.w as F/3. A mode of
sequential scanning at this time can be understood with reference
to FIG. 11G. That is, blanking scanning is started at the time 2F/3
in the period of F/3 after writing scanning is completed in the
period of F/3. Then, blanking scanning is completed when one frame
period is completed. At this time, the lighting ratio R is 2/3.
[0341] When .tau..sub.a=F/4 and b=3F/4, sequential scanning is
performed by setting .tau..sub.w as F/4. A mode of sequential
scanning at this time can be understood with reference to FIG. 11I.
That is, blanking scanning is started right after writing scanning
is completed in a period of F/4, and blanking scanning is completed
at time F/2. At this time, the lighting ratio R is 1/4.
[0342] When .tau..sub.a=3F/4 and .tau..sub.b=F/4, sequential
scanning is performed by setting .tau..sub.w as F/3. A mode of
sequential scanning at this time can be understood with reference
to FIG. 11J. That is, blanking scanning is started at time 3F/4 in
the period of F/2 after writing scanning is completed in the period
of F/4. Then, blanking scanning is completed when one frame period
is completed. At this time, the lighting ratio R is 3/4.
[0343] The mode in which .tau..sub.w is changed in accordance with
a value of the lighting ratio R can be realized by conforming
.tau..sub.w to a period having a smaller value between the image
display period .tau..sub.a and the blanking interval .tau..sub.b,
which lead the lighting ratio R in this manner. Since .tau..sub.w
can be set to a suitable period in accordance with the value of the
lighting ratio R in this manner, operating frequency of a
peripheral circuit such as a scan line driver or a data line driver
can also be set to a suitable value which is in accordance with the
value of the lighting ratio R. Accordingly, power consumption can
be reduced.
[0344] Among the plurality of modes of the method in which writing
scanning and blanking scanning are completed in time shorter than
the one frame period F, a mode which is different from the
above-described mode is a mode in which the period .tau..sub.w
during which writing scanning and blanking scanning are completed
is completed rapidly without depending on the value of the lighting
ratio R.
[0345] In the mode in which the period .tau..sub.w during which
writing scanning and blanking scanning are completed is completed
earlier without depending on the value of the lighting ratio R,
.tau..sub.w is shortened as much as possible. For example,
.tau..sub.w is set to F/4 which is 1/4 of the one frame period F. A
mode of sequential scanning at this time can be understood with
reference to FIGS. 11B, 11D, 11F, 11H, 11I, and 11J.
[0346] When the blanking interval .tau..sub.b is 0, blanking
scanning is not performed. A mode of sequential scanning at this
time can be understood with reference to FIG. 11B. That is,
sequential scanning is performed by setting .tau..sub.w as F/4. At
this time, the lighting ratio R is 1.
[0347] When .tau..sub.a=.tau..sub.b=F/2, sequential scanning is
performed also by setting .tau..sub.w as F/4. A mode of sequential
scanning at this time can be understood with reference to FIG. 11D.
That is, blanking scanning is started at the time F/2 in the period
of F/4 after writing scanning is completed in the period of F/4.
Then, blanking scanning is completed at the time 3F/4. At this
time, the lighting ratio R is 1/2.
[0348] When .tau..sub.a =F/3 and .tau..sub.b=2F/3, sequential
scanning is performed also by setting .tau..sub.w as F/4. A mode of
sequential scanning at this time can be understood with reference
to FIG. 11F. That is, blanking scanning is started at the time F/3
in a period of F/12 after writing scanning is completed in the
period of F/4. T blanking scanning is completed at time 7F/12. At
this time, the lighting ratio R is 1/3.
[0349] When .tau..sub.a=2F/3 and .tau..sub.b=F/3, sequential
scanning is performed also by setting .tau..sub.w as F/4. A mode of
sequential scanning at this time can be understood with reference
to FIG. 11H. That is, blanking scanning is started at the time 2F/3
in a period of 5F/12 after writing scanning is completed in the
period of F/4. Then, blanking scanning is completed at time 11F/12.
At this time, the lighting ratio R is 2/ 3.
[0350] When .tau..sub.a=F/4 and .tau..sub.b=3F/4, sequential
scanning is performed also by setting .tau..sub.w as F/4. The mode
of sequential scanning at this time can be understood with
reference to FIG. 11I. That is, blanking scanning is started right
after writing scanning is completed in the period of F/4. Then,
blanking scanning is completed at the time F/2. At this time, the
lighting ratio R is 1/4.
[0351] When .tau..sub.a=3F/4 and .tau..sub.b=F/4, sequential
scanning is performed also by setting .tau..sub.w as F/3. The mode
of sequential scanning at this time can be understood with
reference to FIG. 11J. That is, blanking scanning is started at
time 3F/4 in the period of F/2 after writing scanning is completed
in the period of F/4. Then, blanking scanning is completed when one
frame period is completed. At this time, the lighting ratio R is
3/4.
[0352] Here, in the mode in which .tau..sub.w is completed earlier
without depending on the value of the lighting ratio R, a mode with
a lighting ratio other than those shown in FIGS. 11B, 11D, 11F,
11H, 11I, and 11J can be easily realized. That is, a period at
which blanking scanning is started can be set freely, so that a
mode with a lighting ratio corresponding to it can be realized. In
addition, a range of the image display period .tau..sub.a which can
be set is equal to or greater than .tau..sub.w and equal to or less
than 1-.tau..sub.w. For example, in an example in which .tau..sub.w
is set as F/4, the lighting ratio R can be freely selected in a
range of equal to or greater than 1/4 and equal to or less than
3/4.
[0353] Note that the mode in which the period .tau..sub.w during
which writing scanning and blanking scanning are completed is
changed in accordance with the value of the lighting ratio R and
the mode in which .tau..sub.w is completed earlier without
depending on the value of the lighting ratio R can be combined. For
example, when the lighting ratio R can be freely selected in a
range of equal to or greater than 1/3 and equal to or less than
2/3, .tau..sub.w is set as F/3. Then, when the lighting ratio R is
needed to be selected in a range of greater than that, .tau..sub.w
is set to be smaller than F/3. For example, when .tau..sub.w is set
as F/4, the lighting ratio R in a range of equal to or greater than
1/4 and equal to or less than 1/3 and the lighting ratio R in a
range of equal to or greater than 2/3 and equal to or less than
3/4, which cannot be selected when .tau..sub.w is set as F/3, can
be selected. Since the value of the lighting ratio R can be
selected in a certain range in this manner and operating frequency
of the peripheral circuit such as a scan line driver or a data line
driver can be set to a suitable value which is in accordance with
the range of the value of the lighting ratio R, power consumption
can be reduced, which is extremely advantageous.
[0354] (1) a method of directly writing blanking data to each
pixel, (2) a method of blinking the whole backlight, and (3) a
method of sequentially blinking a backlight which is divided by
areas can be mainly given as a control method of the lighting ratio
R, which has been described at the beginning of this embodiment
mode. The driving method which is described heretofore is a method
which can be used for the method (1).
[0355] The method (1) can be applied to both of the case where a
display element included in a display device is a self-luminous
element typified by an element included in an EL display, a PDP, or
an EFD and the case where a display element included in a display
device is a non-light emitting element typified by an element
included in a liquid crystal display. Next, driving methods of the
methods (2) and (3) are described.
[0356] The method (2) where the whole backlight is blinked can be
used when a display device includes a member called a backlight. A
backlight corresponds to a light source provided on the back of a
display portion of a display device. In particular, a backlight is
advantageous when a display portion of a display device includes a
non-light emitting display element. As such a display element, a
transmissive liquid crystal element and a semi-transmissive liquid
crystal element can be given, for example. Note that a display
device may include a front light projector, a rear and front
projector, or a light source for a projector without limiting to a
backlight.
[0357] In the case of a non-light emitting display element, a light
source is necessary in addition to the display element because the
display element does not emit light by itself. At this time, a
backlight is used in some cases. A backlight is usually a surface
light source which uniformly illuminates a display portion of a
display device. At this time, the display element has a function of
determining how much light of this light source transmits.
Accordingly, increase and decrease in luminance of the backlight
corresponds to increase and decrease in brightness of the whole
image.
[0358] That is, in a display device which includes a backlight, a
blanking interval can be provided by changing luminance of the
backlight without writing a blanking signal to a display element.
Further, the lighting ratio R can be controlled by controlling the
length of a period during which the luminance of the backlight is
changed.
[0359] FIGS. 12A and 12B each show a mode of a method in which the
lighting ratio R is controlled by controlling luminance of a
backlight. Each of graphs shown in FIGS. 12A and 12B shows a mode
of sequential scanning of a display device and timing at which the
backlight is controlled at the same time, in which a horizontal
axis represents time and a vertical axis represents a scanning
direction of a pixel. Solid lines in the graphs show positions in
which a plurality of scan lines included in the display device are
selected.
[0360] In the method in which the lighting ratio R is controlled by
controlling the luminance of the backlight, switching of the
luminance of the backlight and writing scanning are preferably
controlled at timing which is different in terms of time. This is
because by controlling switching of the luminance of the backlight
and writing scanning at timing which is different in terms of time,
all the pixels are classified into pixels which do not emit light
before data is written and do not emit light right after data is
written or pixels which emit light before data is written and
continuously emit light when data is written. Therefore, since
pixels in different conditions are not displayed concurrently in
the display portion, a problem such as display unevenness can be
reduced. This mode can be understood with reference to the graph
shown in FIG. 12A. In FIG. 12A, a period during which the luminance
of the backlight is changed right after writing scanning is
completed is provided in one frame period. The period corresponds
to a region shown by a slanted line in FIG. 12A. When an image is
displayed by lighting the backlight in the period, the period
corresponds to an image display period. Alternatively, when
blanking is displayed by turning out the backlight or reducing
light of the backlight in the period, the period corresponds to a
blanking interval.
[0361] In addition, the length of the period during which the
luminance of the backlight is changed may be changed. FIG. 12B
shows an example thereof. In FIG. 12B, a period during which
luminance of a backlight is changed is shorter than a similar
period in FIG. 12A. In this manner, the lighting ratio R can be
controlled by the length of the period during which the luminance
of the backlight is changed.
[0362] When the lighting ratio R is controlled by the method of
blinking the whole backlight, the backlight is turned out or light
of the backlight is reduced in the blanking interval, so that there
is an advantage in that power consumption can be reduced to a
corresponding extent. In addition, since a structure of a circuit
is simple, manufacturing cost can be reduced.
[0363] Next, among the control methods of the lighting ratio R, the
method (3) in which a backlight which is divided by areas is
sequentially blinked is described. Here, an example in which the
backlight is divided by areas in a direction which is parallel to a
scan line is described.
[0364] FIGS. 13A to 13C each show a mode of a method in which the
lighting ratio R is controlled by controlling luminance of a
backlight which is divided by areas. Each of graphs shown in FIGS.
13A to 13C shows a mode of sequential scanning of a display device
and timing at which the backlight is controlled at the same time,
in which a horizontal axis represents time and a vertical axis
represents a scanning direction of a pixel. Solid lines in the
graphs show positions in which a plurality of scan lines included
in the display device are selected.
[0365] In the method in which the lighting ratio R is controlled by
controlling the luminance of the backlight which is divided by
areas, switching of the luminance of the backlight and writing
scanning are preferably controlled at timing which is different in
terms of time. This is because by controlling switching of the
luminance of the backlight and writing scanning at timing which is
different in terms of time, all the pixels are classified into
pixels which do not emit light before data is written and do not
emit light right after data is written or pixels which emit light
before data is written and continuously emit light when data is
written. Therefore, since pixels in different conditions are not
displayed at the same time in the display portion, a problem such
as display unevenness can be reduced. This mode can be understood
with reference to the graphs shown in FIG. 13A to 13C.
[0366] In FIG. 13A, the case is described in which a ratio of a
period during which the luminance of the backlight is changed to
one frame period is approximately 2/3. The period corresponds to a
region shown by a slanted line. When an image is displayed by
lighting the backlight in the period, the period corresponds to an
image display period. Alternatively, when blanking is displayed by
turning out the backlight or reducing light of the backlight in the
period, the period corresponds to a blanking interval.
[0367] By using the backlight which is divided by areas, a period
during which the luminance of the backlight is changed in each area
can be varied. In FIG. 13A, the backlight is divided into five
areas and luminance of each area is sequentially controlled.
[0368] In addition, the length of the period during which the
luminance of the backlight which is divided by areas is changed may
be changed. FIG. 13B shows an example thereof. In FIG. 13B, a
period during which luminance of a backlight is changed is shorter
than a similar period in FIG. 13A. In this manner, the lighting
ratio R can be controlled by the length of the period during which
the luminance of the backlight is changed.
[0369] In addition, when the backlight which is divided by areas is
used, control can be performed such that switching of the luminance
of the backlight and writing scanning do not overlap with each
other in terms of time without increasing scan speed of writing
scanning. For example, in FIG. 13A or FIG. 13B, the period during
which the luminance of the backlight is changed can be provided
even when writing scanning is performed in the whole one frame
period. Thus, operating frequency of a peripheral circuit such as a
scan line driver or a data line driver can be set small regardless
of a value of the lighting ratio R. Accordingly, power consumption
can be reduced.
[0370] Note that even when the backlight which is divided by areas
is used, scan speed of writing scanning may be increased. Thus, a
display trouble caused by variation in light-emitting time between
areas can be reduced. This point can be understood with reference
to the graph shown in FIG. 13C. The graph shown in FIG. 13C shows
an example of the case where scan speed of writing scanning is
increased. From the graph shown in FIG. 13C, it can be seen that
variation in light-emitting time between adjacent areas is less
than that of the case where scan speed of writing scanning is not
high (FIG. 13A or FIG. 13B) when scan speed of writing scanning is
increased. When variation in light-emitting time between the
adjacent areas is little, a display trouble caused by variation in
light-emitting time between areas can be reduced.
[0371] As a display trouble caused by variation in light-emitting
time between areas, increase of false light emission caused by
light leakage from the areas, increase of visibility of a boundary
between the areas, or the like can be given, for example.
[0372] When the lighting ratio R is controlled by the method of
controlling the luminance of the backlight which is divided by
areas, the backlight is turned out or light of the backlight is
reduced in the blanking interval, so that there is an advantage in
that power consumption can be reduced to a corresponding
extent.
[0373] Heretofore, a method of controlling the lighting ratio R is
described under a condition that luminance perceived by human eyes
(L/F) is constant. A method for changing luminance perceived by
human eyes is described below.
[0374] In order to change luminance perceived by human eyes, there
are a method of changing the integrated luminance L and a method of
changing the lighting ratio R. Here, when it is assumed that the
lighting ratio R is constant, the integrated luminance L should be
changed in order to change the luminance perceived by human
eyes.
[0375] The integrated luminance L is luminance obtained by time
integrating the instantaneous luminance I (t) as shown in Formula
3. That is, it is necessary that the instantaneous luminance I (t)
be changed in order to change the integrated luminance L.
[0376] Here, in the case where a display element included in a
display device is a self-luminous element such as an element
included in an EL display, a PDP, or an EFD, luminance of the
display element itself changes the instantaneous luminance I (t).
That is, the instantaneous luminance I (t) can be changed by
writing a predetermined signal to each display element.
[0377] On the other hand, luminance of a display element itself
changes the instantaneous luminance I (t) even in the case where
the display element included in a display device is a non-light
emitting element; however, the luminance of the display element
itself can be divided into a plurality of elements in the case
where the display element is a non-light emitting element. That is,
the plurality of factors correspond to backlight luminance B.sub.L
and transmittance T of the display element. Therefore, the
luminance of the display element is a product of the backlight
luminance B.sub.L and the transmittance T. The luminance of the
display element also corresponds to the instantaneous luminance I
(t). When the description is summarized, it can be represented as
Formula 8.
I(t)=B.sub.L(t)T(t) Formula 8]
[0378] Here, Formula 8 is assigned to Formula 3 which leads the
integrated luminance L. Note that when the backlight luminance
B.sub.L and the transmittance T are not dependent on the time t for
simplification, Formula 9 is obtained.
L F = B L T [ Formula 9 ] ##EQU00004##
[0379] A left-hand side of Formula 9 shows the luminance perceived
by human eyes (L/F). Therefore, when the backlight luminance
B.sub.L and the transmittance T are constant, the product of
B.sub.L and T represents the luminance perceived by human eyes.
[0380] In a display device using a liquid crystal element, the
transmittance T is usually controlled by voltage written to a pixel
and the luminance perceived by human eyes is controlled. A numeric
value in which a degree of the luminance perceived by human eyes is
represented by a positive integer is called a gray scale. In
addition, G is used as a sign which represents the gray scale. For
example, when brightness between the darkest brightness and the
brightest brightness is classified into 256 stages, a gray scale
0expresses the darkest brightness and a gray scale 255 expresses
the brightest brightness. An intermediate gray scale expresses
intermediate brightness of the two gray scales.
[0381] It should be noted that when a gray scale is dealt,
brightness expressed by the gray scale dose not necessarily have a
linear relation with physical luminance. That is, when a relation
between a gray scale and luminance is expressed by a graph, the
gray scale and the luminance can be in connection with each other
by a curve having various shapes. This curve showing a relationship
between a gray scale and luminance is called a gamma curve.
[0382] A Typical gamma curve is described with reference to FIG.
14A. FIG. 14A is a graph showing a relation between a gray scale
and luminance, i.e., a gamma curve. A horizontal axis represents a
gray scale and a vertical axis represents luminance. Here,
luminance corresponds to luminance perceived by human eyes (L/F).
That is, from Formula 9, the vertical axis represents the amount
expressed by the product of B.sub.L and T. A curve 1400 shown in
FIG. 14A is a gamma curve when brightness perceived by human eyes
is changed almost linearly. In this manner, an ideal gamma curve is
a curve having convexity below.
[0383] When luminance B.sub.LT is changed by changing a gray scale
G transmittance T is usually changed. This is because although the
transmittance T can be individually controlled by changing voltage
written to each pixel, it is difficult to individually control the
backlight luminance B.sub.L because the backlight luminance B.sub.L
is shared with a plurality of pixels.
[0384] Next, a method of displaying an image normally even when the
backlight luminance B.sub.L decreases by controlling the
transmittance T and the backlight luminance B.sub.L is described.
Since the luminance B.sub.LT is the product of the transmittance T
and the backlight luminance B.sub.L, various gamma curves can be
realized by changing the transmittance T and the backlight
luminance B.sub.L.
[0385] A curve 1401 shown in FIG. 14A is a curve in which the
transmittance T of the curve 1400 increases in each gray scale G
and is represented as a function T.sub.1 (G). In FIG. 14A, since
the backlight luminance B.sub.L is not changed, the luminance
B.sub.LT is higher than the luminance of the curve 1400. In
addition, since the transmittance T has the maximum value and
cannot be made larger than that, the curve 1401 is saturated in a
certain gray scale.
[0386] A curve 1402 shown in FIG. 14B is a gamma curve at the time
when the transmittance T increases as in the curve 1401 shown in
FIG. 14A and the backlight luminance B.sub.L decreases. At this
time, in a region G.sub.1402 of a gray scale, the transmittance T
of which is saturated, there is no difference in luminance and the
curve 1402 is saturated. Luminance at this time is denoted by
a.sub.1. In gray scale regions other than the gray scale region
G.sub.1402, a shape of the curve 1402 preferably corresponds to
that of the curve 1400. Thus, even when power consumption is
reduced by decreasing the backlight luminance B.sub.L, display
which is similar to display at the time when the backlight
luminance B.sub.L is not decreased can be performed in the gray
scale regions other than the gray scale region G.sub.1402.
[0387] Note that an advantage of the method in this document is
that the backlight luminance B.sub.L can be decreased by
controlling the lighting ratio R. Thus, power consumption of a
backlight can be reduced and a blanking interval can be provided,
so that motion blur can be reduced.
[0388] Here, an adverse effect on image display at the time when
display is performed in accordance with a gamma curve in which
luminance is saturated as in the curve 1402 shown in FIG. 14B is
described. When image display is performed in accordance with a
gamma curve in which luminance is saturated as in the curve 1402,
needless to say, all the grays scales included in the gray scale
region G.sub.1402 have the same luminance. At this time, as an
adverse effect on image display, a condition in which there is no
bright gray scale, i.e., a condition called blown-out highlights
can be given.
[0389] However, not all the images cause blown-out highlights. In a
graph shown in FIG. 14C, a horizontal axis represents the gray
scale G and a vertical axis represents the number of data included
in the pixels. Such a graph is called a histogram. In a histogram
1403 shown in FIG. 14C, there is almost no data in the gray scale
region G.sub.1402. That is, as for an image originally having no
data in the gray scale region G.sub.1402, blown-out highlights do
not occur even when the curve 1402 shown in FIG. 14B is used as a
gamma curve.
[0390] On the other hand, a histogram 1404 shown in FIG. 14D shows
the case of an image having a certain number of data in the gray
scale region G.sub.1402. At this time, a certain degree of
blown-out highlights occurs at the time when the curve 1402 shown
in FIG. 14B is used as a gamma curve. However, when the number of
data included in the gray scale region G.sub.1402 is equal to or
less than 1/10 of the total number of data, blown-out highlights
are hardly perceived.
[0391] In this manner, the method in this document analyzes a
histogram of an image and determines whether the number of data of
an image included in a gray scale region in which luminance is
saturated is equal to or less than 1/10 of the total number of
data. When the number of data of the image included in the gray
scale region in which the luminance is saturated is equal to or
less than 1/10 of the total number of data, the transmittance T
increases such that the graph has a gamma curve which is in
accordance with the function T.sub.1 (G), and the backlight
luminance B.sub.L decreases. In addition, the backlight luminance
B.sub.L is preferably decreased by controlling the lighting ratio
R. Thus, power consumption of a backlight can be reduced and a
blanking interval can be provided, so that motion blur can be
reduced.
[0392] Next, the case is described in which the histogram of the
image is analyzed and the number of data of the image included in
the gray scale region in which the luminance is saturated is equal
to or greater than 1/10 of the total number of data.
[0393] In the case where the histogram of the image is analyzed and
the number of data of the image included in the gray scale region
in which the luminance is saturated is equal to or greater than
1/10 of the total number of data, the curve 1400 is not a curve
represented by the function T.sub.1 (G) but a curve represented by
another function when the transmittance T of the curve 1400
increases in each of the gray scales C
[0394] A curve 1405 shown in FIG. 14E is a curve in which the
transmittance T of the curve 1400 increases in each of the gray
scales G and is represented as a function T.sub.2 (G). In FIG. 14E,
since the a backlight luminance B.sub.L is not changed, the
luminance B.sub.LT is higher than the luminance of the curve 1400.
In addition, since the transmittance T has the maximum value and
cannot be made larger than that, the curve 1405 is saturated in a
certain gray scale. Here, as for a relation between the function
T.sub.1 (G) and the function T.sub.2 (G), T.sub.1 (G)>T.sub.2
(G) is satisfied in a gray scale region in which the transmittance
T is not saturated and T.sub.1 (G)=T.sub.2 (G) is satisfied in a
gray scale region in which the transmittance T is saturated.
[0395] A curve 1406 shown in FIG. 14F is a gamma curve at the time
when the transmittance T increases as in the curve 1405 shown in
FIG. 14E and the backlight luminance B.sub.L decreases. At this
time, in a region G.sub.1406 of a gray scale, the transmittance T
of which is saturated, there is no difference in luminance and the
curve 1406 is saturated. Luminance at this time is denoted by
a.sub.2. In gray scale regions other than the gray scale region
G.sub.1406, a shape of the curve 1406 preferably corresponds to
that of the curve 1400. Thus, even when power consumption is
reduced by decreasing the backlight luminance B.sub.L, display
which is similar to display at the time when the backlight
luminance B.sub.L is not decreased can be performed in the gray
scale regions other than the gray scale region G.sub.1406.
[0396] As for the gamma curve 1406 in which the transmittance T is
changed in accordance with not the function T.sub.1 (G) but the
function T.sub.2 (G) and the luminance is made to be a.sub.2 by
decreasing the backlight luminance B.sub.L, the luminance is
saturated in a certain gray scale region similarly to the gamma
curve 1402. However, the size of a gray scale region in which
luminance is saturated is different between the gray scale region
G.sub.1406 in which the luminance is saturated in the gamma curve
1406 and the gray scale region G.sub.1402 in which the luminance is
saturated in the gamma curve 1402. In addition, luminance in a gray
scale region in which the luminance is saturated is different from
each other. That is, G.sub.1402>G.sub.1406 and
a.sub.1<a.sub.2 are satisfied.
[0397] An advantageous effect on a displayed image due to a
difference in the size of the gray scale regions is described.
Although the histogram 1404 shown in FIG. 14G is similar to the
histogram 1404 shown in FIG. 14D, a displayed gray scale region is
not G.sub.1402 but G.sub.1406. When FIGS. 14D and 14G are compared
with each other, it is apparent that the histogram 1404 has a
certain number of data in the gray scale region G.sub.1402 but the
histogram 1404 has almost no data in the gray scale region
G.sub.1406. Therefore, it can be said that an image having a data
distribution represented by the histogram 1404 has a lower degree
of blown-out highlights in the case where the image is displayed in
accordance with the gamma curve 1406 than the case where the image
is displayed in accordance with the gamma curve 1402.
[0398] Therefore, if the number of data included in the gray scale
region G.sub.1402 is equal to or greater than 1/10 of the total
number of data in the image displayed by the histogram 1404, a
degree of blown-out highlights in image display can be made not to
be perceived by changing a gamma curve used for display from the
gamma curve represented by the curve 1402 to the curve represented
by the curve 1406.
[0399] In this manner, the method in this document analyzes a
histogram of an image and determines whether the number of data of
an image included in a gray scale region in which luminance is
saturated is equal to or less than 1/10 of the total number of
data. When the number of data of the image included in the gray
scale region in which the luminance is saturated is equal to or
greater than 1/10 of the total number of data, the transmittance T
increases such that the graph has a gamma curve which is in
accordance with the function T.sub.2 (G) supplying luminance which
is lower than that of the function T.sub.1 (G), and the backlight
luminance B.sub.L decreases. In addition, the backlight luminance
B.sub.L is preferably decreased by controlling the lighting ratio
R. Thus, power consumption of a backlight can be reduced and a
blanking interval can be provided, so that motion blur can be
reduced.
[0400] Note that even in the case of the gamma curve which is in
accordance with the function T.sub.2 (G) supplying the luminance
which is lower than that of the function T.sub.1 (G), display with
a lower degree of blown-out highlights can be performed by not
using the function T.sub.2 (G) but separately preparing a function
supplying luminance which is lower than that of the function
T.sub.2 (G) in the case of an image having a histogram in which the
number of data included in the gray scale region G.sub.1406 in
which the luminance is saturated is equal to or greater than 1/10
of the total number of data (e.g., a histogram shown in FIG.
14H).
[0401] Next, a method in which peak luminance can be improved by
controlling the transmittance T and the backlight luminance B.sub.L
is described. Peak luminance corresponds to the highest luminance
which can be displayed by a display device. When peak luminance is
high, expressive power of an image is improved. For example, an
image where stars twinkle in the night sky, an image where light is
reflected by a car body, or the like can be displayed as expression
which is closer to real objects.
[0402] The highest luminance can be simply increased by just
increasing the backlight luminance. However, luminance on a lower
gray scale side is also increased at the same time when the
backlight luminance is just increased, and a condition where
luminance of a portion displaying black increases (i.e., black
blurring) is caused. Thus, expressive power of an image is not
improved. In order to improve expressive power of an image, it is
important to increase the highest luminance without causing black
blurring. In this document, description "peak luminance is
improved" may mean that the highest luminance increases without
causing black blurring.
[0403] A curve 1501 shown in FIG. 15A is a curve in which the
transmittance T of the curve 1400 decreases in each gray scale G
and is represented as a function T.sub.3 (G). In FIG. 15A, since
the backlight luminance B.sub.L is not changed, the luminance
B.sub.LT is lower than the luminance of the curve 1400. In
addition, the transmittance T in the highest gray scale is the
maximum value which can be obtained by a display element.
[0404] A curve 1502 shown in FIG. 15B is a gamma curve at the time
when the transmittance T decreases as in the curve 1501 shown in
FIG. 15A and the backlight luminance B.sub.L increases. At this
time, a region of a gray scale where luminance of the curve 1502 is
higher than the luminance of the curve 1400 corresponds to a gray
scale region G.sub.1502. The highest luminance is denoted by
a.sub.3. In gray scale regions other than the gray scale region
G.sub.1502, a shape of the curve 1502 preferably corresponds to
that of the curve 1400. Thus, even when the backlight luminance
B.sub.L increases, display which is similar to display when the
backlight luminance B.sub.L is not increased can be performed in
the gray scale regions other than the gray scale region G.sub.1502.
Therefore, black blurring can be suppressed.
[0405] When image display is performed by using the curve 1502 as a
gamma curve, the highest luminance can be increased without causing
black blurring in a low gray scale region. That is, peak luminance
can be improved. Thus, expressive power of an image can be
improved.
[0406] Note that an advantage of the method in this document is
that the backlight luminance B.sub.L can be decreased by
controlling the lighting ratio R. Thus, a suitable blanking
interval can be set, so that a flicker can be reduced and motion
blur can be reduced optimally.
[0407] Here, when a curve represented by the curve 1502 shown in
FIG. 15B is used as a gamma curve, an image having the large number
of data included in the gray scale region G.sub.1502 as in a
histogram 1503 shown in FIG. 15C has a larger effect on improvement
in peak luminance. Specifically, when the number of data included
in the gray scale region G.sub.1502 is equal to or greater than 1/3
of the total number of data, it is more effective. Note that even
when the number of data included in the gray scale region
G.sub.1502 is small, a portion displayed in accordance with data
included in the gray scale region G.sub.1502 is further enhanced
when the image is an image (e.g., an image where stars twinkle in
the night sky) having a histogram where the number of data in the
low gray scale region is considerably large (e.g., a histogram 1504
shown in FIG. 15D), so that it is effective to use the curve
represented by the curve 1502 shown in FIG. 15B as a gamma curve.
Specifically, when the whole gray scale regions are divided equally
into a low gray scale region, an intermediate gray scale region,
and a high gray scale region, it is particularly effective to use
the curve represented by the curve 1502 shown in FIG. 15B as a
gamma curve when data of equal to or greater than 1/2 of the total
number of data is included in the low gray scale region.
[0408] Next, another method of displaying an image normally even
when the backlight luminance B.sub.L decreases by controlling the
transmittance T and the backlight luminance B.sub.L is described.
Since the luminance B.sub.LT is the product of the transmittance T
and the backlight luminance B.sub.L, various gamma curves can be
realized by changing the transmittance T and the backlight
luminance B.sub.L.
[0409] A curve 1601 shown in FIG. 16A is a curve in which the
transmittance T of the curve 1400 increases in each gray scale G
and is represented as a function T.sub.4 (G). In FIG. 16A, since
the backlight luminance B.sub.L is not changed, the luminance
B.sub.LT is higher than the luminance of the curve 1400. In
addition, although the curve 1401 shown in FIG. 14A and the curve
1405 shown in FIG. 14E are each saturated in a certain gray scale,
the curve 1601 shown in FIG. 16A is not saturated and has a
gradient in the gray scale region in which luminance is saturated
in the curve 1401 and the curve 1405.
[0410] A curve 1602 shown in FIG. 16B is a gamma curve at the time
when the transmittance T increases as in the curve 1601 shown in
FIG. 16A and the backlight luminance B.sub.L decreases. Here, a
shape of the curve 1602 preferably corresponds to that of the curve
1400 in gray scale regions other than part of a high gray scale
region. At this time, a gray scale region where the curve 1602 and
the curve 1400 do not correspond to each other is denoted by a gray
scale region G.sub.1602. In addition, the highest luminance of the
curve 1602 is denoted by a.sub.4. Thus, even when power consumption
is reduced by decreasing the backlight luminance B.sub.L, display
which is similar to display at the time when the backlight
luminance B.sub.L is not decreased can be performed in gray scale
regions other than the gray scale region G.sub.1602. Further, since
a certain degree of difference in luminance can be obtained also in
display of gray scales included in the gray scale region
G.sub.1602, blown-out highlights of a displayed image can be
suppressed.
[0411] A curve 1603 shown in FIG. 16C is a curve in which the
transmittance T of the curve 1400 increases in each of the gray
scales G and is represented as a function T.sub.5 (G). In FIG. 16C,
since the backlight luminance B.sub.L is not changed, the luminance
B.sub.LT is higher than the luminance of the curve 1400. In
addition, since the curve 1601 shown in FIG. 16A has the gradient
in part of the high gray scale region, a primary differential
function of the function T.sub.4 (G) is discontinuous at a boundary
between regions having different shapes; however, as for the curve
1603 shown in FIG. 16C, a primary differential function of the
function T.sub.5 (G) is continuous at the boundary between regions
having different shapes and the curve 1603 is smooth.
[0412] A curve 1604 shown in FIG. 16D is a gamma curve at the time
when the transmittance T increases as in the curve 1603 shown in
FIG. 16C and the backlight luminance B.sub.L decreases. Here, a
shape of the curve 1604 preferably corresponds to that of the curve
1400 in gray scale regions other than part of a high gray scale
region. At this time, a gray scale region where the curve 1604 and
the curve 1400 do not correspond to each other is denoted by a gray
scale region G.sub.1604. In addition, the highest luminance of the
curve 1604 is denoted by a.sub.5. Thus, even when power consumption
is reduced by decreasing the backlight luminance B.sub.L, display
which is similar to display at the time when the backlight
luminance B.sub.L is not decreased can be performed in gray scale
regions other than the gray scale region G.sub.1604. Further, since
a certain degree of difference in luminance can be obtained also in
display of gray scales included in the gray scale region
G.sub.1604, blown-out highlights of a displayed image can be
suppressed. Furthermore, since a boundary between a gray scale
region where the curve 1604 and the curve 1400 correspond to each
other and the gray scale region where the curve 1604 and the curve
1400 do not correspond to each other is smooth, there is an
advantage in that a visual boarder line in a Mach band image (a
false contour perceived by human physiology and psychology) cannot
be seen.
[0413] Note that an advantage of the method in this document is
that the backlight luminance B.sub.L can be decreased by
controlling the lighting ratio R. Thus, power consumption of a
backlight can be reduced and a blanking interval can be provided,
so that motion blur can be reduced.
[0414] Although this embodiment mode is described with reference to
various drawings, the contents (or may be part of the contents)
described in each drawing can be freely applied to, combined with,
or replaced with the contents (or may be part of the contents)
described in another drawing. Further, even more drawings can be
formed by combining each part with another part in the
above-described drawings.
[0415] The contents (or may be part of the contents) described in
each drawing of this embodiment mode can be freely applied to,
combined with, or replaced with the contents (or may be part of the
contents) described in a drawing in another embodiment mode.
Further, even more drawings can be formed by combining each part
with part of another embodiment mode in the drawings of this
embodiment mode.
[0416] This embodiment mode shows an example of an embodied case of
the contents (or may be part of the contents) described in other
embodiment modes, an example of slight transformation thereof, an
example of partial modification thereof, an example of improvement
thereof, an example of detailed description thereof, an application
example thereof, an example of related part thereof, or the like.
Therefore, the contents described in other embodiment modes can be
freely applied to, combined with, or replaced with this embodiment
mode.
Embodiment Mode 3
[0417] In this embodiment mode, specific examples of the control
parameter P or Q described in Embodiment Mode 1 are described. In
addition, in this embodiment mode, P is used as a sign showing a
control parameter.
[0418] Here, in this document, there is not a particular
distinction between the case where the sign showing the control
parameter is P, the case where the sign showing the control
parameter is Q, and the case where the sign showing the control
parameter is other than P and Q. The sign showing the control
parameter is just determined for convenience. Therefore, among a
plurality of specific examples of the control parameter, which are
described below, any of the specific examples may be used as the
control parameter P, or any of the specific examples may be used as
the control parameter Q. In addition, the number of the control
parameters is not particularly limited.
[0419] First, the case is described in which the control parameter
P is determined by numerically analyzing image data which is
displayed on a display device.
[0420] The displayed image is divided into an object and a
background by analyzing image data which is input to the display
device. Here, the object corresponds to a portion of the image
where the control parameter P is determined. In addition, the
background corresponds to portions other than the object.
[0421] FIG. 17A is a view showing a calculation method of the
control parameter P when the control parameter P is determined by a
distance of an object in the case where the object moves on the
screen. In FIG. 17A, a region shown by a sign 1701 shows an object
of a current frame. In addition, a region shown by a sign 1702
shows an object of a previous frame. That is, the control parameter
P is determined by a distance in which the object moves when a
displayed image is changed from the previous frame to the current
frame. Here, .DELTA.X in FIG. 17A shows a component in a horizontal
direction of the distance in which the object moves. .DELTA.Y in
FIG. 17A shows a component in a vertical direction of the distance
in which the object moves. A square root of the sum of a square of
.DELTA.X and .DELTA.Y is the distance in which the object moves,
and the control parameter P is determined by the size thereof.
Here, as the distance in which the object moves becomes larger and
the object moves faster, a degree of motion blur increases.
Therefore, as the object moves faster, the control parameter P is
preferably increased. This is because the lighting ratio R is
controlled such that motion blur is further reduced as the control
parameter P becomes larger in Embodiment Mode 1.
[0422] FIGS. 17B to 17D are views each showing the case where a
shape of an object is used as the control parameter P. An object
1711 in FIG. 17B is an object having a shape with no corner such as
a circle or an oval. An object 1712 in FIG. 17C is an object having
a relatively simple shape with several corners such as a quadrangle
or a triangle. An object 1713 in FIG. 17D is an object having a
complicated shape such as hiragana (Japanese syllabary characters),
katakana (square phonetic Japanese syllabary), alphabet, or Chinese
character. Here, as the shape of the object becomes complicated, a
degree of motion blur increases. Therefore, as the shape of the
object becomes complicated, the control parameter P is preferably
increased. This is because the lighting ratio R is controlled such
that motion blur is further reduced as the control parameter P
becomes larger in Embodiment Mode 1.
[0423] FIGS. 17E to 17G are views each showing the case where the
size of an object is used as the control parameter P. An object
1721 in FIG. 17E is an object having a size of approximately 1/100
of an area of a display portion of a display device. An object 1722
in FIG. 17F is an object having a size of approximately 1/100 to
approximately 1/10 of the area of the display portion of the
display device. An object 1723 in FIG. 17G is an object having a
size of approximately 1/10 or more of the area of the display
portion of the display device. Here, as the size of the object
becomes larger, a degree of motion blur increases. Therefore, as
the size of the object becomes larger, the control parameter P is
preferably increased. This is because the lighting ratio R is
controlled such that motion blur is further reduced as the control
parameter P becomes larger in Embodiment Mode 1.
[0424] FIGS. 17H and 171 are views each showing the case where a
position of an object on a display portion is used as the control
parameter P. An object 1731 in FIG. 17H is an object having a
certain distance from the center of a display portion of a display
device. An object 1732 in FIG. 17I is an object located almost in
the center of the display portion of the display device. Here, as
the position of the object becomes closer to the center, a degree
of motion blur increases because the object is noticeable for a
user. Therefore, as the position of the object becomes closer to
the center, the control parameter P is preferably increased. This
is because the lighting ratio R is controlled such that motion blur
is further reduced as the control parameter P becomes larger in
Embodiment Mode 1.
[0425] FIGS. 17J to 17L are views each showing the case where
density of objects is used as the control parameter P. A region
1741 in FIG. 17J is a group of objects in a certain range of a
display portion of a display device. FIG. 17J shows the case where
density of the objects in the region 1741 is low. A region 1742 in
FIG. 17K is a group of objects in a certain range of the display
portion of the display device. FIG. 17K shows the case where
density of the objects in the region 1742 is intermediate. A region
1743 in FIG. 17L is a group of objects in a certain range of the
display portion of the display device. FIG. 17L shows the case
where density of the objects in the region 1743 is high. Here, as
density of the objects becomes higher, a degree of motion blur
increases. Therefore, as density of the objects becomes higher, the
control parameter P is preferably increased. This is because the
lighting ratio R is controlled such that motion blur is further
reduced as the control parameter P becomes larger in Embodiment
Mode 1.
[0426] FIGS. 18A to 18I are views each showing the case where a
difference in luminance between an object and a background is used
as the control parameter P. In addition, FIGS. 18J to 18L are
diagrams in which histograms of images shown in FIGS. 18A to 18I
are compared with each other.
[0427] FIGS. 18A to 18C are views showing images at the time when
luminance of backgrounds 1802, 1804, and 1806 is luminance in a low
gray scale region. FIG. 18A shows the case where luminance of an
object 1801 is luminance in the low gray scale region. FIG. 18B
shows the case where luminance of an object 1803 is luminance in an
intermediate gray scale region. FIG. 18C shows the case where
luminance of an object 1805 is luminance in a high gray scale
region. In addition, histograms of respective images are shown by a
curve 1831, a curve 1832, and a curve 1833 in FIG. 18J.
[0428] In each of FIGS. 18A to 18C, as the difference in luminance
between the object and the background becomes larger, a degree of
motion blur increases because a difference between the object and
the background stands out. That is, the degree of motion blur in
the image shown in FIG. 18C is the largest and the degree of motion
blur in the image shown in FIG. 18A is the smallest. The degree of
motion blur in the image shown in FIG. 18B is intermediate
therebetween. When this is described with reference to FIG. 18J, it
can be said that a degree of motion blur increases as a difference
in gray scales of the image between a portion showing luminance
distribution of the background and a portion showing luminance
distribution of the object becomes larger. Therefore, as the
difference in gray scales of the image between the portion showing
the luminance distribution of the background and the portion
showing the luminance distribution of the object becomes larger,
the control parameter P is preferably increased. This is because
the lighting ratio R is controlled such that motion blur is further
reduced as the control parameter P becomes larger in Embodiment
Mode 1.
[0429] FIGS. 18D to 18F are views showing images at the time when
luminance of backgrounds 1812, 1814, and 1816 is luminance in an
intermediate gray scale region. FIG. 18D shows the case where
luminance of an object 1811 is luminance in the low gray scale
region. FIG. 18E shows the case where luminance of an object 1813
is luminance in an intermediate gray scale region. FIG. 18F shows
the case where luminance of an object 1815 is luminance in a high
gray scale region. In addition, histograms of respective images are
shown by a curve 1834, a curve 1835, and a curve 1836 in FIG.
18K.
[0430] In each of FIGS. 18D to 18F, as the difference in luminance
between the object and the background becomes larger, a degree of
motion blur increases because a difference between the object and
the background stands out. That is, the degree of motion blur in
the images shown in FIGS. 18D and 18F is the largest and the degree
of motion blur in the image shown in FIG. 18E is the smallest. Note
that the degree of motion blur in the images shown in FIGS. 18D and
18F is similar to the degree of motion blur in the image shown in
FIG. 18B. This is because the difference in luminance between the
object and the background in the images shown in FIGS. 18D and 18F
is similar to the difference in luminance between the object and
the background in the image shown in FIG. 18B. When this is
described with reference to FIG. 18K, it can be said that a degree
of motion blur increases as a difference in gray scales of the
image between a portion showing luminance distribution of the
background and a portion showing luminance distribution of the
object becomes larger. Therefore, as the difference in gray scales
of the image between the portion showing the luminance distribution
of the background and the portion showing the luminance
distribution of the object becomes larger, the control parameter P
is preferably increased. This is because the lighting ratio R is
controlled such that motion blur is further reduced as the control
parameter P becomes larger in Embodiment Mode 1.
[0431] FIGS. 18G to 18I are views showing images at the time when
luminance of backgrounds 1822, 1824, and 1826 is luminance in a
high gray scale region. FIG. 18G shows the case where luminance of
an object 1821 is luminance in the low gray scale region. FIG. 18H
shows the case where luminance of an object 1823 is luminance in an
intermediate gray scale region. FIG. 18I shows the case where
luminance of an object 1825 is luminance in a high gray scale
region. In addition, histograms of respective images are shown by a
curve 1837, a curve 1838, and a curve 1839 in FIG. 18L.
[0432] In each of FIGS. 18G to 18I, as the difference in luminance
between the object and the background becomes larger, a degree of
motion blur increases because a difference between the object and
the background stands out. That is, the degree of motion blur in
the image shown in FIG. 18G is the largest and the degree of motion
blur in the image shown in FIG. 18I is the smallest. The degree of
motion blur in the image shown in FIG. 18H is intermediate
therebetween. When this is described with reference to FIG. 18L, it
can be said that a degree of motion blur increases as a difference
in gray scales of the image between a portion showing luminance
distribution of the background and a portion showing luminance
distribution of the object becomes larger. Therefore, as the
difference in gray scales of the image between the portion showing
the luminance distribution of the background and the portion
showing the luminance distribution of the object becomes larger,
the control parameter P is preferably increased. This is because
the lighting ratio R is controlled such that motion blur is further
reduced as the control parameter P becomes larger in Embodiment
Mode 1.
[0433] In this manner, a difference in luminance between an object
and a background is analyzed by a histogram, and the control
parameter P increases (the lighting ratio R decreases) as a
difference between luminance distribution of the object and
luminance distribution of the background becomes larger. Therefore,
motion blur can be reduced.
[0434] Note that the control parameter P can be determined by not
only the difference in luminance between the object and the
background, but also sharpness of change in luminance at a boundary
between the object and the background. That is, the control
parameter P may be determined by a value obtained by secondarily
differentiating a function which corresponds to luminance with
respect to a position in a display portion of a display device on a
line including the boundary between the object and the background.
Here, as the secondary differential value at the boundary between
the object and the background becomes larger, a degree of motion
blur increases because an image is an image where the boundary
between the object and the background stands out. Therefore, as the
secondary differential value at the boundary between the object and
the background becomes larger, the control parameter P is
preferably increased. This is because the lighting ratio R is
controlled such that motion blur is further reduced as the control
parameter P becomes larger in Embodiment Mode 1.
[0435] Next, the case is described in which the control parameter P
is determined by a method other than the method of numerically
analyzing image data which is displayed on a display device.
[0436] As a method of determining the control parameter P other
than the method of numerically analyzing image data which is
displayed on a display device, a method of collecting data on
environment where a display device is set can be given.
[0437] For example, a display device 1900 described in this
document is set in a room as shown in FIG. 19A. The display device
1900 is set on a board 1901. A temperature and humidity control
device 1902 is provided on a wall surface which is an upper part of
the display device 1900. A window 1903 is provided on a wall
surface which is a left part seeing from a display device 1900
side. A lighting device 1904 is provided an upper part of front
seeing from the display device 1900 side. An entrance 1905 is
provided on a wall surface of front seeing from the display device
1900 side. Particularly important items as data on environment
where the display device 1900 is set are heat and light.
[0438] In the environment where the display device 1900 is set,
some change in temperature due to various factors always occurs.
For example, when some kind of electronic and electric device is
put inside the board 1901 or the board 1901 itself is some kind of
electronic and electric device, change in temperature in the
display device 1900 due to heat from a lower part is inevitable. In
addition, when air delivered from the temperature and humidity
control device 1902 directly or indirectly flows to the display
device 1900, change in temperature in the display device 1900 due
to heat or cool air from an upper part is inevitable. The same can
be said for the window 1903 and the entrance 1905.
[0439] When temperature of the environment where the display device
1900 is set is changed, characteristics of a display element is
changed. For example, in the case of a liquid crystal element,
response speed is quickened when temperature is high and the
response speed decreases when the temperature is low. Therefore, as
the temperature of the environment becomes lower, the control
parameter P is preferably increased.
[0440] In this manner, the control parameter P which determines a
control condition of the display device 1900 may be determined in
accordance with change in the temperature of the environment where
the display device 1900 is set. Therefore, the display device 1900
may include a temperature sensor.
[0441] In addition, light which shines on a display portion of the
display device 1900 greatly affects a display condition of the
display device 1900. As light which shines on a display portion of
the display device 1900, light from the lighting device 1904 or
penetration of external light from the window 1903 can be given in
environment shown in FIG. 19A.
[0442] When light shines on the display portion of the display
device 1900, contrast of an image decreases by reflected light of
the light. That is, when the contrast of the image decreases by
increase in the reflected light, a degree of motion blur decreases.
Therefore, as reflected light by the light which shines on the
display portion of the display device 1900 becomes less, the
control parameter P is preferably increased.
[0443] In this manner, the control parameter P which determines the
control condition of the display device 1900 may be determined in
accordance with change in brightness of the environment where the
display device 1900 is set. Therefore, the display device 1900 may
include a photo sensor.
[0444] Next, as a method of determining the control parameter P
other than the method of numerically analyzing image data which is
displayed on a display device, a method of determining the control
parameter P by contents displayed by a display device can be
given.
[0445] A view shown in FIG. 19B shows the case where the display
device 1900 displays a baseball game. In addition, a view shown in
FIG. 19C shows the case where the display device 1900 displays a
soccer game.
[0446] When the display device 1900 displays a baseball game, an
object which is used for determining the control parameter P is a
baseball ball 1910, a bat 1911 of a batter, or the like. When the
display device 1900 displays a soccer game, an object which is used
for determining the control parameter P is a soccer ball 1920, a
movement of the whole image by a pan operation on a imaging device
side, or the like. In each case, a kind of the object is extremely
limited.
[0447] In addition, conditions such as speed of a movement when the
object is displayed, the shape, the size, the position, density,
the difference in luminance of the background, and the sharpness of
change in the luminance at the boundary between the object and the
background are hardly changed during which the contents are
displayed. That is, when a value of the control parameter P, which
should be set, is determined in advance depending on kinds of the
contents, a suitable control parameter P can be determined without
analyzing data on an image which is displayed on the display device
every frame.
[0448] As kinds of the contents other than those shown in FIGS. 19B
and 19C, various kinds of contents such as sports other than
baseball and soccer, movies, cooking programs, news programs,
variety programs, music programs, and animations can be given. The
control parameter P can be set in advance depending on various
kinds of contents.
[0449] In this manner, when a suitable control parameter P can be
set in advance depending on kinds of the contents, a suitable
control parameter P can be determined without analyzing data on an
image which is displayed on the display device every frame.
[0450] Note that as a method for determining kinds of the contents,
information from an electronic program guide (an EPG) may be used
as well as analyzing data on the image which is displayed on the
display device.
[0451] Next, as a method of determining the control parameter P
other than the method of numerically analyzing image data which is
displayed on a display device, a method of determining the control
parameter P by age of the user can be given.
[0452] When the control parameter P is determined by age of the
user of the display device, the control parameter P can be
determined by setting a tendency of kinds of contents displayed
very often depending on age in advance.
[0453] In addition, when the control parameter P is determined by
age of the user of the display device, luminance of a backlight can
be set suitably by age of the user of the display device in order
to reduce burden on eyes of the user. At this time, luminance of
the backlight may be controlled by controlling the lighting ratio
R. Thus, burden on eyes can be reduced and motion blur can be
reduced.
[0454] Further, all the methods for determining the control
parameter P, which are described in this embodiment mode, may be
means which can be set by the user of the display device.
[0455] Although this embodiment mode is described with reference to
various drawings, the contents (or may be part of the contents)
described in each drawing can be freely applied to, combined with,
or replaced with the contents (or may be part of the contents)
described in another drawing. Further, even more drawings can be
formed by combining each part with another part in the
above-described drawings.
[0456] The contents (or may be part of the contents) described in
each drawing of this embodiment mode can be freely applied to,
combined with, or replaced with the contents (or may be part of the
contents) described in a drawing in another embodiment mode.
Further, even more drawings can be formed by combining each part
with part of another embodiment mode in the drawings of this
embodiment mode.
[0457] This embodiment mode shows an example of an embodied case of
the contents (or may be part of the contents) described in other
embodiment modes, an example of slight transformation thereof, an
example of partial modification thereof, an example of improvement
thereof, an example of detailed description thereof, an application
example thereof, an example of related part thereof, or the like.
Therefore, the contents described in other embodiment modes can be
freely applied to, combined with, or replaced with this embodiment
mode.
Embodiment Mode 4
[0458] In this embodiment mode, a method for increasing response
speed of a display element when a display element in which response
to a signal input (response speed) is low, such as a liquid crystal
element and an electrophoretic element, is used as a display
element provided in a display device is described. In particular, a
method effective for the case where a lighting ratio R is changed
is described.
[0459] There are various kinds of methods for a display element
using a liquid crystal element. A method which is most widely used
is a method in which a liquid crystal element is controlled by
analog voltage, such as a TN mode, a VA mode, and an IPS mode. In
these methods, response time (also referred to as response speed)
of a liquid crystal element is several to several tens of ms. One
frame period in the NTSC system is 16.7 ms, and response time of a
liquid crystal element in these modes is often longer than one
frame period. Since one of causes of motion blur is that response
time of a display element is longer than one frame period, the
response time of the display element is preferably at least shorter
than one frame period. Accordingly, for a display element using a
liquid crystal element, a method is used in which voltage V.sub.OD
(voltage providing desired transmittance at or around the time when
predetermined time has passed) which is different from original
voltage V.sub.S (voltage providing desired transmittance after
enough time passes) is applied to the liquid crystal element in
order to reduce response time of the liquid crystal element. This
method is referred to as overdrive in this document. Note that the
voltage V.sub.OD is referred to as overdrive voltage.
[0460] Here, in at least one of driving methods of a display device
according to this document, a blanking interval is provided in one
frame period. Accordingly, response time of a display element is
preferably shorter than an image display period .tau..sub.a and a
blanking interval .tau..sub.b. Note that when a liquid crystal
element or the like is used as the display element, response time
is not always shorter than the image display period .tau..sub.a and
the blanking interval .tau..sub.b. In this embodiment mode, a
method is described in which response time of a liquid crystal
element or the like in accordance with the length of the image
display period .tau..sub.a and the blanking interval .tau..sub.b is
obtained by using overdrive.
[0461] In this document, there are several methods of providing the
blanking interval .tau..sub.b (methods of controlling the lighting
ratio R) as described above, that is, (1) the method where blanking
data is directly written to each pixel, (2) the method where the
whole backlight blinks, and (3) the method where a backlight
divided into areas sequentially blinks. First, in (1) the method
where blanking data is directly written to each pixel, a method
where response time of a liquid crystal element or the like in
accordance with the length of the image display period .tau..sub.a
and the blanking interval .tau..sub.b is obtained is described with
reference to FIGS. 20A to 20C.
[0462] The length of the blanking interval .tau..sub.b can be
changed as appropriate in order to directly write blanking data to
each pixel, as shown in Embodiment Modes 1 and 2. Further, when the
length of the blanking interval .tau..sub.b is changed in
accordance with the control parameters P and Q described in
Embodiment Mode 3, driving in accordance with a state of an image
and an environment can be realized. For example, in the case such
that movement of an object displayed in an image is large or where
luminance difference between a background and an object displayed
in an image is large, motion blur is likely to be seen. Motion blur
can be reduced by increasing the length of the blanking interval
.tau..sub.b. In addition, in the case such that movement of an
object displayed in an image is small or where luminance difference
between a background and an object displayed in an image is small,
motion blur is not likely to be seen. A flicker can be reduced by
reducing the length of the blanking interval .tau..sub.b.
[0463] In each graph shown in FIGS. 20A to 20C, a horizontal axis
represents time, and a vertical axis represents voltage and
transmittance of a liquid crystal element. Voltage is shown by a
solid line, and transmittance is shown by a dashed line. Note that
in this embodiment mode, the voltage refers to voltage in the case
of a positive signal when voltage of a counter electrode is 0 V. In
the case of a negative signal, polarity of voltage is inverted.
Therefore, the voltage in the graph may be considered as an
absolute value of voltage applied to the liquid crystal element. A
range of time used for description is a first frame period and a
second frame period. That is, the graphs shown in FIGS. 20A to 20C
show change in voltage and transmittance over time in a range of
two frame periods.
[0464] A value of the voltage applied to the liquid crystal element
is described. Voltage V.sub.S1 and voltage V.sub.S2 are original
voltages which should be applied in the first frame period and the
second frame period, respectively. Note that the voltages V.sub.S1
and V.sub.S2 have the same value in all graphs in FIGS. 20A to 20C.
Voltages V.sub.OD2001 and V.sub.OD2002, voltages V.sub.OD2011 and
V.sub.OD2012, and voltages V.sub.OD2021 and V.sub.OD2022 are
overdrive voltages in the first frame period and the second frame
period, respectively. The overdrive voltages are preferably
different from each other in the graphs shown in FIGS. 20A to 20C.
Note that in a frame period before the first frame period, voltage
applied to the liquid crystal element in an image display period
and voltage applied in a blanking interval are determined as
appropriate, and they are equal, for example.
[0465] Overdrive intensity refers to difference (an absolute value)
between overdrive voltage and original voltage. First overdrive
intensity refers to overdrive intensity in the first frame period.
Second overdrive intensity refers to overdrive intensity in the
second frame period.
[0466] First, with reference to FIG. 20A, a relation between
voltage applied to the liquid crystal element and transmittance in
each frame period is described in the case where values of the
image display period .tau..sub.a and the blanking interval
.tau..sub.b are the same, that is, in the case where
.tau..sub.a=.tau..sub.b=F/2 is satisfied.
[0467] In the image display period in the first frame period, the
overdrive voltage V.sub.OD2001 is applied at or around the end of
the image display period in the first frame period so that
transmittance of the liquid crystal element becomes transmittance
Ta.sub.2001 corresponding to the original voltage V.sub.S1. Thus,
the transmittance of the liquid crystal element becomes the
transmittance Ta.sub.2001 at or around the end of the image display
period in the first frame period. At this time, the first overdrive
intensity is V.sub.2001, and V.sub.2001=V.sub.OD2001-V.sub.S1 is
satisfied.
[0468] In the blanking interval in the first frame period, it is
preferable that the transmittance of the liquid crystal element
become transmittance for providing luminance in the blanking
interval, at or around the end of the blanking interval in the
first frame period at the latest. However, because of
characteristics of the liquid crystal element, it is difficult to
apply overdrive voltage in a shorter time, which is for reaching
transmittance corresponding to voltage applied to the liquid
crystal element of approximately 0 V. Thus, at or around the end of
the blanking interval in the first frame period, the transmittance
of the liquid crystal element is not necessary to be transmittance
for providing the luminance in the blanking interval. Instead, it
is preferable that transmittance Tb.sub.2001 at the end of the
blanking interval in the first frame period can be estimated from
the transmittance Ta.sub.2001 at the end of the image display
period in the first frame period and the length of the blanking
interval .tau..sub.b, which can be estimated from a plurality of
experiments performed in advance. When the data is stored in a
memory such as a lookup table, the data can be utilized for
adjusting a value of voltage applied to the liquid crystal
element.
[0469] In the image display period in the second frame period, the
overdrive voltage V.sub.OD2002 is applied at or around the end of
the image display period in the second frame period so that the
transmittance of the liquid crystal element becomes transmittance
Ta.sub.2002 corresponding to the original voltage V.sub.S2. Thus,
the transmittance of the liquid crystal element becomes the
transmittance Ta.sub.2002 at or around the end of the image display
period in the second frame period. At this time, the second
overdrive intensity is V.sub.2002, and
V.sub.2002=V.sub.OD2002-V.sub.S2 is satisfied.
[0470] The image display period in the first frame period is
different from the image display period in the second frame period
in the following ways: the transmittance of the liquid crystal
element is the transmittance for providing the luminance in the
blanking interval at or around the start of the image display
period in the first frame period, whereas the transmittance of the
liquid crystal element is not always the transmittance for
providing the luminance in the blanking interval at or around the
start of the image display period in the second frame period. In
that case, the transmittance Ta.sub.2002 depends on not only the
voltage V.sub.OD2002 applied in the image display period in the
second frame period but also the transmittance Tb.sub.2001 at the
end of the blanking interval in the first frame period, so that
appropriate transmittance cannot be obtained.
[0471] In this case, it is extremely useful in the first frame
period that the transmittance Tb.sub.2001 at the end of the
blanking interval in the first frame period can be estimated from
the transmittance Ta.sub.2001 at the end of the image display
period in the first frame period and the length of the blanking
interval .tau..sub.b. This is because even when the transmittance
of the liquid crystal element is not the transmittance for
providing the luminance in the blanking interval at or around the
start of the image display period in the second frame period, the
transmittance Tb.sub.2001 at that time is estimated; thus, the
voltage V.sub.OD2002 applied in the image display period in the
second frame period can be adjusted in accordance with the level of
the transmittance Tb.sub.2001.
[0472] In the blanking interval in the second frame period, similar
to the blanking interval in the first frame period, it is
preferable that transmittance Tb.sub.2002 at the end of the
blanking interval in the second frame period can be estimated from
the transmittance Ta.sub.2002 at the end of the image display
period in the second frame period and the length of the blanking
interval .tau..sub.b. Accordingly, desired transmittance can be
accurately obtained also at the end of an image display period in a
frame period next to the second frame period.
[0473] The length of the blanking interval .tau..sub.b can be
changed as appropriate in accordance with the control parameters P
and Q described in Embodiment Mode 3. With reference to FIG. 20B, a
relation between voltage applied to the liquid crystal element and
transmittance in each frame period is described in the case where
the image display period .tau..sub.a is longer than the blanking
interval .tau..sub.b, that is, in the case where
.tau..sub.a>.tau..sub.b is satisfied.
[0474] In the image display period in the first frame period, the
overdrive voltage V.sub.OD2011 is applied at or around the end of
the image display period in the first frame period so that the
transmittance of the liquid crystal element becomes transmittance
Ta.sub.2011 corresponding to the original voltage V.sub.S1. Thus,
the transmittance of the liquid crystal element becomes the
transmittance Ta.sub.2011 at or around the end of the image display
period in the first frame period. At this time, the first overdrive
intensity is V.sub.2011, and V.sub.2011=V.sub.OD2011-V.sub.S1 is
satisfied.
[0475] In the driving method of a display device according to this
document, it is extremely useful that the first overdrive intensity
V.sub.2001 in the case where .tau..sub.a=.tau..sub.b=F/2 is
satisfied shown in FIG. 20A and the first overdrive intensity
V.sub.2011 in the case where .tau..sub.a>.tau..sub.b is
satisfied shown in FIG. 20B are different and
V.sub.2001>V.sub.2011 is satisfied. This is because the image
display period .tau..sub.a is longer in the case where
.tau..sub.a>.tau..sub.b is satisfied, so that a longer period of
time can be allowed to reach desired transmittance. Accordingly,
desired transmittance can be accurately obtained by applying
overdrive voltage which varies depending on the lighting ratio R
even with the same original voltage V.sub.S1. Note that increase in
length of the image display period .tau..sub.a or reduction in
length of the blanking interval .tau..sub.b is preferably
determined in accordance with the control parameters P and Q
described in Embodiment Mode 3. This is because when it is
estimated by the control parameters P and Q that motion blur is not
likely to be seen from a state of an image (e.g., the case where
movement of an object displayed in the image is small or the case
where luminance difference between a background and an object
displayed in the image is small) and an environment, driving by
which a flicker or the like can be reduced by reducing the length
of the blanking interval .tau..sub.b can be realized.
[0476] In the blanking interval in the first frame period, it is
preferable that the transmittance of the liquid crystal element
become transmittance for providing luminance in the blanking
interval, at the end of the blanking interval in the first frame
period at the latest or at the time close thereto. However, because
of characteristics of the liquid crystal element, it is difficult
to apply overdrive voltage in a shorter time, which is for reaching
transmittance corresponding to voltage applied to the liquid
crystal element of approximately 0 V. Thus, at or around the end of
the blanking interval in the first frame period, the transmittance
of the liquid crystal element is not necessary to be transmittance
for providing the luminance in the blanking interval. Instead, it
is preferable that transmittance Tb.sub.2011 at the end of the
blanking interval in the first frame period can be estimated from
the transmittance Ta.sub.2011 at the end of the image display
period in the first frame period and the length of the blanking
interval .tau..sub.b, which can be estimated from a plurality of
experiments performed in advance. When the data is stored in a
memory such as a lookup table, the data can be utilized for
adjusting a value of voltage applied to the liquid crystal
element.
[0477] Note that as shown in FIG. 20B, the blanking interval is
further reduced in the case where .tau..sub.a>.tau..sub.b is
satisfied, so that difference between the transmittance Tb.sub.2011
at the end of the blanking interval in the first frame period and
transmittance providing the luminance in the blanking interval is
further increased. Accordingly, it is very important that the
transmittance Tb.sub.2011 at the end of the blanking interval in
the first frame period can be estimated.
[0478] In the image display period in the second frame period, the
overdrive voltage V.sub.OD2012 is applied at or around the end of
the image display period in the second frame period so that the
transmittance of the liquid crystal element becomes transmittance
Ta.sub.2012 corresponding to the original voltage V.sub.S2. Thus,
the transmittance of the liquid crystal element becomes the
transmittance Ta.sub.2012 at or around the end of the image display
period in the second frame period. At this time, the second
overdrive intensity is V.sub.2012, and
V.sub.2012=V.sub.OD2012-V.sub.S2 is satisfied.
[0479] The image display period in the first frame period is
different from the image display period in the second frame period
in the following ways: the transmittance of the liquid crystal
element is the transmittance for providing the luminance in the
blanking interval at or around the start of the image display
period in the first frame period, whereas the transmittance of the
liquid crystal element is not always the transmittance for
providing the luminance in the blanking interval at or around the
start of the image display period in the second frame period. In
that case, the transmittance Ta.sub.2012 depends on not only the
voltage V.sub.OD2012 applied in the image display period in the
second frame period but also the transmittance Tb.sub.2011 at the
end of the blanking interval in the first frame period, so that
appropriate transmittance cannot be obtained.
[0480] In this case, it is extremely useful in the first frame
period that the transmittance Tb.sub.2011 at the end of the
blanking interval in the first frame period can be estimated from
the transmittance Ta.sub.2011 at the end of the image display
period in the first frame period and the length of the blanking
interval .tau..sub.b. This is because even when the transmittance
of the liquid crystal element is not the transmittance for
providing the luminance in the blanking interval at or around the
start of the image display period in the second frame period, the
transmittance Tb.sub.2011 at that time is estimated; thus, the
voltage V.sub.OD2012 applied in the image display period in the
second frame period can be adjusted in accordance with the level of
the transmittance Tb.sub.2011.
[0481] In the driving method of a display device according to this
document, it is extremely useful that the second overdrive
intensity V.sub.2002 in the case where .tau..sub.a=.tau..sub.b=F/2
is satisfied shown in FIG. 20A and the second overdrive intensity
V.sub.2012 in the case where .tau..sub.a>.tau..sub.b is
satisfied shown in FIG. 20B are different and
V.sub.2002>V.sub.2012 is satisfied. This is because the image
display period .tau..sub.a is longer in the case where
.tau..sub.a>.tau..sub.b is satisfied, so that a longer period of
time can be allowed to reach desired transmittance. Accordingly,
desired transmittance can be accurately obtained by applying
overdrive voltage which varies depending on the lighting ratio R
even with the same original voltage V.sub.S2. Note that increase in
length of the image display period .tau..sub.a or reduction in
length of the blanking interval .tau..sub.b is preferably
determined in accordance with the control parameters P and Q
described in Embodiment Mode 3. This is because when it is
estimated by the control parameters P and Q that motion blur is not
likely to be seen from a state of an image (e.g., the case where
movement of an object displayed in the image is small or the case
where luminance difference between a background and an object
displayed in the image is small) and an environment, driving by
which a flicker or the like can be reduced by reducing the length
of the blanking interval .tau..sub.b can be realized.
[0482] In addition, the second overdrive intensity V.sub.2012 is
preferably further reduced in the case where
.tau..sub.a>.tau..sub.b is satisfied because the transmittance
Tb.sub.2011 at the start of the second frame period is larger than
the transmittance Tb.sub.2001 at the start of the second frame
period in the case where .tau..sub.a=.tau..sub.b is satisfied. That
is, the second overdrive intensity V.sub.2012 in the case where
.tau..sub.a>.tau..sub.b is satisfied is preferably smaller than
the second overdrive intensity V.sub.2002 in the case where
.tau..sub.a=.tau..sub.b is satisfied not only because of increase
in the image display period .tau..sub.a but also because of
increase in the transmittance Tb.sub.2011 at the start of the
second frame period.
[0483] In the blanking interval in the second frame period, similar
to the blanking interval in the first frame period, it is
preferable that transmittance Tb.sub.2012 at the end of the
blanking interval in the second frame period can be estimated from
the transmittance Ta.sub.2012 at the end of the image display
period in the second frame period and the length of the blanking
interval .tau..sub.b. Accordingly, desired transmittance can be
accurately obtained also at the end of an image display period in a
frame period next to the second frame period.
[0484] Next, with reference to FIG. 20C, a relation between voltage
applied to the liquid crystal element and transmittance in each
frame period is described in the case where the image display
period .tau..sub.a is shorter than the blanking interval
.tau..sub.b, that is, in the case where .tau..sub.a<.tau..sub.b
is satisfied.
[0485] In the image display period in the first frame period, the
overdrive voltage V.sub.OD2021 is applied at or around the end of
the image display period in the first frame period so that the
transmittance of the liquid crystal element becomes transmittance
Ta.sub.2021 corresponding to the original voltage V.sub.S1. Thus,
the transmittance of the liquid crystal element becomes the
transmittance Ta.sub.2021 at or around the end of the image display
period in the first frame period. At this time, the first overdrive
intensity is V.sub.2021, and V.sub.2021=V.sub.OD2021-V.sub.S1 is
satisfied.
[0486] In the driving method of a display device according to this
document, it is extremely useful that the first overdrive intensity
V.sub.2001 in the case where .tau..sub.a=.tau..sub.b=F/2 is
satisfied shown in FIG. 20A and the first overdrive intensity
V.sub.2021 in the case where .tau..sub.a<.tau..sub.b is
satisfied shown in FIG. 20C are different and
V.sub.2001<V.sub.2021 is satisfied. This is because the image
display period .tau..sub.a is shorter in the case where
.tau..sub.a<.tau..sub.b is satisfied, so that a shorter period
of time needs to be allowed to reach desired transmittance.
Accordingly, desired transmittance can be accurately obtained by
applying overdrive voltage which varies depending on the lighting
ratio R even with the same original voltage V.sub.S1. Note that
reduction in length of the image display period .tau..sub.a or
increase in length of the blanking interval .tau..sub.b is
preferably determined in accordance with the control parameters P
and Q described in Embodiment Mode 3. This is because when it is
estimated by the control parameters P and Q that motion blur is
likely to be seen from a state of an image (e.g., the case where
movement of an object displayed in the image is large or the case
where luminance difference between a background and an object
displayed in the image is large) and an environment, driving by
which motion blur can be reduced by increasing the length of the
blanking interval .tau..sub.b can be realized.
[0487] In the blanking interval in the first frame period, it is
preferable that the transmittance of the liquid crystal element
become transmittance for providing luminance in the blanking
interval, at the end of the blanking interval in the first frame
period at the latest or at the time close thereto. However, because
of characteristics of the liquid crystal element, it is difficult
to apply overdrive voltage in a shorter time, which is for reaching
transmittance corresponding to voltage applied to the liquid
crystal element of approximately 0 V. Thus, at or around the end of
the blanking interval in the first frame period, the transmittance
of the liquid crystal element is not necessary to be transmittance
for providing the luminance in the blanking interval. Instead, it
is preferable that transmittance Tb.sub.2021 at the end of the
blanking interval in the first frame period can be estimated from
the transmittance Ta.sub.2021 at the end of the image display
period in the first frame period and the length of the blanking
interval .tau..sub.b.
[0488] Note that as shown in FIG. 20C, the blanking interval is
further increased in the case where .tau..sub.a<.tau..sub.b is
satisfied, so that difference between the transmittance Tb.sub.2021
at the end of the blanking interval in the first frame period and
transmittance providing the luminance in the blanking interval is
reduced. Accordingly, the transmittance Tb.sub.2021 at the end of
the blanking interval in the first frame period may be estimated or
the estimate may be omitted.
[0489] In the image display period in the second frame period, the
overdrive voltage V.sub.OD2022 is applied at or around the end of
the image display period in the second frame period so that the
transmittance of the liquid crystal element becomes transmittance
Ta.sub.2022 corresponding to the original voltage V.sub.S2. Thus,
the transmittance of the liquid crystal element becomes the
transmittance Ta.sub.2022 at or around the end of the image display
period in the second frame period. At this time, the second
overdrive intensity is V.sub.2022, and
V.sub.2022=V.sub.OD2022-V.sub.S2 is satisfied.
[0490] The image display period in the first frame period is
different from the image display period in the second frame period
in the following ways: the transmittance of the liquid crystal
element is the transmittance for providing the luminance in the
blanking interval at or around the start of the image display
period in the first frame period, whereas the transmittance of the
liquid crystal element is not always the transmittance for
providing the luminance in the blanking interval at or around the
start of the image display period in the second frame period. In
that case, the transmittance Ta.sub.2022 depends on not only the
voltage V.sub.OD2022 applied in the image display period in the
second frame period but also the transmittance Tb.sub.2021 at the
end of the blanking interval in the first frame period, so that
appropriate transmittance cannot be obtained.
[0491] In this case, in the first frame period, the transmittance
Tb.sub.2021 at the end of the blanking interval in the first frame
period may be estimated from the transmittance Ta.sub.2021 at the
end of the image display period in the first frame period and the
length of the blanking interval .tau..sub.b. This is because even
when the transmittance of the liquid crystal element is not the
transmittance for providing the luminance in the blanking interval
at or around the start of the image display period in the second
frame period, the transmittance Tb.sub.2021 at that time is
estimated; thus, the voltage V.sub.OD2022 applied in the image
display period in the second frame period can be adjusted in
accordance with the level of the transmittance Tb.sub.2021.
[0492] In the driving method of a display device according to this
document, it is extremely useful that the second overdrive
intensity V.sub.2002 in the case where .tau..sub.a=.tau..sub.b=F/2
is satisfied shown in FIG. 20A and the second overdrive intensity
V.sub.2022 in the case where .tau..sub.a<.tau..sub.b is
satisfied shown in FIG. 20C are different and
V.sub.2002<V.sub.2022 is satisfied. This is because the image
display period .tau..sub.a is shorter in the case where
.tau..sub.a<.tau..sub.b is satisfied, so that a shorter period
of time needs to be allowed to reach desired transmittance.
Accordingly, desired transmittance can be accurately obtained by
applying overdrive voltage which varies depending on the lighting
ratio R even with the same original voltage V.sub.S2. Note that
reduction in length of the image display period .tau..sub.a or
increase in length of the blanking interval .tau..sub.b is
preferably determined in accordance with the control parameters P
and Q described in Embodiment Mode 3. This is because when it is
estimated by the control parameters P and Q that motion blur is
likely to be seen from a state of an image (e.g., the case where
movement of an object displayed in the image is large or the case
where luminance difference between a background and an object
displayed in the image is large) and an environment, driving by
which motion blur can be reduced by increasing the length of the
blanking interval .tau..sub.b can be realized.
[0493] In addition, the second overdrive intensity V.sub.2022 is
preferably further increased in the case where
.tau..sub.a<.tau..sub.b is satisfied because the transmittance
Tb.sub.2021 at the start of the second frame period is smaller than
the transmittance Tb.sub.2001 at the start of the second frame
period in the case where .tau..sub.a=.tau..sub.b is satisfied. That
is, the second overdrive intensity V.sub.2022 in the case where
.tau..sub.a<.tau..sub.b is satisfied is preferably larger than
the second overdrive intensity V.sub.2002 in the case where
.tau..sub.a=.tau..sub.b is satisfied not only because of reduction
in the image display period .tau..sub.a but also because of
reduction in the transmittance Tb.sub.202l at the start of the
second frame period.
[0494] In the blanking interval in the second frame period, similar
to the blanking interval in the first frame period, it is
preferable that transmittance Tb.sub.2022 at the end of the
blanking interval in the second frame period can be estimated from
the transmittance Ta.sub.2022 at the end of the image display
period in the second frame period and the length of the blanking
interval .tau..sub.b. Accordingly, desired transmittance can be
accurately obtained also at the end of an image display period in a
frame period next to the second frame period.
[0495] Note that in the case where .tau..sub.a<.tau..sub.b is
satisfied, difference between the transmittance Tb.sub.2022 at the
end of the blanking interval in the second frame period and
transmittance providing the luminance in the blanking interval is
smaller. Accordingly, the transmittance Tb.sub.2022 at the end of
the blanking interval in the second frame period may be estimated
or the estimate may be omitted.
[0496] In the method where blanking data is directly written to
each pixel, backlight luminance may be changed. For example, when
the level of a data signal written to a pixel is the same,
luminance which human eyes perceive becomes lower as the image
display period .tau..sub.a becomes shorter and the blanking
interval .tau..sub.b becomes longer. Accordingly, in accordance
with the length of the image display period .tau..sub.a and the
length of the blanking interval .tau..sub.b (i.e., the lighting
ratio R), the backlight luminance is reduced when the lighting
ratio R is high, whereas the backlight luminance is increased when
the lighting ratio R is low. Thus, luminance which human eyes
perceive can be constant. Further, the lighting ratio R preferably
depends on the control parameters P and Q described in Embodiment
Mode 3. This is because the lighting ratio R can be controlled as
appropriate by perceivability of motion blur in an image to be
displayed.
[0497] Next, in (2) the method where the whole backlight blinks
among the methods of controlling the lighting ratio R, a method
where response time of a liquid crystal element is increased is
described.
[0498] In (2) the method where the whole backlight blinks, a period
when data written to a pixel is updated is referred to as one frame
period. At this time, in the case where overdrive is used to
increase response speed of a liquid crystal element, the overdrive
voltage V.sub.OD is applied to the liquid crystal element so that
the liquid crystal element has desired transmittance at or around
the time when one frame period passes after voltage is applied to
the liquid crystal element.
[0499] However, in (2) the method where the whole backlight blinks,
timing when voltage is applied to the liquid crystal element in a
backlight lighting period varies depending on a scan position.
Accordingly, even when the same overdrive voltage V.sub.OD is
applied to the liquid crystal element, luminance varies depending
on a position of a scan line to which the liquid crystal element is
connected. Accordingly, in (2) the method where the whole backlight
blinks, it is effective to determine the overdrive, voltage
V.sub.OD in consideration of this point. Further, luminance can be
corrected by correcting a gray scale to be displayed depending on a
position of a scan line, other than by a method of controlling the
overdrive voltage V.sub.OD.
[0500] This is described with reference to FIGS. 21A to 21F. FIG.
21A is a graph showing timing of writing data and timing of
blinking the whole backlight on the same time axis with respect to
a position of a scan line.
[0501] In the method shown in FIG. 21A, at or around the start of
one frame period, data writing starts sequentially from a pixel
connected to a scan line in the first row. Then, writing to pixels
connected to all scan lines ends at or around the time when a half
of one frame period passes. Then, the backlight is lit when writing
to the pixels connected to all the scan lines ends or at the time
close thereto, and the backlight is turned off when one frame
period ends or at the time close thereto.
[0502] FIG. 21B is a graph showing change in voltage applied to the
liquid crystal element and transmittance in the pixel connected to
the scan line in the first row (a position described as (B) in FIG.
21A). Note that a time axis of the graph of FIG. 21B corresponds to
that of the graph of FIG. 21A. Voltage V.sub.OD2101 (original
voltage V.sub.S2101) is applied in a first frame period, and the
voltage V.sub.S2101 is applied in a second frame period.
[0503] In the first frame period, the transmittance in the graph of
FIG. 21B gradually changes from the time when data is written, and
the transmittance becomes desired transmittance when one frame
period passes or at the time close thereto. At this time, the
backlight lighting period starts before change in transmittance
ends and the backlight lighting period ends when change in
transmittance ends. Here, luminance which human eyes perceive in
the first frame period depends on the area of a portion L.sub.2101
shown by oblique lines in the first frame period.
[0504] In the second frame period, the transmittance in the graph
of FIG. 21B is already desired transmittance before data is
written. At this time, the transmittance dose not change in the
backlight lighting period. Luminance which human eyes perceive in
the second frame period depends on the area of a portion L.sub.2102
shown by oblique lines in the second frame period.
[0505] Desired luminance for display is the same in the first frame
period and the second frame period. However, the area of the
oblique line portion L.sub.2101 and the area of the oblique line
portion L.sub.2102 are different from each other, so that luminance
which human eyes perceive is different in the first frame period
and the second frame period.
[0506] In (2) the method where the whole backlight blinks, the
original voltage V.sub.S2101 in the first frame period may be
changed to correct luminance difference between frames. That is,
luminance difference between frames can be corrected by correcting
gray scale data itself to be written to each pixel. Luminance
difference between frames which may cause color shading in
displaying a moving image and motion blur can be reduced by the
method according to this document.
[0507] As a method for correcting data, a method shown in FIG. 23A
can be used, for example. In the method shown in FIG. 23A, the
original voltage in the first frame period is corrected from
V.sub.S2302 to V.sub.S2301 in order that the area of oblique line
regions L.sub.2301 and L.sub.2302, which represent luminance in the
first frame period and the second frame period, are the same. At
this time, as overdrive voltage V.sub.OD2301 written to each pixel,
voltage calculated from the original voltage V.sub.S2301 after
correction by using a normal method can be used. By the original
voltage V.sub.S2301 after correction, the area of oblique line
region L.sub.2301 and the area of oblique line region L.sub.2302
are corrected to be the same. That is, the original voltage
V.sub.S2301 is determined so that the area of two regions
L.sub.2301a and L.sub.2301b which are surrounded by an actual
transmittance curve changed by the overdrive voltage V.sub.OD2301
and a straight line representing transmittance in saturation when
the original voltage V.sub.S2302 is applied have approximately the
same area. Note that it is preferable to correct gray scale data so
that luminance of a pixel connected to a scan line in which timing
of writing is lower becomes higher. That is, it is preferable to
increase the amount of correction of the gray scale data gradually
in accordance with sequential scanning so that luminance of a pixel
connected to a scan line in the last row is the highest.
[0508] Description is made with reference to the graph of FIG. 21B
again. In (2) the method where the whole backlight blinks, the
overdrive voltage V.sub.OD2101 in the first frame period may be
changed in order to correct luminance difference between frames. In
general, overdrive voltage is only for making transmittance when
next writing starts in a pixel closer to desired transmittance. In
the method according to this document, overdrive voltage can also
be used for correcting luminance difference between frames.
Luminance difference between frames which may cause color shading
in displaying a moving image and motion blur can be reduced by the
method according to this document.
[0509] As a method for correcting overdrive voltage, a method shown
in FIG. 23C can be used, for example. In the method shown in FIG.
23C, the overdrive voltage is corrected to V.sub.OD2321 in order
that the area of oblique line regions L.sub.2321 and L.sub.2322,
which represent luminance in the first frame period and the second
frame period, are the same. At this time, as the overdrive voltage
V.sub.OD2321, voltage obtained from a special lookup table
considering correction can be used. By the overdrive voltage
V.sub.OD2321 after correction, the area of oblique line region
L.sub.2321 and the area of oblique line region L.sub.2322 are
corrected to be the same. That is, the overdrive voltage
V.sub.OD2321 is determined so that the area of two regions
L.sub.4321a and L.sub.2321b which are surrounded by an actual
transmittance curve changed by the overdrive voltage V.sub.OD2321
and a straight line representing transmittance in saturation when
original voltage V.sub.S2321 is applied have approximately the same
area. Note that it is preferable to correct overdrive voltage so
that luminance of a pixel connected to a scan line in which timing
of writing is lower becomes higher. That is, it is preferable to
increase the amount of correction of the overdrive voltage
gradually in accordance with sequential scanning so that luminance
of a pixel connected to a scan line in the last row is the
highest.
[0510] Next, change in voltage applied to a liquid crystal element
and transmittance in a pixel connected to the scan line near the
center (a position described as (C) in FIG. 21A) is described with
reference to the graph shown in FIG. 21C. Note that a time axis of
the graph of FIG. 21C corresponds to that of the graph of FIG. 21A.
Voltage V.sub.OD2111 (original voltage V.sub.S2111) is applied in a
first frame period, and the voltage V.sub.S2111 is applied in a
second frame period
[0511] In the first frame period, the transmittance in the graph of
FIG. 21C gradually changes from the time when data is written, and
the transmittance becomes desired transmittance when one frame
period passes or at the time close thereto. At this time, the
backlight lighting period starts before change in transmittance
ends and the backlight lighting period ends before change in
transmittance ends. Here, luminance which human eyes perceive in
the first frame period depends on the area of a portion L.sub.2111
shown by oblique lines in the first frame period.
[0512] Here, since timing when writing starts is different
depending on a position of the scan line, it should be noted that
the area of the oblique line portion L.sub.2111 in the first frame
period is different from the area of an oblique line portion in the
first frame period in another scan line. This is why luminance
varies depending on a position of a scan line to which the liquid
crystal element is connected even when the same overdrive voltage
V.sub.OD is applied to the liquid crystal element.
[0513] Variation in luminance depending on a scan position is
perceived as luminance unevenness in a display portion as it is, so
that it is a significant image defect and should be improved with
priority. Accordingly, in (2) the method where the whole backlight
blinks, the original voltage V.sub.S2111 in the first frame period
may be changed in order to correct luminance difference depending
on a scan position. That is, luminance difference depending on a
scan position can be corrected by correcting gray scale data itself
to be written to each pixel.
[0514] As a method for correcting data, a method shown in FIG. 23B
can be used, for example. In the method shown in FIG. 23B, the
original voltage in the first frame period is corrected from
V.sub.S2302 to V.sub.S2311 in order that the area of oblique line
regions L.sub.2311 and L.sub.2312, which represent luminance in the
first frame period and the second frame period, are the same and
each integrated luminance of pixels connected to a different scan
line in the same frame period is the same. At this time, as
overdrive voltage V.sub.OD2311 written to each pixel, voltage
calculated from the original voltage V.sub.S2311 after correction
by using a normal method can be used. By the original voltage
V.sub.S2311 after correction, the area of oblique line region
L.sub.2311 and the area of oblique line region L.sub.2312 are
corrected to be the same. That is, the original voltage V.sub.S2311
is determined so that the areas of two regions L.sub.311a and
L.sub.3111b which are surrounded by an actual transmittance curve
changed by the overdrive voltage V.sub.OD2311 and a straight line
representing transmittance in saturation when the original voltage
V.sub.S2302 is applied are approximately the same. Further, in
order to prevent increase in area of the region shown by the
oblique lines in the second frame period as the overdrive voltage
V.sub.OD2311 in the first frame period increases, the original
voltage in the second frame period may also be corrected in a
similar manner. At this time, corrected original voltage is
V.sub.S2312, and overdrive voltage obtained from the corrected
original voltage V.sub.S2312 is V.sub.OD2312. In the second frame
period also, the original voltage V.sub.S2312 is determined so that
the areas of two regions L.sub.2311a and L.sub.2311b are
approximately the same, similarly in the first frame period. Note
that it is preferable to correct gray scale data so that luminance
of a pixel connected to a scan line in which timing of writing is
lower becomes higher. That is, it is preferable to increase the
amount of correction of the gray scale data gradually in accordance
with sequential scanning so that luminance of a pixel connected to
a scan line in the last row is the highest.
[0515] Description is made with reference to the graph of FIG. 21C
again. In (2) the method where the whole backlight blinks, the
overdrive voltage V.sub.OD2111 in the first frame period may be
changed in order to correct luminance difference depending on a
scan position. In general, overdrive voltage is only for making
transmittance at the start of next writing in a pixel closer to
desired transmittance. In the method according to this document,
overdrive voltage can also be used for correcting luminance
difference depending on a scan position. Accordingly, luminance
difference depending on a scan position can be corrected by
correcting overdrive voltage for a gray scale to be written to each
pixel.
[0516] As a method for correcting overdrive voltage, a method shown
in FIG. 23D can be used, for example. In the method shown in FIG.
23D, the overdrive voltage in the first frame period is corrected
to V.sub.OD2331 in order that the area of oblique line regions
L.sub.2331 and L.sub.2332, which represent luminance in the first
frame period and the second frame period, are the same and each
integrated luminance of pixels connected to a different r scan line
in the same frame period is the same. At this time, as the
overdrive voltage V.sub.OD2331 which is written to each pixel,
voltage obtained from a special lookup table considering correction
can be used. By the overdrive voltage V.sub.OD2331 after
correction, the area of oblique line region L.sub.2331 and the area
of oblique line region L.sub.2332 are corrected to be the same.
That is, the overdrive voltage V.sub.OD2331 is determined so that
the areas of two regions L.sub.2331a and L.sub.2331b which are
surrounded by an actual transmittance curve changed by the
overdrive voltage V.sub.OD2331 and a straight line representing
transmittance in saturation when original voltage V.sub.S2331 is
applied are approximately the same. Further, in order to prevent
increase in area of the region shown by the oblique lines in the
second frame period as the overdrive voltage V.sub.OD2331 in the
first frame period increases, the original voltage in the second
frame period may also be corrected in a similar manner. At this
time, corrected overdrive voltage is V.sub.OD2332. In the second
frame period also, the overdrive voltage V.sub.OD2332 is determined
so that the areas of two regions L.sub.2331a and L.sub.2331b are
approximately the same, similarly in the first frame period. Note
that it is preferable to correct gray scale data so that luminance
of a pixel connected to a scan line in which timing of writing is
lower becomes higher. That is, it is preferable to increase the
amount of correction of the gray scale data gradually in accordance
with sequential scanning so that luminance of a pixel connected to
a scan line in the last row is the highest.
[0517] Description is made with reference to the graph of FIG. 21C
again. In the second frame period, the transmittance in the graph
of FIG. 21C is already desired transmittance before data is
written. At this time, the transmittance dose not change in the
backlight lighting period. Luminance which human eyes perceive in
the second frame period depends on the area of a portion L.sub.2112
shown by oblique lines in the second frame period.
[0518] Desired luminance for display is the same in the first frame
period and the second frame period. However, the area of the
oblique line portion L.sub.2111 and the area of the oblique line
portion L.sub.2112 are different from each other, so that luminance
which human eyes perceive is different in the first frame period
and the second frame period.
[0519] In (2) the method where the whole backlight blinks, the
original voltage V.sub.S2111 in the first frame period may be
changed to correct luminance difference between frames. That is,
luminance difference between frames can be corrected by correcting
gray scale data itself to be written to each pixel. Luminance
difference between frames which may cause color shading in
displaying a moving image and motion blur can be reduced by the
method according to this document. As a method for correcting data,
the method shown in FIG. 23B can be used.
[0520] In addition, in (2) the method where the whole backlight
blinks, the overdrive voltage V.sub.OD2111 in the first frame
period may be changed in order to correct luminance difference
between frames. In general, overdrive voltage is only for making
transmittance when next writing starts in a pixel closer to desired
transmittance. In the method according to this document, overdrive
voltage can also be used for correcting luminance difference
between frames. Luminance difference between frames which may cause
color shading in displaying a moving image and motion blur can be
reduced by the method according to this document. As a method for
correcting overdrive voltage, the method shown in FIG. 23D can be
used.
[0521] Next, change in voltage applied to a liquid crystal element
and transmittance in a pixel connected to the scan line at the
bottom (a position described as (D) in FIG. 21A) is described with
reference to a graph shown in FIG. 21D. Note that a time axis of
the graph of FIG. 21D corresponds to that of the graph of FIG. 21A.
Voltage V.sub.OD2121 (original voltage V.sub.S2121) is applied in a
first frame period, and the voltage V.sub.S2121 is applied in a
second frame period.
[0522] In the first frame period, the transmittance in the graph of
FIG. 21D gradually changes from the time when data is written, and
the transmittance becomes desired transmittance when one frame
period passes or at the time close thereto. At this time, the
backlight lighting period starts when change in transmittance
starts and the backlight lighting period ends long before change in
transmittance ends. Luminance which human eyes perceive in the
first frame period depends on the area of a portion L.sub.2121
shown by oblique lines in the first frame period.
[0523] Here, since timing when writing starts is different
depending on a position of the scan line, it should be noted that
the area of the oblique line portion L.sub.2111 in the first frame
period is different from the area of an oblique line portion in
another scan line. This is why luminance varies depending on a
position of a scan line to which the liquid crystal element is
connected even when the same overdrive voltage V.sub.OD is applied
to the liquid crystal element.
[0524] Variation in luminance depending on a scan position is
perceived as luminance unevenness in a display portion as it is, so
that it is a significant image defect and should be improved with
priority. Therefore, in (2) the method where the whole backlight
blinks, the original voltage V.sub.S2121 in the first frame period
may be changed in order to correct luminance difference depending
on a scan position. That is, luminance difference depending on a
scan position can be corrected by correcting gray scale data itself
to be written to each pixel. As a method for correcting data, the
method shown in FIG. 23B can be used.
[0525] In addition, in (2) the method where the whole backlight
blinks, the overdrive voltage V.sub.OD2121 in the first frame
period may be changed in order to correct luminance difference
depending on a scan position. In general, overdrive voltage is only
for making transmittance when next writing starts in a pixel closer
to desired transmittance. In the method according to this document,
overdrive voltage can also be used for correcting luminance
difference depending on a scan position. Accordingly, luminance
difference depending on a scan position can be corrected by
correcting overdrive voltage for a gray scale to be written to each
pixel. As a method for correcting overdrive voltage, the method
shown in FIG. 23D can be used.
[0526] Luminance difference depending on a scan position increases
as positions of scan lines are distant from each other.
Accordingly, both in a method of changing original voltage V.sub.S
and in a method of changing overdrive voltage V.sub.OD, it is
effective to increase the amount of change in voltage as the
positions of scan lines are distant from each other.
[0527] In the second frame period, the transmittance in the graph
of FIG. 21D is already desired transmittance before data is
written. At this time, the transmittance dose not change in the
backlight lighting period. Luminance which human eyes perceive in
the second frame period depends on the area of a portion L.sub.2122
shown by oblique lines in the second frame period.
[0528] Desired luminance for display is the same in the first frame
period and the second frame period. However, the area of the
oblique line portion L.sub.2121 and the area of the oblique line
portion L.sub.2122 are different from each other, so that luminance
which human eyes perceive is different in the first frame period
and the second frame period.
[0529] In (2) the method where the whole backlight blinks, the
original voltage V.sub.S2121 in the first frame period may be
changed to correct luminance difference between frames. That is,
luminance difference between frames can be corrected by correcting
gray scale data itself to be written to each pixel. Luminance
difference between frames which may cause color shading in
displaying a moving image and motion blur can be reduced by the
method according to this document. As a method for correcting data,
the method shown in FIG. 23B can be used.
[0530] In addition, in (2) the method where the whole backlight
blinks, the overdrive voltage V.sub.OD2121 in the first frame
period may be changed in order to correct luminance difference
between frames. In general, overdrive voltage is only for making
transmittance when next writing starts in a pixel closer to desired
transmittance. In the method according to this document, overdrive
voltage can also be used for correcting luminance difference
between frames. Luminance difference between frames which may cause
color shading in displaying a moving image and motion blur can be
reduced by the method according to this document. As a method for
correcting overdrive voltage, the method shown in FIG. 23D can be
used.
[0531] Luminance difference depending on a scan position increases
as positions of scan lines are distant from each other.
Accordingly, both in a method of changing the original voltage
V.sub.S and in a method of changing the overdrive voltage V.sub.OD,
it is effective to increase the amount of change in voltage as the
positions of scan lines are distant from each other.
[0532] Note that the method of changing the original voltage
V.sub.S can be realized by the flow of data processing shown in
FIG. 21E. First, a gray scale of data input is corrected by a gray
scale correction portion which corrects a gray scale depending on a
scan position. Thereafter, the corrected data is output to a pixel
as the overdrive voltage V.sub.OD by a lookup table (ODLUT) which
performs normal overdrive.
[0533] Note that the method of changing the overdrive voltage
V.sub.OD can be realized by the flow of data processing shown in
FIG. 21F. That is, data input is processed by a special lookup
table (ODLUT), which can also correct a gray scale depending on a
scan position at the same time, and thereafter, is output to a
pixel as the overdrive voltage V.sub.OD.
[0534] Next, in (3) the method where a backlight divided into areas
sequentially blinks among the methods of controlling the lighting
ratio R, a method where response time of a liquid crystal element
or the like is increased is described. Note that areas of a
backlight in this embodiment mode may be one-dimensionally or
two-dimensionally divided. When a backlight is one-dimensionally
divided, a linear light source such as a cold cathode fluorescent
lamp (CCFL) or a hot cathode fluorescent lamp (HSFL) can be used,
and the backlight can be arranged in parallel or perpendicular to a
scan line. When a backlight is two-dimensionally divided, a point
light source such as an LED or a sheet light source such as EL can
be used, and the light source can be arranged in matrix, honeycomb
arrangement, Bayer arrangement, or the like. Further, a structure
may be employed in which light sources for respective colors such
as RGB are provided and the backlight can be controlled for each
color.
[0535] In the (3) the method where a backlight divided into areas
sequentially blinks, a period when data written to a pixel is
updated is referred to as one frame period. At this time, in the
case where overdrive is used to increase response speed of a
liquid. crystal element, the overdrive voltage V.sub.OD is applied
to the liquid crystal element so that the liquid crystal element
has desired transmittance at or around the time when one frame
period passes after voltage is applied to the liquid crystal
element.
[0536] However, in (3) the method where a backlight divided into
areas sequentially blinks, timing when voltage is applied to the
liquid crystal element in a backlight lighting period in the areas
varies depending on a scan position. Accordingly, even when the
same overdrive voltage V.sub.OD is applied to the liquid crystal
element, luminance varies depending on a position of a scan line to
which the liquid crystal element is connected. Accordingly, in (3)
the method where a backlight divided into areas sequentially
blinks, it is effective to determine the overdrive voltage V.sub.OD
in consideration of this point. Further, luminance can be corrected
by correcting a gray scale to be displayed depending on a position
of a scan line, other than by the method of controlling the
overdrive voltage V.sub.OD.
[0537] Large difference between (3) the method where a backlight
divided into areas sequentially blinks and (2) the method where the
whole backlight blinks is whether there are a plurality of areas
with different luminance in a display portion. That is, in (3) the
method where a backlight divided into areas sequentially blinks,
pixels with different luminance are adjacent to each other at a
boundary between a certain area and an area adjacent thereto. Thus,
luminance difference in the display portion is extremely easily
perceived. That is, luminance difference depending on a scan
position in (3) the method where a backlight divided into areas
sequentially blinks causes more serious image quality degradation
than luminance difference depending on a scan position in (2) the
method where the whole backlight blinks. Accordingly, the method
according to this document in (3) the method where a backlight
divided into areas sequentially blinks is very effective in
improving image quality.
[0538] This is described with reference to FIGS. 22A to 22D. FIG.
22A is a graph showing timing of writing data with respect to a
position of a scan line and timing of sequentially blinking a
backlight divided into areas on the same time axis.
[0539] In a method shown in FIG. 22A, at or around the start of one
frame period, data writing starts sequentially from a pixel
connected to a scan line in the first row. Then, the top area of
the backlight is lit at or around the time when a half of one frame
period passes. Thereafter, the backlight in each area sequentially
starts lighting while the other pixels are sequentially scanned and
data is written to the pixels. Then, the top area of the backlight
is turned off when one frame period ends or at the time close
thereto. After that, the backlight in each area is sequentially
turned off while writing and scanning of next frame start and data
is written to pixels from the top.
[0540] FIG. 22B is a graph showing change in voltage applied to the
liquid crystal element and transmittance in the pixel connected to
the scan line in the first row (a position described as (B) in FIG.
22A). Note that a time axis of the graph of FIG. 22B corresponds to
that of the graph of FIG. 22A. Voltage V.sub.OD2201 (original
voltage V.sub.S2201) is applied in a first frame period, and the
voltage V.sub.S2201 is applied in a second frame period.
[0541] In the first frame period, the transmittance in the graph of
FIG. 22B gradually changes from the time when data is written, and
the transmittance becomes desired transmittance when one frame
period passes or at the time close thereto. At this time, the
backlight lighting period starts before change in transmittance
ends and the backlight lighting period ends when change in
transmittance ends. Here, luminance which human eyes perceive in
the first frame period depends on the area of a portion L.sub.2201
shown by oblique lines in the first frame period.
[0542] In the second frame period, the transmittance in the graph
of FIG. 22B is already desired transmittance before data is
written. At this time, the transmittance dose not change in the
backlight lighting period. Luminance which human eyes perceive in
the second frame period depends on the area of a portion L.sub.2202
shown by oblique lines in the second frame period.
[0543] Desired luminance for display is the same in the first frame
period and the second frame period. However, the area of the
oblique line portion L.sub.2201 and the area of the oblique line
portion L.sub.2202 are different from each other, so that luminance
which human eyes perceive is different in the first frame period
and the second frame period.
[0544] In (3) the method where a backlight divided into areas
sequentially blinks, the original voltage V.sub.S2201 in the first
frame period may be changed to correct luminance difference between
frames. That is, luminance difference between frames can be
corrected by correcting gray scale data itself to be written to
each pixel. Luminance difference between frames which may cause
color shading in displaying a moving image and motion blur can be
reduced by the method according to this document. As a method for
correcting data, the method shown in FIG. 23A can be used.
[0545] In addition, in (3) the method where a backlight divided
into areas sequentially blinks, the overdrive voltage V.sub.OD2201
in the first frame period may be changed in order to correct
luminance difference between frames. In general, overdrive voltage
is only for making transmittance when next writing starts in a
pixel closer to desired transmittance. In the method according to
this document, overdrive voltage can also be used for correcting
luminance difference between frames. Luminance difference between
frames which may cause color shading in displaying a moving image
and motion blur can be reduced by the method according to this
document. As a method for correcting overdrive voltage, the method
shown in FIG. 23C can be used.
[0546] Next, change in voltage applied to a liquid crystal element
and transmittance in a pixel connected to the scan line at the
bottom (a position described as (C) in FIG. 22A) among pixels
belonging to the top area of the backlight is described with
reference to a graph shown in FIG. 22C. Note that a time axis of
the graph of FIG. 22C corresponds to that of the graph of FIG. 22A.
Voltage V.sub.OD2211 (original voltage V.sub.S2211) is applied in a
first frame period, and the voltage V.sub.S2211 is applied in a
second frame period.
[0547] In the first frame period, the transmittance in the graph of
FIG. 22C gradually changes from the time when data is written, and
the transmittance becomes desired transmittance when one frame
period passes or at the time close thereto. At this time, the
backlight lighting period starts before change in transmittance
ends and the backlight lighting period ends before change in
transmittance ends. Further, luminance which human eyes perceive in
the first frame period depends on the area of a portion L.sub.2211
shown by oblique lines in the first frame period.
[0548] Here, since timing when writing starts is different
depending on a position of the scan line, it should be noted that
the area of the oblique line portion L.sub.2211 in the first frame
period is different from the area of an oblique line portion in
another scan line belonging to the same area. This is why luminance
varies depending on a position of a scan line to which the liquid
crystal element is connected even when the same overdrive voltage
V.sub.OD is applied to the liquid crystal element.
[0549] Variation in luminance depending on a scan position is
perceived as luminance unevenness in a display portion as it is, so
that it is a significant image defect and should be improved with
priority. Accordingly, in (3) the method where a backlight divided
into areas sequentially blinks, the original voltage V.sub.S2211 in
the first frame period may be changed in order to correct luminance
difference depending on a scan position. That is, luminance
difference depending on a scan position can be corrected by
correcting gray scale data itself to be written to each pixel. As a
method for correcting data, the method shown in FIG. 23B can be
used.
[0550] In addition, in (3) the method where a backlight divided
into areas sequentially blinks, the overdrive voltage V.sub.OD2211
in the first frame period may be changed in order to correct
luminance difference depending on a scan position. In general,
overdrive voltage is only for making transmittance when next
writing starts in a pixel closer to desired transmittance. In the
method according to this document, overdrive voltage can also be
used for correcting luminance difference depending on a scan
position. Accordingly, luminance difference depending on a scan
position can be corrected by correcting gray scale data itself to
be written to each pixel. As a method for correcting overdrive
voltage, the method shown in FIG. 23D can be used.
[0551] In the second frame period, the transmittance in the graph
of FIG. 22C is already desired transmittance before data is
written. At this time, the transmittance dose not change in the
backlight lighting period. Luminance which human eyes perceive in
the second frame period depends on the area of a portion L.sub.2212
shown by oblique lines in the second frame period.
[0552] Desired luminance for display is the same in the first frame
period and the second frame period. However, the area of the
oblique line portion L.sub.2211 and the area of the oblique line
portion L.sub.2212 are different from each other, so that luminance
which human eyes perceive is different in the first frame period
and the second frame period.
[0553] In (3) the method where a backlight divided into areas
sequentially blinks, the original voltage V.sub.S2211 in the first
frame period may be changed to correct luminance difference between
frames. That is, luminance difference between frames can be
corrected by correcting gray scale data itself to be written to
each pixel. Luminance difference between frames which may cause
color shading in displaying a moving image and motion blur can be
reduced by the method according to this document. As a method for
correcting data, the method shown in FIG. 23B can be used.
[0554] In addition, in (3) the method where a backlight divided
into areas sequentially blinks, the overdrive voltage V.sub.OD2211
in the first frame period may be changed in order to correct
luminance difference between frames. In general, overdrive voltage
is only for making transmittance when next writing starts in a
pixel closer to desired transmittance. In the method according to
this document, overdrive voltage can also be used for correcting
luminance difference between frames. Luminance difference between
frames which may cause color shading in displaying a moving image
and motion blur can be reduced by the method according to this
document. As a method for correcting overdrive voltage, the method
shown in FIG. 23D can be used.
[0555] Next, change in voltage applied to a liquid crystal element
and transmittance in a pixel connected to the scan line in the
first row (a position described as (D) in FIG. 22A) among pixels
belonging to the second top area of the backlight is described with
reference to a graph shown in FIG. 22D. Note that a time axis of
the graph of FIG. 22D corresponds to that of the graph of FIG. 22A.
Voltage V.sub.OD2221 (original voltage. V.sub.S2221) is applied in
a first frame period, and the voltage V.sub.S2221 is applied in a
second frame period.
[0556] In the first frame period, the transmittance in the graph of
FIG. 22D gradually changes from the time when data is written, and
the transmittance becomes desired transmittance when one frame
period passes or at the time close thereto. At this time, the
backlight lighting period starts before change in transmittance
ends and the backlight lighting period ends when change in
transmittance ends. Here, luminance which human eyes perceive in
the first frame period depends on the area of a portion L.sub.2221
shown by oblique lines in the first frame period.
[0557] In the second frame period, the transmittance in the graph
of FIG. 22D is already desired transmittance before data is
written. At this time, the transmittance dose not change in the
backlight lighting period. Luminance which human eyes perceive in
the second frame period depends on the area of a portion L.sub.2222
shown by oblique lines in the second frame period.
[0558] Desired luminance for display is the same in the first frame
period and the second frame period. However, the area of the
oblique line portion L.sub.2221 and the area of the oblique line
portion L.sub.2222 are different from each other, so that luminance
which human eyes perceive is different in the first frame period
and the second frame period.
[0559] In (3) the method where a backlight divided into areas
sequentially blinks, the original voltage V.sub.S2221 in the first
frame period may be changed to correct luminance difference between
frames. That is, luminance difference between frames can be
corrected by correcting gray scale data itself to be written to
each pixel. Luminance difference between frames which may cause
color shading in displaying a moving image and motion blur can be
reduced by the method according to this document. As a method for
correcting data, the method shown in FIG. 23A can be used.
[0560] In addition, in (3) the method where a backlight divided
into areas sequentially blinks, the overdrive voltage V.sub.OD2221
in the first frame period may be changed in order to correct
luminance difference between frames. In general, overdrive voltage
is only for making transmittance when next writing starts in a
pixel closer to desired transmittance. In the method according to
this document, overdrive voltage can also be used for correcting
luminance difference between frames. Luminance difference between
frames which may cause color shading in displaying a moving image
and motion blur can be reduced by the method according to this
document. As a method for correcting overdrive voltage, the method
shown in FIG. 23C can be used.
[0561] As described above, in the pixel shown in FIG. 22B, which is
connected to the scan line in the first row (the position described
as (B) in FIG. 22A) in the top area, and the pixel shown in FIG.
22D, which is connected to the scan line in the first row (the
position described as (D) in FIG. 22A) in the second top area,
timing when voltage is written is different; however, the length of
the time from when voltage is written to when a backlight lighting
period starts is the same. Accordingly, the areas of both of the
oblique line portions (L.sub.2201 and L.sub.2221), which represent
integrated luminance, are equal, so that luminance of both pixels
which human eyes perceive is equal.
[0562] That is, it can be said that luminance which human eyes
perceive is determined by the time from when voltage is written to
when a backlight lighting period starts. The luminance which human
eyes perceive is increased as time from when voltage is written to
when a backlight lighting period starts is longer, whereas the
luminance which human eyes perceive is decreased as the time from
when voltage is written to when a backlight lighting period starts
is shorter.
[0563] Here, in the example of FIGS. 21A to 21D, which describe (2)
the method where the whole backlight blinks, time from when voltage
is written to when a backlight lighting period starts in the scan
line in the first row is a half of one frame period, whereas time
from when voltage is written to when a backlight lighting period
starts in the scan line at the bottom is approximately 0. That is,
in the example of FIGS. 21A to 21D, which describe (2) the method
where the whole backlight blinks, the maximum length of the time
between when voltage is written and when a backlight lighting
period starts is a half of one frame period.
[0564] On the other hand, in the example of FIGS. 22A to 22D, which
describe (3) the method where a backlight divided into areas
sequentially blinks, the maximum length of the time from when
voltage is written to when a backlight lighting period starts is a
half of one frame period, which is the same as the example of FIGS.
21A to 21D, which describe (2) the method where the whole backlight
blinks. Meanwhile, even when the time from when voltage is written
to when a backlight lighting period starts is the shortest (in the
scan line at the bottom in each area), it does not become 0.
Accordingly, in the example of FIGS. 22A to 22D, which describe (3)
the method where a backlight divided into areas sequentially
blinks, the maximum value of difference between when voltage is
written and when a backlight lighting period starts is less than a
half of one frame period.
[0565] Accordingly, when the lighting ratio R is the same in (2)
the method where the whole backlight blinks and (3) the method
where a backlight divided into areas sequentially blinks, (3) the
method where a backlight divided into areas sequentially blinks has
a smaller maximum value of luminance difference depending on a scan
position.
[0566] However, as a factor in deciding image quality of a display
device, not only the maximum value of luminance difference
depending on a scan position but also a distribution of luminance
difference is important. In the example of FIGS. 21A to 21F, which
describe (2) the method where the whole backlight blinks, the
maximum value of luminance difference depending on a scan position
is large, and a distribution of luminance difference is gradual.
Thus, luminance difference gently appears in the whole image. For
example, when display is performed with uniform luminance in all of
pixels and after that, the same amount of luminance is changed in
all of the pixels all at once, luminance difference with gradation
from an upper side to a lower side of a display portion is observed
in a transient state.
[0567] On the other hand, in the example of FIG. 22A to 22D, which
describe (3) the method where a backlight divided into areas
sequentially blinks, the maximum value of luminance difference
depending on a scan position is small, and a distribution of
luminance difference is sharp at a boundary between different
areas. Further, a distribution of luminance difference within each
area is gradual. For example, when display is performed with
uniform luminance in all of pixels and after that, the same amount
of luminance is changed in all of the pixels all at once, luminance
difference with gradation from an upper side to a lower side of
each area appears in a transient state. The luminance difference
with gradation is the same in each area. Accordingly, sharp
luminance difference appears at a boundary of each area. The sharp
luminance difference can be extremely easily perceived as compared
with the case where luminance difference with gradation appears in
the whole display portion, and thus causes significant reduction in
image quality.
[0568] By the method according to this document, a problem of
reduction in image quality in (3) the method where a backlight
divided into areas sequentially blinks can be reduced. The original
voltage V.sub.S may be changed in order to correct luminance
difference depending on a scan position. That is, luminance
difference depending on a scan position can be corrected by
correcting gray scale data itself to be written to each pixel. In
particular, the amount of correction of original voltage in a pixel
to which data is written at the end of each area is made to be the
largest in the area to which the pixel belongs, so that sharp
luminance difference at a boundary of areas can be corrected.
[0569] In addition, the overdrive voltage V.sub.OD may be changed
in order to correct luminance difference depending on a scan
position. In general, overdrive voltage is only for making
transmittance when next writing starts in a pixel closer to desired
transmittance. In the method according to this document, overdrive
voltage can also be used for correcting luminance difference
depending on a scan position. Accordingly, luminance difference
depending on a scan position can be corrected by correcting gray
scale data itself to be written to each pixel. In particular, the
amount of correction of overdrive voltage in a pixel to which data
is written at the end of each area is made to be the largest in the
area to which the pixel belongs, so that sharp luminance difference
at a boundary of areas can be corrected.
[0570] Next, in a method of changing the lighting ratio R which is
one of methods of controlling a display device according to this
document, a method of controlling a display device in frame periods
before and after the lighting ratio R is changed is described in
detail. Here, as described in Embodiment Modes 1 and 2, changing
the lighting ratio R refers to changing the length of the blanking
interval .tau..sub.b as appropriate. Further, driving in accordance
with a state of an image and an environment can be realized by
changing the length of the blanking interval .tau..sub.b in
accordance with the control parameters P and Q described in
Embodiment Mode 3. For example, in the case such that movement of
an object displayed in an image is large or where luminance
difference between a background and an object displayed in an image
is large, motion blur is likely to be seen. Motion blur can be
reduced by increasing the length of the blanking interval
.tau..sub.b. In addition, in the case such that movement of an
object displayed in an image is small or where luminance difference
between a background and an object displayed in an image is small,
motion blur is not likely to be seen. Accordingly, a flicker can be
reduced by reducing the length of the blanking interval
.tau..sub.b. Note that here described is a purpose of preventing
luminance which human eyes perceive from being changed in frame
periods before and after the lighting ratio R is changed, even when
the lighting ratio R is changed.
[0571] Methods for preventing luminance which human eyes perceive
from being changed in frame periods before and after the lighting
ratio R is changed are broadly classified into two methods: a
method where voltage written to a pixel is controlled under a
condition that backlight luminance is constant when a backlight is
lit; and a method where backlight luminance is changed.
[0572] In each method, a controlling method of a display device is
different depending on a method for providing the blanking interval
.tau..sub.b (a method of controlling the lighting ratio R).
Accordingly, in this document, the case where a method of
controlling the lighting ratio R is different in each method is
also individually described in detail.
[0573] Note that as the method for providing the blanking interval
.tau..sub.b (the method of controlling the lighting ratio R), (1) a
method where blanking data is directly written to each pixel, (2) a
method where the whole backlight blinks, (3) a method where a
backlight divided into areas sequentially blinks, and a combination
of these methods can be used.
[0574] First, the case of using (1) the method where blanking data
is directly written to each pixel among the methods where voltage
written to a pixel is controlled under a condition that backlight
luminance is constant when a backlight is lit is described with
reference to FIGS. 24A and 24B.
[0575] FIG. 24A is a graph showing timing of writing data and
timing of writing blanking data on the same time axis with respect
to a position of a scan line when the lighting ratio R is different
in the first frame period and the second frame period. Here, for
explanation, an image display period and a blanking interval in the
first frame period are denoted by .tau..sub.a2401 and
.tau..sub.b2401, and an image display period and a blanking
interval in the second frame period are denoted by .tau..sub.a2402
and .tau..sub.b2402.
[0576] FIG. 24B is a graph showing original voltages V.sub.S2401
and V.sub.S2402 and overdrive voltages V.sub.OD2401 and
V.sub.OD2402 written to each pixel, and transmittance with respect
to each voltage on the same time axis when the lighting ratio R is
different in the first frame period and the second frame period.
Here, the image display period and the blanking interval in the
first frame period and the image display period and the blanking
interval in the second frame period are similar to those in FIG.
24A. Each area of oblique line regions L.sub.2401 and L.sub.2402
represents the level of luminance which human eyes perceive
(integrated luminance). Each of voltage V.sub.2401 and voltage
V.sub.2402 is overdrive intensity in the image display periods in
the first frame period and the second frame period, and
V.sub.2401=V.sub.OD2401-V.sub.S2401 and
V.sub.2402=V.sub.OD2402-V.sub.S2402 are satisfied.
[0577] In the case of using (1) the method where blanking data is
directly written to each pixel among the methods where voltage
written to a pixel is controlled under a condition that backlight
luminance when the backlight is lit is constant before and after
the lighting ratio R is changed, driving can be realized by
changing timing of writing blanking data in the first frame period
and the second frame period, as shown in FIG. 24A. In addition, a
relation between the voltage written to each pixel at this time and
transmittance can be understood with reference to FIG. 24B.
[0578] When the overdrive voltage V.sub.OD2401 is applied to each
pixel at or around the start of the image display period in the
first frame period, transmittance of a display element becomes
transmittance corresponding to the original voltage V.sub.S2401 at
the time when the image display period in the first frame period
ends or at the time close thereto. Thereafter, blanking writing is
performed. Thus, integrated luminance in the first frame period is
represented by the area of the oblique line region L.sub.2401.
[0579] Then, when the overdrive voltage V.sub.OD2402 is applied to
each pixel at or around the start of the image display period in
the second frame period, the transmittance of the display element
becomes transmittance corresponding to the original voltage
V.sub.S2402 at the time when the image display period in the second
frame period ends or at the time close thereto. Thereafter,
blanking writing is performed. Thus, integrated luminance in the
second frame period is represented by the area of the oblique line
region L.sub.2402.
[0580] At this time, it is important that values of the voltages
applied to a pixel vary in the first frame period and the second
frame period. That is, in the case where the lighting ratio R is
changed under a condition that backlight luminance is constant when
a backlight is lit, it is preferable to write different voltage in
the first frame period and the second frame period, not the same
voltage, if luminance of the pixel which human eyes perceive is not
desired to be changed.
[0581] Accordingly, in one of methods according to this document,
the original voltage and the overdrive voltage are changed in
accordance with the lighting ratio R in order that the area of the
oblique line region L.sub.2401 in the first frame period and the
area of the oblique line region L.sub.2402 in the second frame
period are approximately the same. Specifically, it is preferable
to reduce the original voltage and the overdrive voltage as the
lighting ratio R increases. In addition, in one of the methods
according to this document, the overdrive intensity V.sub.2401 in
the first frame period and the overdrive intensity V.sub.2402 in
the second frame period may be changed in accordance with the
lighting ratio R. Specifically, it is preferable to reduce the
overdrive intensity as the lighting ratio R increases. This is
because increase in the lighting ratio R means increase in length
of the image display period .tau..sub.a, and increase in length of
the image display period .tau..sub.a can be allowed to have a
longer period of time for reaching intended transmittance of a
liquid crystal element. Moreover, when the length of the image
display period .tau..sub.a is increased, intended transmittance of
a liquid crystal element itself can be reduced, so that the
original voltage V.sub.S is reduced, and further, the overdrive
intensity can be reduced.
[0582] By driving a display device in such a manner, backlight
luminance can be constant even in the case where luminance of the
pixel which human eyes perceive is not desired to be changed when
the lighting ratio R is changed. Thus, a structure of a circuit for
driving a backlight is simplified, so that manufacturing cost can
be reduced. Further, luminance unevenness and a flicker in
displaying an image can be reduced. Moreover, provision of the
blanking interval .tau..sub.b can reduce motion blur, and image
quality of a moving image can be improved.
[0583] Next, the case of using (2) the method where the whole
backlight blinks among the methods where voltage written to a pixel
is controlled under a condition that backlight luminance is
constant when the backlight is lit is described with reference to
FIGS. 25A to 25C.
[0584] FIG. 25A is a graph showing timing of writing data and
timing of blinking a backlight on the same time axis with respect
to a position of a scan line when the lighting ratio R is different
in the first frame period and the second frame period. Here, for
explanation, a backlight lighting period in the first frame period
is denoted by .tau..sub.a2501, and a backlight lighting period in
the second frame period is denoted by .tau..sub.a2502.
[0585] FIG. 25B is a graph showing original voltages V.sub.S2501
and V.sub.S2502 and overdrive voltages V.sub.OD2501 and
V.sub.OD2502 written to each pixel, and transmittance with respect
to each voltage on the same time axis when the lighting ratio R is
different in the first frame period and the second frame period.
Here, the backlight lighting periods in the first frame period and
the second frame period are similar to those in FIG. 25A. Each area
of oblique line regions L.sub.2501 and L.sub.2502 represents the
level of luminance which human eyes perceive (integrated
luminance). Each of voltage V.sub.2501 and voltage V.sub.2502 is
overdrive intensity in the first frame period and the second frame
period, and V.sub.2501=V.sub.OD2501-V.sub.S2501 and
V.sub.2502=V.sub.OD2502-V.sub.S2502 are satisfied.
[0586] In the case of using (2) the method where the whole
backlight blinks among the methods where voltage written to a pixel
is controlled under a condition that backlight luminance is
constant when the backlight is lit before and after the lighting
ratio R is changed, driving can be realized by changing the length
and timing of the backlight lighting period in the first frame
period and the second frame period, as shown in FIG. 25A. In
addition, a relation between the voltage written to each pixel at
this time and transmittance can be understood with reference to
FIG. 25B.
[0587] When the overdrive voltage V.sub.OD2501 is written to a
pixel by data writing scanning in the first frame period,
transmittance of a display element becomes transmittance
corresponding to the original voltage V.sub.S2501 at the time when
the next data is written by data writing scanning in the second
frame period or at the time close thereto. In that period, a
backlight lighting period is provided in all pixels all at once.
Thus, integrated luminance in the first frame period is represented
by the area of the oblique line region L.sub.2501, which is
surrounded by the backlight lighting period and the
transmittance.
[0588] Then, when the overdrive voltage V.sub.OD2502 is written to
the pixel by data writing scanning in the second frame period, the
transmittance of the display element becomes transmittance
corresponding to the original voltage V.sub.S2502 at the time when
the next data is written by data writing scanning in the next frame
period or at the time close thereto. In that period, a backlight
lighting period is provided in all the pixels all at once. Thus,
integrated luminance in the second frame period is represented by
the area of the oblique line region L.sub.2502, which is surrounded
by the backlight lighting period and the transmittance.
[0589] At this time, it is important that values of the voltages
applied to a pixel vary in the first frame period and the second
frame period. That is, in the case where the lighting ratio R is
changed under a condition that backlight luminance is constant when
a backlight is lit, it is preferable to write different voltage in
the first frame period and the second frame period, not the same
voltage, if luminance of the pixel which human eyes perceive is not
desired to be changed.
[0590] Accordingly, in one of methods according to this document,
the original voltage and the overdrive voltage are changed in
accordance with the lighting ratio R in order that the area of the
oblique line region L.sub.2501 in the first frame period and the
area of the oblique line region L.sub.2502 in the second frame
period are approximately the same. Specifically, it is preferable
to reduce the original voltage and the overdrive voltage as the
lighting ratio R increases. In addition, in one of the methods
according to this document, the overdrive intensity V.sub.2501 in
the first frame period and the overdrive intensity V.sub.2502 in
the second frame period may be changed in accordance with the
lighting ratio R. Specifically, it is preferable to reduce the
overdrive intensity as the lighting ratio R increases. This is
because increase in the lighting ratio R means increase in length
of the image display period .tau..sub.a, and increase in length of
the image display period .tau..sub.a can be allowed to have a
longer period of time for reaching intended transmittance of a
liquid crystal element. Moreover, when the length of the image
display period .tau..sub.a is increased, intended transmittance of
a liquid crystal element itself can be reduced, so that the
original voltage V.sub.S is reduced, and further, the overdrive
intensity can be reduced.
[0591] By driving a display device in such a manner, backlight
luminance can be constant even in the case where luminance of the
pixel which human eyes perceive is not desired to be changed when
the lighting ratio R is changed. Thus, a structure of a circuit for
driving a backlight is simplified, so that manufacturing cost can
be reduced. Further, luminance unevenness and a flicker in
displaying an image can be reduced. Moreover, provision of the
blanking interval .tau..sub.b can reduce motion blur, and image
quality of a moving image can be improved.
[0592] FIG. 25C is a graph showing original voltages V.sub.S2511
and V.sub.S2512 and overdrive voltages V.sub.OD2511 and
V.sub.OD2512 written to each pixel, and transmittance with respect
to each voltage on the same time axis in a pixel connected to a
scan line different from that shown in FIG. 25B when the lighting
ratio R is different in the first frame period and the second frame
period. Here, the backlight lighting periods in the first frame
period and the second frame period are similar to those in FIG.
25A. Each area of oblique line regions L.sub.2511 and L.sub.2512
represents the level of luminance which human eyes perceive
(integrated luminance). Each of voltage V.sub.2511 and voltage
V.sub.2512 is overdrive intensity in the first frame period and the
second frame period, and V.sub.2511=V.sub.OD2511-V.sub.S2511 and
V.sub.2512=V.sub.OD2512-V.sub.S2512 are satisfied.
[0593] Although details of a controlling method shown in FIG. 25C
are similar to those shown in FIG. 25B, the length of time from
when data is written to when a backlight lighting period starts is
different. Thus, each area of the oblique line regions L.sub.2511
and L.sub.2512, which represents integrated luminance, is different
from each area of the oblique line regions L.sub.2501 and
L.sub.2502 in FIG. 25B. Accordingly, the original voltage and
overdrive voltage V may be changed in order to correct luminance
difference depending on a scan position. As a method for correcting
original voltage, the method shown in FIG. 23B can be used. As a
method for correcting overdrive voltage, the method shown in FIG.
23D can be used. Thus, color shading and motion blur in displaying
a moving image can be reduced.
[0594] Next, the case of using (3) the method where a backlight
divided into areas sequentially blinks among the methods where
voltage written to a pixel is controlled under a condition that
backlight luminance is constant when the backlight is lit is
described with reference to FIGS. 26A to 26C.
[0595] FIG. 26A is a graph showing timing of writing data and
timing of sequentially blinking a backlight on the same time axis
with respect to a position of a scan line when the lighting ratio R
is different in the first frame period and the second frame period.
Here, for explanation, a backlight lighting period in the first
frame period is denoted by .tau..sub.a2601, and a backlight
lighting period in the second frame period is denoted by
.tau..sub.a2602.
[0596] FIG. 26B is a graph showing original voltages V.sub.S2601
and V.sub.S2602 and overdrive voltages V.sub.OD2601 and
V.sub.OD2602 written to each pixel, and transmittance with respect
to each voltage on the same time axis when the lighting ratio R is
different in the first frame period and the second frame period.
Here, the backlight lighting periods in the first frame period and
the second frame period are similar to those in FIG. 26A. Each area
of oblique line regions L.sub.2601 and L.sub.2602 represents the
level of luminance which human eyes perceive (integrated
luminance). Each of voltage V.sub.2601 and voltage V.sub.2602 is
overdrive intensity in the first frame period and the second frame
period, and V.sub.2601=V.sub.OD2601-V.sub.S2601 and
V.sub.2602=V.sub.OD2602-V.sub.S2602 are satisfied.
[0597] In the case of using (3) the method where a backlight
divided into areas sequentially blinks among the methods where
voltage written to a pixel is controlled under a condition that
backlight luminance is constant when the backlight is lit before
and after the lighting ratio R is changed, driving can be realized
by changing the length and timing of the backlight lighting period
in the first frame period and the second frame period, as shown in
FIG. 26A. In addition, a relation between the voltage written to
each pixel at this time and transmittance can be understood with
reference to FIG. 26B.
[0598] When the overdrive voltage V.sub.OD2601 is written to a
pixel by data writing scanning in the first frame period,
transmittance of a display element becomes transmittance
corresponding to the original voltage V.sub.S2601 at the time when
the next data is written by data writing scanning in the second
frame period or at the time close thereto. In that period, a
backlight lighting period is sequentially provided for each area.
Thus, integrated luminance in the top area in the first frame
period is represented by the area of the oblique line region
L.sub.2601, which is surrounded by the backlight lighting period
and the transmittance.
[0599] Then, when the overdrive voltage V.sub.OD2602 is written to
the pixel by data writing scanning in the second frame period, the
transmittance of the display element becomes transmittance
corresponding to the original voltage V.sub.S2602 at the time when
the next data is written by data writing scanning in the next frame
period or at the time close thereto. In that period, a backlight
lighting period is sequentially provided for each area. Thus,
integrated luminance in the top area in the second frame period is
represented by the area of the oblique line region L.sub.2602 which
is surrounded by the backlight lighting period and the
transmittance.
[0600] At this time, it is important that values of the voltages
applied to a pixel vary in the first frame period and the second
frame period. That is, in the case where the lighting ratio R is
changed under a condition that backlight luminance is constant when
a backlight is lit, it is preferable to write different voltage in
the first frame period and the second frame period, not the same
voltage, if luminance of the pixel which human eyes perceive is not
desired to be changed.
[0601] Accordingly, in one of methods according to this document,
the original voltage and the overdrive voltage are changed in
accordance with the lighting ratio R in order that the area of the
oblique line region L.sub.2601 in the first frame period and the
area of the oblique line region L.sub.2602 in the second frame
period are approximately the same. Specifically, it is preferable
to reduce the original voltage and the overdrive voltage as the
lighting ratio R increases. In addition, in one of the methods
according to this document, the overdrive intensity V.sub.2601 in
the first frame period and the overdrive intensity V.sub.2602 in
the second frame period may be changed in accordance with the
lighting ratio R. Specifically, it is preferable to reduce the
overdrive intensity as the lighting ratio R increases. This is
because increase in the lighting ratio R means increase in length
of the image display period .tau..sub.a, and increase in length of
the image display period .tau..sub.a can be allowed to have a
longer period of time for reaching intended transmittance of a
liquid crystal element. Moreover, when the length of the image
display period .tau..sub.a is increased, intended transmittance of
a liquid crystal element itself can be reduced, so that the
original voltage V.sub.S is reduced, and further, the overdrive
intensity can be reduced.
[0602] By driving a display device in such a manner, backlight
luminance can be constant even in the case where luminance of the
pixel which human eyes perceive is not desired to be changed when
the lighting ratio R is changed. Thus, a structure of a circuit for
driving a backlight is simplified, so that manufacturing cost can
be reduced. Further, luminance unevenness and a flicker in
displaying an image can be reduced. Moreover, provision of the
blanking interval .tau..sub.b can reduce motion blur, and image
quality of a moving image can be improved.
[0603] FIG. 26C is a graph showing original voltages V.sub.S2611
and V.sub.S2612 and overdrive voltages V.sub.OD2611 and
V.sub.OD2612 written to each pixel, and transmittance with respect
to each voltage on the same time axis in a pixel connected to a
scan line different from that shown in FIG. 26B when the lighting
ratio R is different in the first frame period and the second frame
period. Here, the backlight lighting periods in the first frame
period and the second frame period are similar to those in FIG.
26A. Each area of oblique line regions L.sub.2611 and L.sub.2612
represents the level of luminance which human eyes perceive
(integrated luminance). Each of voltage V.sub.2611 and voltage
V.sub.2612 is overdrive intensity in the first frame period and the
second frame period, and V.sub.2611=V.sub.OD2611-V.sub.S2611 and
V.sub.2612=V.sub.OD2612-V.sub.S2612 are satisfied.
[0604] Although details of a controlling method shown in FIG. 26C
are similar to those shown in FIG. 26B, the length of time from
when data is written to when a backlight lighting period starts is
different. Thus, each area of the oblique line regions L.sub.2611
and L.sub.2612, which represents integrated luminance, is different
from each area of the oblique line regions L.sub.2601 and
L.sub.2602 in FIG. 26B. Accordingly, the original voltage and the
overdrive voltage V may be changed in order to correct luminance
difference depending on a scan position. As a method for correcting
original voltage, the method shown in FIG. 23B can be used. As a
method for correcting overdrive voltage, the method shown in FIG.
23D can be used. Thus, color shading and motion blur in displaying
a moving image can be reduced.
[0605] Note that the driving methods shown in FIGS. 26B and 26C are
similar in other areas. At this time, the amount of correction of
the original voltage and the overdrive voltage in a pixel to which
data is written at the end of each area is made to be the largest
in the area to which the pixel belongs, so that sharp luminance
difference at a boundary of areas can be corrected.
[0606] Next, the case of a combination of (2) the method where the
whole backlight blinks and (1) the method where blanking data is
directly written to each pixel among the methods where voltage
written to a pixel is controlled under a condition that backlight
luminance is constant when the backlight is lit is described with
reference to FIGS. 27A to 27C.
[0607] FIG. 27A is a graph showing timing of writing data, timing
of blinking a backlight, and timing of writing blanking date on the
same time axis with respect to a position of a scan line when the
lighting ratio R is different in the first frame period and the
second frame period. Here, for explanation, a backlight lighting
period in the first frame period is denoted by .tau..sub.a2701, and
a backlight lighting period in the second frame period is denoted
by .tau..sub.a2702.
[0608] FIG. 27B is a graph showing original voltages V.sub.S2701
and V.sub.S2702 and overdrive voltages V.sub.OD2701 and
V.sub.OD2702 written to each pixel, and transmittance with respect
to each voltage on the same time axis when the lighting ratio R is
different in the first frame period and the second frame period.
Here, the backlight lighting periods in the first frame period and
the second frame period are similar to those in FIG. 27A. Each area
of oblique line regions L.sub.2701 and L.sub.2702 represents the
level of luminance which human eyes perceive (integrated
luminance). Each of voltage V.sub.2701 and voltage V.sub.2702 is
overdrive intensity in the image display periods in the first frame
period and the second frame period, and
V.sub.2701=V.sub.OD2701-V.sub.S2701 and
V.sub.2702=V.sub.OD2702-V.sub.S2702 are satisfied.
[0609] In the case of the combination of (2) the method where the
whole backlight blinks and (1) the method where blanking data is
directly written to each pixel among the methods where voltage
written to a pixel is controlled under a condition that backlight
luminance is constant when the backlight is lit before and after
the lighting ratio R is changed, driving can be realized by
changing the length and timing of the backlight lighting period in
the first frame period and the second frame period and performing
blanking writing scanning in addition to data writing scanning, as
shown in FIG. 27A. Here, although the case is shown in which data
writing scanning and blanking writing scanning are performed at the
same timing in each frame period, a driving method according to
this document is not limited thereto, and various types of writing
timing can be used. For example, data writing scanning may be
changed in accordance with the lighting ratio R. As a method where
data writing scanning is changed in accordance with the lighting
ratio R, the length of time from blanking writing scanning to data
writing scanning in the same frame period may be increased as the
lighting ratio R is decreased. A relation between the voltage
written to each pixel at this time and transmittance can be
understood with reference to FIG. 27B.
[0610] When the overdrive voltage VOD.sub.2701 is written to a
pixel by data writing scanning in the first frame period,
transmittance of a display element becomes transmittance
corresponding to the original voltage V.sub.S2701 at the time when
the next data is written by blanking writing scanning in the second
frame period or at the time close thereto. In that period, a
backlight lighting period is provided in all pixels all at once.
Thus, integrated luminance in the first frame period is represented
by the area of the oblique line region L.sub.2701, which is
surrounded by the backlight lighting period and the
transmittance.
[0611] Then, when the overdrive voltage V.sub.OD2702 is written to
the pixel by data writing scanning in the second frame period after
blanking writing scanning in the second frame period, the
transmittance of the display element becomes transmittance
corresponding to the original voltage V.sub.S2702 at the time when
the next data is written by blanking writing scanning in the next
frame period or at the time close thereto. In that period, a
backlight lighting period is provided in all the pixels all at
once. Thus, integrated luminance in the second frame period is
represented by the area of the oblique line region L.sub.2702,
which is surrounded by the backlight lighting period and the
transmittance.
[0612] At this time, it is important that values of the voltages
applied to a pixel vary in the first frame period and the second
frame period. That is, in the case where the lighting ratio R is
changed under a condition that backlight luminance is constant when
a backlight is lit, it is preferable to write different voltage in
the first frame period and the second frame period, not the same
voltage, if luminance of the pixel which human eyes perceive is not
desired to be changed.
[0613] Accordingly, in one of methods according to this document,
the original voltage and the overdrive voltage are changed in
accordance with the lighting ratio R in order that the area of the
oblique line region L.sub.2701 in the first frame period and the
area of the oblique line region L.sub.2702 in the second frame
period are approximately the same. Specifically, it is preferable
to reduce the original voltage and the overdrive voltage as the
lighting ratio R increases. In addition, in one of the methods
according to this document, the overdrive intensity V.sub.2701 in
the first frame period and the overdrive intensity V.sub.2702 in
the second frame period may be changed in accordance with the
lighting ratio R. Specifically, it is preferable to reduce the
overdrive intensity as the lighting ratio R increases. This is
because increase in the lighting ratio R means increase in length
of the image display period .tau..sub.a, and increase in length of
the image display period .tau..sub.a can be allowed to have a
longer period of time for reaching intended transmittance of a
liquid crystal element. Moreover, when the length of the image
display period .tau..sub.a is increased, intended transmittance of
a liquid crystal element itself can be reduced, so that the
original voltage V.sub.S is reduced, and further, the overdrive
intensity can be reduced.
[0614] By driving a display device in such a manner, backlight
luminance can be constant even in the case where luminance of the
pixel which human eyes perceive is not desired to be changed when
the lighting ratio R is changed. Thus, a structure of a circuit for
driving a backlight is simplified, so that manufacturing cost can
be reduced. Further, luminance unevenness and a flicker in
displaying an image can be reduced. Moreover, provision of the
blanking interval .tau..sub.b can reduce motion blur, and image
quality of a moving image can be improved. Furthermore, since
blanking writing is performed in a period other than the backlight
lighting period, light leakage can be reduced. Thus, black blurring
in displaying an image can be reduced, so that a contrast ratio of
the display device can be improved.
[0615] FIG. 27C is a graph showing original voltages V.sub.S2711
and V.sub.S2712 and overdrive voltages V.sub.OD2711 and
V.sub.OD2712 written to each pixel, and transmittance with respect
to each voltage on the same time axis in a pixel connected to a
scan line different from that shown in FIG. 27B when the lighting
ratio R is different in the first frame period and the second frame
period. Here, the backlight lighting periods in the first frame
period and the second frame period are similar to those in FIG.
27A. Each area of oblique line regions L.sub.2711 and L.sub.2712
represents the level of luminance which human eyes perceive
(integrated luminance). Each of voltage V.sub.2711 and voltage
V.sub.2712 is overdrive intensity in the first frame period and the
second frame period, and V.sub.2711=V.sub.OD2711-V.sub.S2711 and
V.sub.2712=V.sub.OD2712-V.sub.S2712 are satisfied.
[0616] Although details of a controlling method shown in FIG. 27C
are similar to those shown in FIG. 27B, the length of time from
when data is written to when a backlight lighting period starts is
different. Thus, each area of the oblique line regions L.sub.2711
and L.sub.2712, which represents integrated luminance, is different
from each area of the oblique line regions L.sub.2701 and
L.sub.2702 in FIG. 27B. Accordingly, the original voltage and the
overdrive voltage V may be changed in order to correct luminance
difference depending on a scan position. As a method for correcting
original voltage, the method shown in FIG. 23B can be used. As a
method for correcting overdrive voltage, the method shown in FIG.
23D can be used. Thus, color shading and motion blur in displaying
a moving image can be reduced.
[0617] Next, the case of a combination of (3) the method where a
backlight divided into areas sequentially blinks and (1) the method
where blanking data is directly written to each pixel among the
methods where voltage written to a pixel is controlled under a
condition that backlight luminance is constant when the backlight
is lit is described with reference to FIGS. 28A to 28C.
[0618] FIG. 28A is a graph showing timing of writing data, timing
of writing blank data, and timing of sequentially blinking a
backlight on the same time axis with respect to a position of a
scan line when the lighting ratio R is different in the first frame
period and the second frame period. Here, for explanation, a
backlight lighting period in the first frame period is denoted by
.tau..sub.a2802, and a backlight lighting period in the second
frame period is denoted by .tau..sub.a2802.
[0619] FIG. 28B is a graph showing original voltages V.sub.S2801
and V.sub.S2802 and overdrive voltages V.sub.OD2801 and
V.sub.OD2802 written to each pixel, and transmittance with respect
to each voltage on the same time axis when the lighting ratio R is
different in the first frame period and the second frame period.
Here, the backlight lighting periods in the first frame period and
the second frame period are similar to those in FIG. 28A. Each area
of oblique line regions L.sub.2801 and L.sub.2802 represents the
level of luminance which human eyes perceive (integrated
luminance). Each of voltage V.sub.2801 and voltage V.sub.2802 is
overdrive intensity in the image display periods in the first frame
period and the second frame period, and
V.sub.2801=V.sub.OD2801-V.sub.S2801 and
V.sub.2802=V.sub.OD2802-V.sub.S2802 are satisfied.
[0620] In the case of the combination of (3) the method where a
backlight divided into areas sequentially blinks and (1) the method
where blanking data is directly written to each pixel among the
methods where voltage written to a pixel is controlled under a
condition that backlight luminance is constant when the backlight
is lit before and after the lighting ratio R is changed, driving
can be realized by changing the length and timing of the backlight
lighting period in the first frame period and the second frame
period and performing blanking writing scanning in addition to data
writing scanning, as shown in FIG. 28A. Here, although the case is
shown in which data writing scanning and blanking writing scanning
are performed at the same timing in each frame period, a driving
method according to this document is not limited thereto, and
various types of writing timing can be used. For example, data
writing scanning may be changed in accordance with the lighting
ratio R. As a method where data writing scanning is changed in
accordance with the lighting ratio R, the length of time from
blanking writing scanning to data writing scanning in the same
frame period may be increased as the lighting ratio R is decreased.
A relation between the voltage written to each pixel at this time
and transmittance can be understood with reference to FIG. 28B.
[0621] When the overdrive voltage V.sub.OD2801 is written to a
pixel by data writing scanning in the first frame period,
transmittance of a display element becomes transmittance
corresponding to the original voltage V.sub.S2801 at the time when
the next data is written by blanking writing scanning in the second
frame period or at the time close thereto. In that period, a
backlight lighting period is sequentially provided for each area.
Thus, integrated luminance in the top area in the first frame
period is represented by the area of the oblique line region
L.sub.2801, which is surrounded by the backlight lighting period
and the transmittance.
[0622] Then, when the overdrive voltage V.sub.OD2802 is written to
the pixel by data writing scanning in the second frame period after
blanking writing scanning in the second frame period, the
transmittance of the display element becomes transmittance
corresponding to the original voltage V.sub.S2802 at the time when
the next data is written by data writing scanning in the next frame
period or at the time close thereto. In that period, a backlight
lighting period is sequentially provided for each area. Thus,
integrated luminance in the top area in the second frame period is
represented by the area of the oblique line region L.sub.2802,
which is surrounded by the backlight lighting period and the
transmittance.
[0623] At this time, it is important that values of the voltages
applied to a pixel vary in the first frame period and the second
frame period. That is, in the case where the lighting ratio R is
changed under a condition that backlight luminance is constant when
a backlight is lit, it is preferable to write different voltage in
the first frame period and the second frame period, not the same
voltage, if luminance of the pixel which human eyes perceive is not
desired to be changed.
[0624] Accordingly, in one of methods according to this document,
the original voltage and the overdrive voltage are changed in
accordance with the lighting ratio R in order that the area of the
oblique line region L.sub.2801 in the first frame period and the
area of the oblique line region L.sub.2802 in the second frame
period are approximately the same. Specifically, it is preferable
to reduce the original voltage and the overdrive voltage as the
lighting ratio R increases. In addition, in one of the methods
according to this document, the overdrive intensity V.sub.2801 in
the first frame period and the overdrive intensity V.sub.2802 in
the second frame period may be changed in accordance with the
lighting ratio R. Specifically, it is preferable to reduce the
overdrive intensity as the lighting ratio R increases. This is
because increase in the lighting ratio R means increase in length
of the image display period .tau..sub.a, and increase in length of
the image display period .tau..sub.a can be allowed to have a
longer period of time for reaching intended transmittance of a
liquid crystal element. Moreover, when the length of the image
display period .tau..sub.a is increased, intended transmittance of
a liquid crystal element itself can be reduced, so that the
original voltage V.sub.S is reduced, and further, the overdrive
intensity can be reduced.
[0625] By driving a display device in such a manner, backlight
luminance can be constant even in the case where luminance of the
pixel which human eyes perceive is not desired to be changed when
the lighting ratio R is changed. Thus, a structure of a circuit for
driving a backlight is simplified, so that manufacturing cost can
be reduced. Further, luminance unevenness and a flicker in
displaying an image can be reduced. Moreover, provision of the
blanking interval .tau..sub.b can reduce motion blur, and image
quality of a moving image can be improved. Furthermore, since
blanking writing is performed in a period other than the backlight
lighting period, light leakage in a non-lighting period of the
backlight can be reduced. Thus, black blurring in displaying an
image can be reduced, so that a contrast ratio of the display
device can be improved.
[0626] FIG. 28C is a graph showing original voltages V.sub.S2811
and V.sub.S2812 and overdrive voltages V.sub.OD2811 and
V.sub.OD2812 written to each pixel, and transmittance with respect
to each voltage on the same time axis in a pixel connected to a
scan line different from that shown in FIG. 28B when the lighting
ratio R is different in the first frame period and the second frame
period. Here, the backlight lighting periods in the first frame
period and the second frame period are similar to those in FIG.
28A. Each area of oblique line regions L.sub.2811 and L.sub.2812
represents the level of luminance which human eyes perceive
(integrated luminance). Each of voltage V.sub.2811 and voltage
V.sub.2812 is overdrive intensity in the first frame period and the
second frame period, and V.sub.2811=V.sub.OD2811-V.sub.S2811 and
V.sub.2812=V.sub.OD2812-V.sub.S2812 are satisfied.
[0627] Although details of a controlling method shown in FIG. 28C
are similar to those shown in FIG. 28B, the length of time from
when data is written to when a backlight lighting period starts is
different. Thus, each area of the oblique line regions L.sub.2811
and L.sub.2812, which represents integrated luminance, is different
from each area of the oblique line regions L.sub.2801 and
L.sub.2802 in FIG. 28B. Accordingly, the original voltage and the
overdrive voltage V may be changed in order to correct luminance
difference depending on a scan position. As a method for correcting
original voltage, the method shown in FIG. 23B can be used. As a
method for correcting overdrive voltage, the method shown in FIG.
23D can be used. Thus, color shading and motion blur in displaying
a moving image can be reduced.
[0628] Note that the driving methods shown in FIGS. 28B and 28C are
similar in other areas. At this time, the amount of correction of
the original voltage and the overdrive voltage in a pixel to which
data is written at the end of each area is made to be the largest
in the area to which the pixel belongs, so that sharp luminance
difference at a boundary of areas can be corrected.
[0629] Next, in a method of changing backlight luminance, the case
where the method for providing the blanking interval .tau..sub.b
(the method of controlling the lighting ratio R) is different is
individually described in detail. Note that in the method of
changing backlight luminance, by controlling transmittance of a
display element, backlight luminance can have extremely various
values in order to prevent change in luminance which human eyes
perceive in frame periods before and after the lighting ratio R is
changed. Here, the case where voltage written to each pixel is not
changed when the lighting ratio R is changed is described. This is
because this can provide a beneficial effect in driving a display
device.
[0630] First, the case of using (2) the method where the whole
backlight blinks among the methods of changing backlight luminance
is described with reference to FIGS. 29A to 29C.
[0631] FIG. 29A is a graph showing timing of writing data and
timing of blinking a backlight on the same time axis with respect
to a position of a scan line when the lighting ratio R is different
in the first frame period and the second frame period. Here, for
explanation, a backlight lighting period in the first frame period
is denoted by .tau..sub.a2901, and a backlight lighting period in
the second frame period is denoted by .tau..sub.a2902.
[0632] FIG. 29B is a graph showing original voltage V.sub.S2901 and
overdrive voltage V.sub.OD2901 written to each pixel, and
transmittance with respect to each voltage on the same time axis
when the lighting ratio R is different in the first frame period
and the second frame period. Here, the backlight lighting periods
in the first frame period and the second frame period are similar
to those in FIG. 29A. Each area of oblique line regions L.sub.2901
and L.sub.2902 represents the level of luminance which human eyes
perceive (integrated luminance). Voltage V.sub.2901 is overdrive
intensity in the first frame period, and
V.sub.2901=V.sub.OD2901-V.sub.S2901 is satisfied.
[0633] In the case of using (2) the method where the whole
backlight blinks among the methods of changing backlight luminance
before and after the lighting ratio R is changed, driving can be
realized by changing backlight luminance and the length and timing
of the backlight lighting period, as shown in FIG. 29A. A relation
between the voltage written to each pixel at this time and
transmittance can be understood with reference to FIG. 29B.
[0634] When the overdrive voltage V.sub.OD2901 is written to a
pixel by data writing scanning in the first frame period,
transmittance of a display element becomes transmittance
corresponding to the original voltage V.sub.S2901 at the time when
the next data is written by data writing scanning in the second
frame period or at the time close thereto. In that period, a
backlight lighting period is provided in all pixels all at once.
Thus, integrated luminance in the first frame period is represented
by the area of the oblique line region L.sub.2901, which is
surrounded by the backlight lighting period and the
transmittance.
[0635] In the second frame period, the transmittance in the graph
of FIG. 29B is already desired transmittance before data is
written. At this time, the transmittance dose not change in the
backlight lighting period. Luminance which human eyes perceive in
the second frame period depends on the area of the oblique line
region L.sub.2902 in the second frame period.
[0636] At this time, it is important that luminance in the
backlight lighting periods varies in the first frame period and the
second frame period. That is, when the lighting ratio R is changed,
display can be performed without change in luminance of a pixel
which human eyes perceive by changing backlight luminance even in
the case where luminance of the pixel is not desired to be
changed.
[0637] Accordingly, in one of methods according to this document,
backlight luminance in the backlight lighting period is determined
by difference between the area of the oblique line region
L.sub.2901 in the first frame period and the area of the oblique
line region L.sub.2902 in the second frame period. Specifically,
when the lighting ratio R is changed and the backlight lighting
period in the second frame period is 1/X (X is a positive number)
of the backlight lighting period in the first frame period, it is
preferable that backlight luminance be X times as high as that in
the first frame period. Then, in one of the methods according to
this document, it is preferable that the original voltage
V.sub.S2901 in the first frame period be approximately the same in
the first frame period and the second frame period.
[0638] By driving a display device in such a manner, when the
lighting ratio R is changed, the original voltage V.sub.S2901 can
be approximately the same in the first frame period and the second
frame period even in the case where luminance of the pixel which
human eyes perceive is not desired to be changed. Thus, a structure
of a circuit which processes image data and is included in the
display device is simplified, so that manufacturing cost and power
consumption of the display device can be reduced. Further, in the
case where the same luminance is desired to be displayed when the
lighting ratio R is changed, voltage written to each pixel does not
have to be changed from that in the previous frame; thus, power
consumption in writing data can be reduced.
[0639] Note that overdrive voltage and overdrive intensity do not
have to be approximately the same in the first frame period and the
second frame period. This is because overdrive voltage and
overdrive intensity are obtained from original voltages and
transmittance in one frame and the previous frame; thus, when
original voltage and transmittance in each previous frame are
different in the first frame period and the second frame period,
various values are obtained as a matter of course.
[0640] FIG. 29C is a graph showing original voltage V.sub.S2911 and
overdrive voltage V.sub.OD2911 written to each pixel, and
transmittance with respect to each voltage on the same time axis in
a pixel connected to a scan line different from that shown in FIG.
29B when the lighting ratio R is different in the first frame
period and the second frame period. Here, the backlight lighting
periods in the first frame period and the second frame period are
similar to those in FIG. 29A. Each area of oblique line regions
L.sub.2911 and L.sub.2912 represents the level of luminance which
human eyes perceive (integrated luminance). Voltage V.sub.2911 is
overdrive intensity in the first frame period, and
V.sub.2911=V.sub.OD2911-V.sub.S2911 is satisfied.
[0641] Although details of a controlling method shown in FIG. 29C
are similar to those shown in FIG. 29B, the length of time from
when data is written to when a backlight lighting period starts is
different. Thus, each area of the oblique line regions L.sub.2911
and L.sub.2912, which represents integrated luminance, is different
from each area of the oblique line regions L.sub.2901 and
L.sub.2902 in FIG. 29B. Accordingly, the original voltage and the
overdrive voltage V may be changed in order to correct luminance
difference depending on a scan position. As a method for correcting
original voltage, the method shown in FIG. 23B can be used. As a
method for correcting overdrive voltage, the method shown in FIG.
23D can be used. Thus, color shading and motion blur in displaying
a moving image can be reduced.
[0642] Next, the case of using (3) the method where a backlight
divided into areas sequentially blinks among the methods of
changing backlight luminance is described with reference to FIGS.
30A to 30C.
[0643] FIG. 30A is a graph showing timing of writing data and
timing of sequentially blinking a backlight on the same time axis
with respect to a position of scan lines when the lighting ratio R
is different in the first frame period and the second frame period.
Here, for explanation, a backlight lighting period in the first
frame period is denoted by .tau..sub.a3001, and a backlight
lighting period in the second frame period is denoted by
.tau..sub.a3002.
[0644] FIG. 30B is a graph showing original voltage V.sub.S3001 and
overdrive voltage V.sub.OD3001 written to each pixel, and
transmittance with respect to each voltage on the same time axis
when the lighting ratio R is different in the first frame period
and the second frame period. Here, the backlight lighting periods
in the first frame period and the second frame period are similar
to those in FIG. 30A. Each area of oblique line regions L.sub.3001
and L.sub.3002 represents the level of luminance which human eyes
perceive (integrated luminance). Voltage V.sub.3001 is overdrive
intensity in the first frame period, and
V.sub.3001=V.sub.OD3001-V.sub.S3001 is satisfied.
[0645] In the case of using (3) the method where a backlight
divided into areas sequentially blinks among the methods of
changing backlight luminance before and after the lighting ratio R
is changed, driving can be realized by changing backlight luminance
and the length and timing of the backlight lighting period, as
shown in FIG. 30A. A relation between the voltage written to each
pixel at this time and transmittance can be understood with
reference to FIG. 30B.
[0646] When the overdrive voltage V.sub.OD3001 is written to a
pixel by data writing scanning in the first frame period,
transmittance of a display element becomes transmittance
corresponding to the original voltage V.sub.S3001 at the time when
the next data is written by data writing scanning in the second
frame period or at the time close thereto. In that period, a
backlight lighting period is sequentially provided for each area.
Thus, integrated luminance in the top area in the first frame
period is represented by the area of the oblique line region
L.sub.3001, which is surrounded by the backlight lighting period
and the transmittance.
[0647] In the second frame period, the transmittance in the graph
of FIG. 30B is already desired transmittance before data is
written. At this time, the transmittance dose not change in the
backlight lighting period. Luminance which human eyes perceive in
the second frame period depends on the area of the oblique line
region L.sub.3002 in the second frame period.
[0648] At this time, it is important that luminance in the
backlight lighting periods varies in the first frame period and the
second frame period. That is, when the lighting ratio R is changed,
display can be performed without change in luminance of a pixel
which human eyes perceive by changing backlight luminance even in
the case where luminance of the pixel is not desired to be
changed.
[0649] Accordingly, in one of methods according to this document,
backlight luminance in the backlight lighting period is determined
by difference between the area of the oblique line region
L.sub.3001 in the first frame period and the area of the oblique
line region L.sub.3002 in the second frame period. Specifically,
when the lighting ratio R is changed and the backlight lighting
period in the second frame period is 1/X (X is a positive number)
of the backlight lighting period in the first frame period, it is
preferable that backlight luminance be X times as high as that in
the first frame period. Then, in one of the methods according to
this document, it is preferable that the original voltage
V.sub.S3001 in the first frame period be approximately the same in
the first frame period and the second frame period.
[0650] By driving a display device in such a manner, when the
lighting ratio R is changed, the original voltage V.sub.S3001 can
be approximately the same in the first frame period and the second
frame period even in the case where luminance of the pixel which
human eyes perceive is not desired to be changed. Thus, a structure
of a circuit which processes image data and is included in the
display device is simplified, so that manufacturing cost and power
consumption of the display device can be reduced. Further, in the
case where the same luminance is desired to be displayed when the
lighting ratio R is changed, voltage written to each pixel does not
have to be changed from that in the previous frame; thus, power
consumption in writing data can be reduced.
[0651] Note that overdrive voltage and overdrive intensity do not
have to be approximately the same in the first frame period and the
second frame period. This is because overdrive voltage and
overdrive intensity are obtained from original voltages and
transmittance in one frame and the previous frame; thus, when
original voltage and transmittance in each previous frame are
different in the first frame period and the second frame period,
various values are obtained as a matter of course.
[0652] FIG. 30C is a graph showing original voltage V.sub.S3011 and
overdrive voltage V.sub.OD3011 written to each pixel, and
transmittance with respect to each voltage on the same time axis in
a pixel connected to a scan line different from that shown in FIG.
30B when the lighting ratio R is different in the first frame
period and the second frame period. Here, the backlight lighting
periods in the first frame period and the second frame period are
similar to those in FIG. 30A. Each area of oblique line regions
L.sub.3011 and L.sub.3012 represents the level of luminance which
human eyes perceive (integrated luminance). Voltage V.sub.3011 is
overdrive intensity in the first frame period, and
V.sub.3011=V.sub.OD3011-V.sub.S3011 is satisfied.
[0653] Although details of a controlling method shown in FIG. 30C
are similar to those shown in FIG. 30B, the length of time from
when data is written to when a backlight lighting period starts is
different. Thus, each area of the oblique line regions L.sub.3011
and L.sub.3012, which represents integrated luminance, is different
from each area of the oblique line regions L.sub.3001 and
L.sub.3002 in FIG. 30B. Accordingly, the original voltage and the
overdrive voltage V may be changed in order to correct luminance
difference depending on a scan position. As a method for correcting
original voltage, the method shown in FIG. 23B can be used. As a
method for correcting overdrive voltage, the method shown in FIG.
23D can be used. Thus, color shading and motion blur in displaying
a moving image can be reduced.
[0654] Note that the driving methods shown in FIGS. 30B and 30C are
similar in other areas. At this time, the amount of correction of
the original voltage and the overdrive voltage in a pixel to which
data is written at the end of each area is made to be the largest
in the area to which the pixel belongs, so that sharp luminance
difference at a boundary of areas can be corrected.
[0655] Next, the case of a combination of (2) the method where the
whole backlight blinks and (1) the method where blanking data is
directly written to each pixel among the methods of changing
backlight luminance is described with reference to FIGS. 31A to
31C.
[0656] FIG. 31A is a graph showing timing of writing data, timing
of writing blank data, and timing of blinking a backlight on the
same time axis with respect to a position of a scan line when the
lighting ratio R is different in the first frame period and the
second frame period. Here, for explanation, a backlight lighting
period in the first frame period is denoted by .tau..sub.a3101, and
a backlight lighting period in the second frame period is denoted
by .tau..sub.a3102.
[0657] FIG. 31B is a graph showing original voltage V.sub.S3101 and
overdrive voltages V.sub.OD3101 and V.sub.OD3102 written to each
pixel, and transmittance with respect to each voltage on the same
time axis when the lighting ratio R is different in the first frame
period and the second frame period. Here, the backlight lighting
periods in the first frame period and the second frame period are
similar to those in FIG. 31A. Each area of oblique line regions
L.sub.3101 and L.sub.3102 represents the level of luminance which
human eyes perceive (integrated luminance). Each of voltage
V.sub.3101 and voltage V.sub.3102 is overdrive intensity in image
display periods in the first frame period and the second frame
period, and V.sub.3101=V.sub.OD3101-V.sub.S3101 and
V.sub.3102=V.sub.OD3102-V.sub.S3101 are satisfied.
[0658] In the case of the combination of (2) the method where the
whole backlight blinks and (1) the method where blanking data is
directly written to each pixel among the methods of changing
backlight luminance before and after the lighting ratio R is
changed, driving can be realized by changing backlight luminance
and the length and timing of the backlight lighting period, and
performing blanking writing scanning in addition to data writing
scanning, as shown in FIG. 31A. Here, although the case is shown in
which data writing scanning and blanking writing scanning are
performed at the same timing in each frame period, a driving method
according to this document is not limited thereto, and various
types of writing timing can be used. For example, data writing
scanning may be changed in accordance with the lighting ratio R. As
a method where data writing scanning is changed in accordance with
the lighting ratio R, the length of time from blanking writing
scanning to data writing scanning in the same frame period may be
increased as the lighting ratio R is decreased. A relation between
the voltage written to each pixel at this time and transmittance
can be understood with reference to FIG. 31B.
[0659] When the overdrive voltage V.sub.OD3101 is written to a
pixel by data writing scanning in the first frame period,
transmittance of a display element becomes transmittance
corresponding to the original voltage V.sub.S3101 at the time when
the next data is written by blanking writing scanning in the second
frame period or at the time close thereto. In that period, a
backlight lighting period is provided in all pixels all at once.
Thus, integrated luminance in the first frame period is represented
by the area of the oblique line region L.sub.3101, which is
surrounded by the backlight lighting period and the
transmittance.
[0660] Then, when the overdrive voltage V.sub.OD3102 is written to
the pixel by data writing scanning in the second frame period after
blanking writing scanning in the second frame period, the
transmittance of the display element becomes transmittance
corresponding to the original voltage V.sub.S3101 at the time when
the next data is written by blanking writing scanning in the next
frame period or at the time close thereto. In that period, a
backlight lighting period is provided in all the pixel all at once.
Thus, integrated luminance in the second frame period is
represented by the area of the oblique line region L.sub.3102,
which is surrounded by the backlight lighting period and the
transmittance.
[0661] At this time, it is important that luminance in the
backlight lighting periods varies in the first frame period and the
second frame period. That is, when the lighting ratio R is changed,
display can be performed without change in luminance of a pixel
which human eyes perceive by changing backlight luminance even in
the case where luminance of the pixel is not desired to be
changed.
[0662] Accordingly, in one of methods according to this document,
backlight luminance in the backlight lighting period is determined
by difference between the area of the oblique line region
L.sub.3101 in the first frame period and the area of the oblique
line region L.sub.3102 in the second frame period. Specifically,
when the lighting ratio R is changed and the backlight lighting
period in the second frame period is 1/X (X is a positive number)
of the backlight lighting period in the first frame period, it is
preferable that backlight luminance be X times as high as that in
the first frame period. Then, in one of the methods according to
this document, it is preferable that the original voltage
V.sub.S3101 in the first frame period be approximately the same in
the first frame period and the second frame period.
[0663] By driving a display device in such a manner, the original
voltage V.sub.S3101 can be the same in the first frame period and
the second frame period even in the case where luminance of the
pixel which human eyes perceive is not desired to be changed when
the lighting ratio R is changed. Thus, a structure of a circuit
which processes image data, which is included in the display
device, is simplified, so that manufacturing cost and power
consumption of the display device can be reduced. Further, since
blanking writing is performed in a period other than the backlight
lighting period, light leakage in a non-lighting period of the
backlight can be reduced. Thus, black blurring in displaying an
image can be reduced, so that a contrast ratio of the display
device can be improved.
[0664] Note that overdrive voltage and overdrive intensity do not
have to be approximately the same in the first frame period and the
second frame period. This is because overdrive voltage and
overdrive intensity are obtained from original voltages and
transmittance in one frame and the previous frame; thus, when
original voltage and transmittance in each previous frame are
different in the first frame period and the second frame period,
various values are obtained as a matter of course.
[0665] FIG. 31C is a graph showing original voltage V.sub.S3111 and
overdrive voltages V.sub.OD3111 and V.sub.OD3111 written to each
pixel, and transmittance with respect to each voltage on the same
time axis in a pixel connected to a scan line different from that
shown in FIG. 31B when the lighting ratio R is different in the
first frame period and the second frame period. Here, the backlight
lighting periods in the first frame period and the second frame
period are similar to those in FIG. 31A. Each area of oblique line
regions L.sub.3111 and L.sub.3112 represents the level of luminance
which human eyes perceive (integrated luminance). Each of voltage
V.sub.3111 and voltage V.sub.3112 is overdrive intensity in the
first frame period and the second frame period, and
V.sub.3111=V.sub.OD3111-V.sub.S3111 and V.sub.3112
V.sub.OD3112-V.sub.S3111 are satisfied.
[0666] Although details of a controlling method shown in FIG. 31C
are similar to those shown in FIG. 31B, the length of time from
when data is written to when a backlight lighting period starts is
different. Thus, each area of the oblique line regions L.sub.3111
and L.sub.3112, which represents integrated luminance, is different
from each area of the oblique line regions L.sub.3101 and
L.sub.3102 in FIG. 31B. Accordingly, the original voltage and the
overdrive voltage V may be changed in order to correct luminance
difference depending on a scan position. As a method for correcting
original voltage, the method shown in FIG. 23B can be used. As a
method for correcting overdrive voltage, the method shown in FIG.
23D can be used. Thus, color shading and motion blur in displaying
a moving image can be reduced.
[0667] Next, the case of a combination of (3) the method where a
backlight divided into areas sequentially blinks and (1) the method
where blanking data is directly written to each pixel among the
methods of changing backlight luminance is described with reference
to FIGS. 32A to 32C.
[0668] FIG. 32A is a graph showing timing of writing data, timing
of writing blank data, and timing of sequentially blinking a
backlight on the same time axis with respect to a position of a
scan line when the lighting ratio R is different in the first frame
period and the second frame period. Here, for explanation, a
backlight lighting period in the first frame period is denoted by
.tau..sub.a3201, and a backlight lighting period in the second
frame period is denoted by .tau..sub.a3202.
[0669] FIG. 32B is a graph showing original voltage V.sub.S3201 and
overdrive voltages V.sub.OD3201 and V.sub.OD3202 written to each
pixel, and transmittance with respect to each voltage on the same
time axis when the lighting ratio R is different in the first frame
period and the second frame period. Here, the backlight lighting
periods in the first frame period and the second frame period are
similar to those in FIG. 32A. Each area of oblique line regions
L.sub.3201 and L.sub.3202 represents the level of luminance which
human eyes perceive (integrated luminance). Each of voltage
V.sub.3201 and voltage V.sub.3202 is overdrive intensity in image
display periods in the first frame period and the second frame
period, and V.sub.3201=V.sub.OD3201-V.sub.S3201 and
V.sub.3202=V.sub.OD3202 -V.sub.S3201 are satisfied.
[0670] In the case of the combination of (3) the method where a
backlight divided into areas sequentially blinks and (1) the method
where blanking data is directly written to each pixel among the
methods of changing backlight luminance before and after the
lighting ratio R is changed, driving can be realized by changing
backlight luminance and the length and timing of the backlight
lighting period, and performing blanking writing scanning in
addition to data writing scanning, as shown in FIG. 32A. Here,
although the case is shown in which data writing scanning and
blanking writing scanning are performed at the same timing in each
frame period, a driving method according to this document is not
limited thereto, and various types of writing timing can be used.
For example, data writing scanning may be changed in accordance
with the lighting ratio R. As a method where data writing scanning
is changed in accordance with the lighting ratio R, the length of
time from blanking writing scanning to data writing scanning in the
same frame period may be increased as the lighting ratio R is
decreased. A relation between the voltage written to each pixel at
this time and transmittance can be understood with reference to
FIG. 32B. In addition, although timing of data writing scanning and
blanking writing scanning does not overlap with the backlight
lighting period in the graph of FIG. 32A, a method according to
this document is not limited thereto, and the timing thereof may
overlap with the backlight lighting period. For example, writing
scanning and blanking scanning at all scan positions may overlap
with the backlight lighting periods. In this case, the backlight is
already lit when data is written or at the time close thereto, and
at or around the time when blanking data is written, a blanking
interval starts even when the backlight is lit. Accordingly, time
from when writing is performed to when the backlight lighting
period starts is the same at all the scan positions, so that
luminance difference of pixels depending on a scan position
disappears, and luminance unevenness in displaying an image can be
reduced. Further, since a period when the backlight is not lit is
in the blanking interval, light leakage in the blanking interval
can be reduced. Thus, black blurring in displaying an image can be
reduced, so that a contrast ratio of the display device can be
improved. Moreover, the length of the blanking interval .tau..sub.b
can be controlled by changing timing of blanking writing, instead
of changing a state of sequential scanning of the backlight so that
the length of the backlight lighting period is changed. At this
time, since timing of blanking writing can be changed in each one
gate selection period, the length of the blanking interval
.tau..sub.b can be finely adjusted, and the degree of reduction in
motion blur can be finely changed. Accordingly, the lighting ratio
R depending on the control parameters P and Q can be further
optimally controlled.
[0671] When the overdrive voltage V.sub.OD3201 is written to a
pixel by data writing scanning in the first frame period,
transmittance of a display element becomes transmittance
corresponding to the original voltage V.sub.S3201 at the time when
the next data is written by blanking writing scanning in the second
frame period or at the time close thereto. In that period, a
backlight lighting period is sequentially provided for each area.
Thus, integrated luminance in the top area in the first frame
period is represented by the area of the oblique line region
L.sub.3201, which is surrounded by the backlight lighting period
and the transmittance.
[0672] Then, when the overdrive voltage V.sub.OD3202 is written to
the pixel by data writing scanning in the second frame period after
blanking writing scanning in the second frame period, the
transmittance of the display element becomes transmittance
corresponding to the original voltage V.sub.S3201 at the time when
the next data is written by blanking writing scanning in the next
frame period or at the time close thereto. In that period, a
backlight lighting period is sequentially provided for each area.
Thus, integrated luminance in the top area in the second frame
period is represented by the area of the oblique line region
L.sub.3202, which is surrounded by the backlight lighting period
and the transmittance.
[0673] At this time, it is important that luminance in the
backlight lighting periods varies in the first frame period and the
second frame period. That is, when the lighting ratio R is changed,
display can be performed without change in luminance of a pixel
which human eyes perceive by changing backlight luminance even in
the case where luminance of the pixel is not desired to be
changed.
[0674] Accordingly, in one of methods according to this document,
backlight luminance in a backlight lighting period is determined by
difference between the area of the oblique line region L.sub.3201
in the first frame period and the area of the oblique line region
L.sub.3202 in the second frame period. Specifically, when the
lighting ratio R is changed and the backlight lighting period in
the second frame period is 1/X (X is a positive number) of the
backlight lighting period in the first frame period, it is
preferable that backlight luminance be X times as high as that in
the first frame period. Then, in one of the methods according to
this document, it is preferable that the original voltage
V.sub.S3201 in the first frame period be approximately the same in
the first frame period and the second frame period.
[0675] By driving a display device in such a manner, the original
voltage V.sub.S3201 can be the same in the first frame period and
the second frame period even in the case where luminance of the
pixel which human eyes perceive is not desired to be changed when
the lighting ratio R is changed. Thus, a structure of a circuit
which processes image data, which is included in the display
device, is simplified, so that manufacturing cost and power
consumption of the display device can be reduced. Further, since
blanking writing is performed in a period other than the backlight
lighting period, light leakage in a non-lighting period of the
backlight can be reduced. Thus, black blurring in displaying an
image can be reduced, so that a contrast ratio of the display
device can be improved.
[0676] Note that overdrive voltage and overdrive intensity do not
have to be approximately the same in the first frame period and the
second frame period. This is because overdrive voltage and
overdrive intensity are obtained from original voltages and
transmittance in one frame and the previous frame; thus, when
original voltage and transmittance in each previous frame are
different in the first frame period and the second frame period,
various values are obtained as a matter of course.
[0677] FIG. 32C is a graph showing original voltage V.sub.S3211 and
overdrive voltages V.sub.OD3211 and V.sub.OD3212 written to each
pixel, and transmittance with respect to each voltage on the same
time axis in a pixel connected to a scan line different from that
shown in FIG. 32B when the lighting ratio R is different in the
first frame period and the second frame period. Here, the backlight
lighting periods in the first frame period and the second frame
period are similar to those in FIG. 32A. Each area of oblique line
regions L.sub.3211 and L.sub.3212 represents the level of luminance
which human eyes perceive (integrated luminance). Each of voltage
V.sub.3211 and voltage V.sub.3212 is overdrive intensity in the
first frame period and the second frame period, and
V.sub.3211=V.sub.OD3211-V.sub.S3211 and
V.sub.3212=V.sub.OD3212-V.sub.S3211 are satisfied.
[0678] Although details of a controlling method shown in FIG. 32C
are similar to those shown in FIG. 32B, the length of time from
when data is written to when a backlight lighting period starts is
different. Thus, each area of the oblique line regions L.sub.3211
and L.sub.3212, which represents integrated luminance, is different
from each area of the oblique line regions L.sub.3201 and
L.sub.3202 in FIG. 32B. Accordingly, the original voltage and the
overdrive voltage V may be changed in order to correct luminance
difference depending on a scan position. As a method for correcting
original voltage, the method shown in FIG. 23B can be used. As a
method for correcting overdrive voltage, the method shown in FIG.
23D can be used. Thus, color shading and motion blur in displaying
a moving image can be reduced.
[0679] Note that the driving methods shown in FIGS. 32B and 32C are
similar in other areas. At this time, the amount of correction of
the original voltage and the overdrive voltage in a pixel to which
data is written at the end of each area is made to be the largest
in the area to which the pixel belongs, so that sharp luminance
difference at a boundary of areas can be corrected.
[0680] At least one of the methods of driving a display device
according to this document can be used when a pixel provided in the
display device includes a plurality of subpixels. At this time,
reduction in display quality, such as motion blur, can be further
reduced by driving with the lighting ratio R different in each
subpixel.
[0681] When a pixel includes a plurality of subpixels, a function
of the pixel can be extended, and properties of a display device
can be improved. For example, the number of gray scales which the
pixel can display can be increased by changing luminance in each
subpixel and combining such luminance (i.e., area gray scale). In
addition, when a display element is a liquid crystal element, there
are problems such as reduction in contrast of display, color shift,
and luminance inversion depending on an angle at which a display
portion of the display device is seen (i.e., a narrow viewing
angle). When the pixel includes a plurality of subpixels and
voltages slightly different from each other are applied to each
subpixel, a viewing angle of the display device can be increased.
Accordingly, various beneficial effects can be obtained by a
structure where each pixel provided in the display device includes
a plurality of subpixels, and properties of the display device can
be further improved by using the method described in this
embodiment mode.
[0682] An example of a pixel including a plurality of subpixels is
described with reference to FIG. 33A. A pixel 3350 shown in FIG.
33A includes a first subpixel 3351 and a second subpixel 3352.
Here, the first subpixel 3351 and the second subpixel 3352 are also
referred to as a subpixel I and a subpixel II.
[0683] A plurality of wirings are connected to the first subpixel
3351 and the second subpixel 3352, and various connection methods
can be used. As a structure example of wirings connected to a
plurality of subpixels, a structure shown in FIG. 33A can be used,
for example. In the structure shown in FIG. 33A, a data line DATA
which is a signal line for transmitting a data signal is connected
to the plurality of subpixels in common. Further, scan lines
GATEI.sub.n and GATEII.sub.n which are signal lines for selecting
the subpixel I and the subpixel II are separately connected to
respective subpixels. Here, n is a positive integer representing
the number of scan lines.
[0684] For a pixel structure, various structures other than the
structure shown in FIG. 33A can be used. For example, the data
lines DATA may be separately connected to a plurality of subpixels,
and a scan line GATE may be connected to the plurality of subpixels
in common. Alternatively, both the data lines DATA and the scan
lines GATE may be separately connected to a plurality of subpixels.
Here, description of structures other than the structure shown in
FIG. 33A is omitted.
[0685] Note that the structures shown in FIGS. 9G and 9H can be
used for the inside of the first subpixel 3351 and the second
subpixel 3352.
[0686] As a method where a data signal is written to each subpixel,
sequential scanning is usually performed. That is, GATEI.sub.1,
GATEII.sub.1, GATEI.sub.2, and GATEII.sub.2 are sequentially
selected, GATEI.sub.X and GATEII.sub.X are selected, and scanning
finishes. Here, X represents the number of pixels in a
perpendicular direction. This sequential scanning may be performed
when writing scanning and blanking scanning are performed.
[0687] When writing scanning and blanking scanning are performed by
a scanning method shown in FIG. 33B, driving with the lighting
ratio R different in each subpixel can be realized.
[0688] FIG. 33B is a timing chart with a horizontal axis
representing time and a vertical axis representing voltage with
respect to each signal line. The data line DATA represents voltage
written to a pixel. The scan lines GATEI.sub.n and GATEII.sub.n
represent a non-selected state when at low level and a selected
state when at high level.
[0689] In the scanning method shown in FIG. 33B, one gate selection
period is divided into two periods, and the first half of one gate
selection period represents a period in which a data signal is
written to a pixel and the latter half thereof represents a period
in which blanking data is written. In the first half of one gate
selection period, a data signal is written to each pixel by
sequentially scanning scan lines, whereas in the latter half of one
gate selection period, the scan lines may be scanned with timing
depending on the lighting ratio R of each subpixel without
sequential scanning of the scan lines.
[0690] Specifically, after a data signal is written to GATEII.sub.1
a data signal is written to GATEII.sub.1 in the first half of the
next gate selection period. Next, GATEI.sub.2 and GATEII.sub.2 are
sequentially selected and scanned. Then, at the time when an image
display period of GATEI.sub.1 ends, blanking data is written to
GATEI.sub.1 in the latter half of the gate selection period. Then,
at the time when an image display period of GATEII.sub.1 ends,
blanking data is written to GATEII.sub.1 in the latter half of the
gate selection period. In such a manner, writing scanning is
sequentially performed and temporally-discrete blanking scanning is
performed on each subpixel, so that driving with the lighting ratio
R different in each subpixel can be realized. Further, an image
display period .tau..sub.a3301 of the scan line GATEI.sub.n at this
time is a period from writing scanning to blanking scanning, and a
blanking interval .tau..sub.b3301 is a period from blanking
scanning to writing scanning in the next frame. Similarly, an image
display period .tau..sub.a3311 of the scan line GATEII.sub.n is a
period from writing scanning to blanking scanning, and a blanking
interval .tau..sub.b3311 is a period from blanking scanning to
writing scanning in the next frame.
[0691] Here, a data signal is written to a pixel in the first half
of one gate selection period, and blanking data is written to the
pixel in the latter half thereof; on the contrary, blanking data
may be written to a pixel in the first half of one gate selection
period and a data signal may be written to the pixel in the latter
half thereof.
[0692] Voltage V.sub.blank of blanking data may vary in a period
when blanking data is written to the subpixel I and a period when
blanking data is written to the subpixel II. Accordingly, luminance
of a pixel in the blanking interval may freely vary in each
subpixel.
[0693] In particular, a method where the lighting ratio R can vary
in each subpixel is beneficial to a display device in which a
viewing angle is increased by displaying a bright image in one of
subpixels and a dark image in the other of the subpixels. This is
because an effect of reducing motion blur can be obtained in a
bright pixel in which motion blur is likely to be seen and a gray
scale on the lower gray scale level can be sufficiently displayed
in a dark pixel in which a gray scale on the lower gray scale level
is likely to be damaged, by reducing the lighting ratio R in a
subpixel for displaying a bright image and increasing the lighting
ratio R in a subpixel for displaying a dark image.
[0694] As an example where the lighting ratio R freely varies in
each subpixel, the length of image display periods .tau..sub.a3401
and .tau..sub.a3402 of the subpixel I can be different from the
length of image display periods .tau..sub.a3411 and .tau..sub.a3412
of the subpixel II, as shown in FIGS. 34A and 34B. Accordingly, an
effect of reducing motion blur can be obtained in a bright pixel in
which motion blur is likely to be seen and a gray scale on the
lower gray scale level can be sufficiently displayed in a dark
pixel in which a gray scale on the lower gray scale level is likely
to be damaged.
[0695] FIG. 34A is a graph showing timing of writing data and
timing of writing blanking date in the first frame period and the
second frame period on the same time axis with respect to a
position of a scan line. The image display periods of the subpixel
I in the first frame period and the second frame period are denoted
by .tau..sub.a3401 and .tau..sub.a3402. Blanking intervals of the
subpixel I in the first frame period and the second frame period
are denoted by .tau..sub.b3401 and .tau..sub.b3402. The image
display periods of the subpixel II in the first frame period and
the second frame period are denoted by .tau..sub.a3411 and
.tau..sub.a3412. Blanking intervals of the subpixel II in the first
frame period and the second frame period are denoted by
.tau..sub.b3411 and .tau..sub.b3412.
[0696] FIG. 34BI is a graph showing original voltage V.sub.S3401
and overdrive voltages V.sub.OD3401 and V.sub.OD3402 written to
each pixel, and transmittance with respect to each voltage in the
first frame period and the second frame period on the same time
axis. Here, the image display periods and the blanking intervals in
the first frame period and the second frame period are similar to
those in FIG. 34A. Each of voltage V.sub.3401 and voltage
V.sub.3402 is overdrive intensity in the image display periods in
the first frame period and the second frame period, and
V.sub.3401=V.sub.OD3401-V.sub.S3401 and
V.sub.3402=V.sub.OD3402-V.sub.S3401 are satisfied.
[0697] FIG. 34BII is a graph showing original voltage V.sub.S3411
and overdrive voltages V.sub.OD3411 and V.sub.OD3412 written to
each pixel, and transmittance with respect to each voltage in the
first frame period and the second frame period on the same time
axis. Here, the image display periods and the blanking intervals in
the first frame period and the second frame period are similar to
those in FIG. 34A. Each of voltage V.sub.3411 and voltage
V.sub.3412 is overdrive intensity in the image display periods in
the first frame period and the second frame period, and
V.sub.3411=V.sub.OD3411-V.sub.S3411 and
V.sub.3412=V.sub.OD3412-V.sub.S3411 are satisfied.
[0698] As shown in FIG. 34B, when the lighting ratio R varies in
the subpixel I and the subpixel II, it is preferable to reduce the
overdrive intensity in each frame as the lighting ratio R
increases. This is because increase in the lighting ratio R means
increase in length of the image display period .tau..sub.a, and
increase in length of the image display period .tau..sub.a can be
allowed to have a longer period of time for reaching intended
transmittance of a liquid crystal element. Moreover, when the
length of the image display period .tau..sub.a is increased,
intended transmittance of a liquid crystal element itself can be
reduced, so that the original voltage V.sub.S is reduced, and
further, the overdrive intensity can be reduced.
[0699] Note that transmittance at or around the time when each
frame ends changes depending on the length of the blanking
interval. Specifically, the transmittance at or around the time
when each frame ends increases as the blanking interval is reduced.
Thus, it is preferable to further reduce overdrive intensity of one
frame as the blanking interval of the previous frame is
shorter.
[0700] In addition, difference in the lighting ratio R of the
subpixels I and II is preferably determined in accordance with the
control parameter P. Specifically, it is preferable to increase
difference in the lighting ratio R of the subpixels I and II as the
control parameter P increases. This is because an effect of
reducing motion blur can be obtained in a bright pixel in which
motion blur is likely to be seen, whereas a gray scale on the lower
gray scale level can be sufficiently displayed in a dark pixel in
which a gray scale on the lower gray scale level is likely to be
damaged.
[0701] Other examples of a method where the lighting ratio R can
vary between subpixels include a method where the lighting ratio R
is changed in one of subpixels and not changed in the other of the
subpixels in accordance with the magnitude of the control
parameters P and Q (see FIGS. 35A and 35B), and a method where the
lighting ratio R is changed in one of subpixels and is also changed
in the other of the subpixels in accordance with the magnitude of
the control parameters P and Q (see FIGS. 36A and 36B). Thus, an
optimal driving method in accordance with a state of an image can
be set. Specifically, since a bright subpixel can increase the
whole luminance and has a property that motion blur is likely to be
seen, it is preferable to reduce the lighting ratio R as the
control parameter P increases. Since a dark subpixel cannot
sufficiently display a gray scale on the lower gray scale level and
has a property that motion blur is not likely to be seen, motion
blur is hardly seen even when the control parameter P increases.
Thus, when the control parameter P increases, the lighting ratio R
can be increased. By increasing the lighting ratio R, a gray scale
on the lower gray scale level can be sufficiently displayed in a
dark pixel in which a gray scale on the lower gray scale level is
likely to be damaged. Accordingly, it is very beneficial to
optimally control the lighting ratio R with respect to the control
parameter P depending on properties of each subpixel.
[0702] Note that optimal driving can also be realized by changing
backlight luminance at this time. For example, when the level of a
data signal written to a pixel is the same, luminance which human
eyes perceive becomes lower as the image display period .tau..sub.a
becomes shorter and the blanking interval .tau..sub.b becomes
longer Accordingly, in accordance with the length of the image
display period .tau..sub.a and the length of the blanking interval
.tau..sub.b (i.e., the lighting ratio R), the backlight luminance
is reduced when the lighting ratio R is high, whereas the backlight
luminance is increased when the lighting ratio R is low; thus,
luminance which human eyes perceive can be constant. Further, the
lighting ratio R preferably depends on the control parameters P and
Q described in Embodiment Mode 3. This is because the lighting
ratio R can be controlled as appropriate by perceivability of
motion blur in an image to be displayed.
[0703] FIG. 35A is a graph showing timing of writing data and
timing of writing blanking data in the first frame period and the
second frame period on the same time axis with respect to a
position of a scan line. Image display periods of the subpixel I in
the first frame period and the second frame period are denoted by
.tau..sub.a3501 and .tau..sub.a3502. Blanking intervals of the
subpixel I in the first frame period and the second frame period
are denoted by .tau..sub.b3501 and 96 .sub.b3502. Image display
periods of the subpixel II in the first frame period and the second
frame period are denoted by .tau..sub.a3511 and .tau..sub.a3512.
Blanking intervals of the subpixel II in the first frame period and
the second frame period are denoted by .tau..sub.b3511 and
.tau..sub.b3512.
[0704] FIG. 35BI is a graph showing original voltages V.sub.S3501
and V.sub.S3502 and overdrive voltages V.sub.OD3501 and
V.sub.OD3502 written to each pixel, and transmittance with respect
to each voltage on the same time axis when the lighting ratio R is
different in the first frame period and the second frame period.
Here, image display periods and blanking intervals in the first
frame period and the second frame period are similar to those in
FIG. 35A. Each area of oblique line regions L.sub.3501 and
L.sub.3502 represents the level of luminance which human eyes
perceive (integrated luminance). Each of voltage V.sub.3501 and
voltage V.sub.3502 is overdrive intensity in the image display
periods in the first frame period and the second frame period, and
V.sub.3501=V.sub.OD3501-V.sub.S3501 and
V.sub.3502=V.sub.OD3502-V.sub.S3502 are satisfied.
[0705] FIG. 35BII is a graph showing original voltage V.sub.S3511
and overdrive voltages V.sub.OD3511 and V.sub.OD3512 written to
each pixel, and transmittance with respect to each voltage in the
first frame period and the second frame period on the same time
axis. Here, image display periods and blanking intervals in the
first frame period and the second frame period are similar to those
in FIG. 35A. Each of voltage V.sub.3511 and voltage V.sub.3512 is
overdrive intensity in the image display periods in the first frame
period and the second frame period, and
V.sub.3511=V.sub.OD3511-V.sub.S3511 and
V.sub.3512=V.sub.OD3512-V.sub.S3511 are satisfied.
[0706] In FIG. 35BI, the area of the oblique line region L.sub.3501
and the area of the oblique line region L.sub.3502 are made
approximately the same by controlling the original voltage and the
overdrive voltage as appropriate, so that luminance which human
eyes perceive can be approximately the same even when the lighting
ratio R is different.
[0707] FIG. 36A is a graph showing timing of writing data and
timing of writing blanking data in the first frame period and the
second frame period on the same time axis with respect to a
position of a scan line. Image display periods of the subpixel I in
the first frame period and the second frame period are denoted by
.tau..sub.a3601 and .tau..sub.a3602. Blanking intervals of the
subpixel I in the first frame period and the second frame period
are denoted by .tau..sub.b3601 and .tau..sub.b3602. Image display
periods of the subpixel II in the first frame period and the second
frame period are denoted by .tau..sub.a3611 and .tau..sub.a3612.
Blanking intervals of the subpixel II in the first frame period and
the second frame period are denoted by .tau..sub.b3611 and
.tau..sub.b3612.
[0708] FIG. 36BI is a graph showing original voltages V.sub.S3601
and V.sub.S3602 and overdrive voltages V.sub.OD3601 and
V.sub.OD3602 written to each pixel, and transmittance with respect
to each voltage on the same time axis when the lighting ratio R is
different in the first frame period and the second frame period.
Here, image display periods and blanking intervals in the first
frame period and the second frame period are similar to those in
FIG. 36A. Each area of oblique line regions L.sub.3601 and
L.sub.3602 represents the level of luminance which human eyes
perceive (integrated luminance). Each of voltage V.sub.3601 and
voltage V.sub.3602 is overdrive intensity in the image display
periods in the first frame period and the second frame period, and
V.sub.3601=V.sub.OD3601-V.sub.S3601 and
V.sub.3602=V.sub.OD3602-V.sub.S3602 are satisfied.
[0709] FIG. 36BII is a graph showing original voltages V.sub.S3611
and V.sub.S3612 and overdrive voltages V.sub.OD3611 and
V.sub.OD3612 written to each pixel, and transmittance with respect
to each voltage on the same time axis when the lighting ratio R is
different in the first frame period and the second frame period.
Here, backlight lighting periods and blanking intervals in the
first frame period and the second frame period are similar to those
in FIG. 36A. Each area of oblique line regions L.sub.3611 and
L.sub.3612 represents the level of luminance which human eyes
perceive (integrated luminance). Each of voltage V.sub.3611 and
voltage V.sub.3612 is overdrive intensity in image display periods
in the first frame period and the second frame period, and
V.sub.3611=V.sub.OD3611-V.sub.S3611 and V.sub.3612=V.sub.OD3612
-V.sub.S3612 are satisfied.
[0710] In FIG. 36BI, the area of the oblique line region L.sub.3601
and the area of the oblique line region L.sub.3602 are made
approximately the same by controlling the original voltage and the
overdrive voltage as appropriate, so that luminance which human
eyes perceive can be approximately the same even when the lighting
ratio R is different.
[0711] In FIG. 36BII also, the area of the oblique line region
L.sub.3611 and the area of the oblique line region L.sub.3612 are
made approximately the same by controlling the original voltage and
the overdrive voltage as appropriate, so that luminance which human
eyes perceive can be approximately the same even when the lighting
ratio R is different.
[0712] Note that the methods shown in FIGS. 35A, 35B, 36A, and 36B,
it is effective to combine the method where the lighting ratio R
freely varies between subpixels with the control parameter P
described in another embodiment mode. For example, by increasing
difference between the lighting ratios R of the subpixels as the
control parameter P increases, an effect of reducing motion blur
can be obtained in a bright pixel in which motion blur is likely to
be seen and a gray scale on the lower gray scale level can be
sufficiently displayed in a dark pixel in which a gray scale on the
lower gray scale level is likely to be damaged.
[0713] Although this embodiment mode is described with reference to
various drawings, the contents (or part of the contents) described
in each drawing can be freely applied to, combined with, or
replaced with the contents (or part of the contents) described in
another drawing. Further, much more drawings can be formed by
combining each part with another part in the above-described
drawings.
[0714] The contents (or part of the contents) described in each
drawing in this embodiment mode can be freely applied to, combined
with, or replaced with the contents (or part of the contents)
described in a drawing in another embodiment mode. Further, much
more drawings can be formed by combining each part in each drawing
in this embodiment mode with part of another embodiment mode.
[0715] This embodiment mode shows examples of embodying, slightly
transforming, partially modifying, improving, describing in detail,
or applying the contents (or part of the contents) described in
other embodiment modes, an example of related part thereof, or the
like. Therefore, the contents described in other embodiment modes
can be freely applied to, combined with, or replaced with this
embodiment mode.
Embodiment Mode 5
[0716] In this embodiment mode, a pixel structure of a display
device is described. In particular, a pixel structure of a liquid
crystal display device is described.
[0717] A pixel structure in the case where each liquid crystal mode
and a transistor are combined is described with reference to
cross-sectional views of a pixel.
[0718] Note that as the transistor, a thin film transistor (a TFT)
or the like including a non-single crystalline semiconductor layer
typified by amorphous silicon, polycrystalline silicon, micro
crystalline (also referred to as semi-amorphous) silicon, or the
like can be used.
[0719] As a structure of the transistor, a top-gate structure, a
bottom-gate structure, or the like can be used. Note that a
channel-etched transistor, a channel-protective transistor, or the
like can be used as a bottom-gate transistor.
[0720] FIG. 37 is an example of a cross-sectional view of a pixel
in the case where a TN mode and a transistor are combined. By
applying the pixel structure shown in FIG. 37 to a liquid crystal
display device, a liquid crystal display device can be formed at
low cost.
[0721] Features of the pixel structure shown in FIG. 37 are
described. Liquid crystal molecules 10118 shown in FIG. 37 are long
and narrow molecules each having a major axis and a minor axis. In
FIG. 37, a direction of each of the liquid crystal molecules 10118
is expressed by the length thereof. That is, the direction of the
major axis of the liquid crystal molecule 10118, which is expressed
as long, is parallel to the page, and as the liquid crystal
molecule 10118 is expressed to be shorter, the direction of the
major axis becomes closer to a normal direction of the page. That
is, among the liquid crystal molecules 10118 shown in FIG. 37, the
direction of the major axis of the liquid crystal molecule 10118
which is close to the first substrate 10101 and the direction of
the major axis of the liquid crystal molecule 10118 which is close
to the second substrate 10116 are different from each other by 90
degrees, and the directions of the major axes of the liquid crystal
molecules 10118 located therebetween are arranged so as to link the
above two directions smoothly. That is, the liquid crystal
molecules 10118 shown in FIG. 37 are aligned to be twisted by 90
degrees between the first substrate 10101 and the second substrate
10116.
[0722] Here, the case is described in which a bottom-gate
transistor using an amorphous semiconductor is used as the
transistor In the case where a transistor using an amorphous
semiconductor is used, a liquid crystal display device can be
formed at low cost by using a large substrate.
[0723] A liquid crystal display device includes a basic portion
displaying images, which is called a liquid crystal panel. The
liquid crystal panel is manufactured as follows: two processed
substrates are attached to each other with a gap of several .mu.m
therebetween, and a liquid crystal material is injected into a
space between the two substrates. In FIG. 37, the two substrates
correspond to the first substrate 10101 and the second substrate
10116. A transistor and a pixel electrode are formed over the first
substrate. A light-shielding film 10114, a color filter 10115, a
fourth conductive layer 10113, a spacer 10117, and a second
alignment film 10112 are formed on the second substrate.
[0724] The light-shielding film 10114 is not necessarily formed on
the second substrate 10116. When the light-shielding film 10114 is
not formed, the number of steps is reduced, so that manufacturing
cost can be reduced. In addition, since a structure is simple,
yield can be improved. On the other hand, when the light-shielding
film 10114 is formed, a display device with little light leakage at
the time of black display can be obtained.
[0725] The color filter 10115 is not necessarily formed on the
second substrate 10116. When the color filter 10115 is not formed,
the number of steps is reduced, so that manufacturing cost can be
reduced. In addition, since a structure is simple, yield can be
improved. Note that even when the color filter 10115 is not formed,
a display device which can perform color display can be obtained by
field sequential driving. On the other hand, needless to say, when
the color filter 10115 is formed, a display device which can
perform color display can be obtained.
[0726] Spherical spacers may be dispersed on the second substrate
10116 instead of forming the spacer 10117. When the spherical
spacers are dispersed, the number of steps is reduced, so that
manufacturing cost can be reduced. In addition, since a structure
is simple, yield can be improved. On the other hand, when the
spacer 10117 is formed, a distance between the two substrates can
be uniform because a position of the spacer is not varied, so that
a display device with little display unevenness can be
obtained.
[0727] Subsequently, a process to be performed to the first
substrate 10101 is described.
[0728] First, a first insulating film 10102 is formed over the
first substrate 10101 by sputtering, a printing method, a coating
method, or the like. The first insulating film 10102 has a function
of preventing change in characteristics of the transistor due to an
impurity from the substrate which affects a semiconductor layer.
Note that the first insulating film 10102 is not necessarily
formed.
[0729] Next, a first conductive layer 10103 is formed over the
first insulating film 10102 by photolithography, a laser direct
writing method, an inkjet method, or the like.
[0730] Next, a second insulating film 10104 is formed over the
entire surface by sputtering, a printing method, a coating method,
or the like. The second insulating film 10104 has a function of
preventing change in characteristics of the transistor due to an
impurity from the substrate which affects the semiconductor
layer.
[0731] Next, a first semiconductor layer 10105 and a second
semiconductor layer 10106 are formed. Note that the first
semiconductor layer 10105 and the second semiconductor layer 10106
are formed sequentially and shapes thereof are processed at the
same time.
[0732] Next, a second conductive layer 10107 is formed by
photolithography, a laser direct writing method, an inkjet method,
or the like. Note that as a method for etching which processes a
shape of the second conductive layer 10107, dry etching is
preferable. Note that as the second conductive layer 10107, a
light-transmitting material may be used or a reflective material
may be used.
[0733] Next, a channel region of the transistor is formed. Here, an
example of a step thereof is described. The second semiconductor
layer 10106 is etched by using the second conductive layer 10107 as
a mask. Alternatively, the second semiconductor layer 10106 is
etched by using a mask for processing the shape of the second
conductive layer 10107. Then, the first conductive layer 10103 at a
position where the second semiconductor layer 10106 is removed
serves as the channel region of the transistor. Thus, the number of
masks can be reduced, so that manufacturing cost can be
reduced.
[0734] Next, a third insulating film 10108 is formed and a contact
hole is selectively formed in the third insulating film 10108. Note
that a contact hole may be formed also in the second insulating
film 10104 at the same time as forming the contact hole in the
third insulating film 10108. Note also that a surface of the third
insulating film 10108 is preferably as even as possible. This is
because alignment of the liquid crystal molecules are affected by
unevenness of a surface with which the liquid crystal is in
contact.
[0735] Next, a third conductive layer 10109 is formed by
photolithography, a laser direct writing method, an inkjet method,
or the like.
[0736] Next, a first alignment film 10110 is formed. Note that
after the first alignment film 10110 is formed, rubbing may be
performed so as to control the alignment of the liquid crystal
molecules. Rubbing is a step of forming stripes on an alignment
film by rubbing the alignment film with a cloth. By performing
rubbing, the alignment film can have alignment properties.
[0737] The first substrate 10101 which is manufactured as described
above and the second substrate 10116 on which the light-shielding
film 10114, the color filter 10115, the fourth conductive layer
10113, the spacer 10117, and the second alignment film 10112 are
formed are attached to each other by a sealant with a gap of
several .mu.m therebetween. Then, liquid crystals 10111 which
include the liquid crystal molecules 10118 are injected into a
space between the two substrates. Note that in the TN mode, the
fourth conductive layer 10113 is formed over the entire surface of
the second substrate 10116.
[0738] FIG. 38A is an example of a cross-sectional view of a pixel
in the case where an MVA (Multi-domain Vertical Alignment) mode and
a transistor are combined. By applying the pixel structure shown in
FIG. 38A to a liquid crystal display device, a liquid crystal
display device having a wide viewing angle, high response speed,
and high contrast can be obtained.
[0739] Features of the pixel structure of an MVA-mode liquid
crystal panel shown in FIG. 38A are described. Liquid crystal
molecules 10218 shown in FIG. 38A are long and narrow molecules
each having a major axis and a minor axis. In FIG. 38A, a direction
of each of the liquid crystal molecules 10218 is expressed by the
length thereof. That is, the direction of the major axis of the
liquid crystal molecule 10218, which is expressed as long, is
parallel to the page, and as the liquid crystal molecule 10218 is
expressed to be shorter, the direction of the major axis becomes
closer to a normal direction of the page. That is, the liquid
crystal molecules 10218 shown in FIG. 38A are aligned such that the
direction of the major axis is normal to the alignment film. Thus,
the liquid crystal molecules 10218 at a position where an alignment
control projection 10219 is formed are aligned radially with the
alignment control projection 10219 as a center. With this state, a
liquid crystal display device having a wide viewing angle can be
obtained.
[0740] Here, the case is described in which a bottom-gate
transistor using an amorphous semiconductor is used as the
transistor. In the case where a transistor using an amorphous
semiconductor is used, a liquid crystal display device can be
formed at low cost by using a large substrate.
[0741] A liquid crystal display device includes a basic portion
displaying images, which is called a liquid crystal panel. The
liquid crystal panel is manufactured as follows: two processed
substrates are attached to each other with a gap of several .mu.m
therebetween, and a liquid crystal material is injected into a
space between the two substrates. In FIG. 38A, the two substrates
correspond to the first substrate 10201 and the second substrate
10216. A transistor and a pixel electrode are formed over the first
substrate. A light-shielding film 10214, a color filter 10215, a
fourth conductive layer 10213, a spacer 10217, a second alignment
film 10212, and an alignment control projection 10219 are formed on
the second substrate.
[0742] The light-shielding film 10214 is not necessarily formed on
the second substrate 10216. When the light-shielding film 10214 is
not formed, the number of steps is reduced, so that manufacturing
cost can be reduced. In addition, since a structure is simple,
yield can be improved. On the other hand, when the light-shielding
film 10214 is formed, a display device with little light leakage at
the time of black display can be obtained.
[0743] The color filter 10215 is not necessarily formed on the
second substrate 10216. When the color filter 10215 is not formed,
the number of steps is reduced, so that manufacturing cost can be
reduced. In addition, since a structure is simple, yield can be
improved. Note that even when the color filter 10215 is not formed,
a display device which can perform color display can be obtained by
field sequential driving. On the other hand, needless to say, when
the color filter 10215 is formed, a display device which can
perform color display can be obtained.
[0744] Spherical spacers may be dispersed on the second substrate
10216 instead of forming the spacer 10217. When the spherical
spacers are dispersed, the number of steps is reduced, so that
manufacturing cost can be reduced. In addition, since a structure
is simple, yield can be improved. On the other hand, when the
spacer 10217 is formed, a distance between the two substrates can
be uniform because a position of the spacer is not varied, so that
a display device with little display unevenness can be
obtained.
[0745] Subsequently, a process to be performed to the first
substrate 10201 is described.
[0746] First, a first insulating film 10202 is formed over the
first substrate 10201 by sputtering, a printing method, a coating
method, or the like. The first insulating film 10202 has a function
of preventing change in characteristics of the transistor due to an
impurity from the substrate which affects a semiconductor layer
Note that the first insulating film 10202 is not necessarily
formed.
[0747] Next, a first conductive layer 10203 is formed over the
first insulating film 10202 by photolithography, a laser direct
writing method, an inkjet method, or the like.
[0748] Next, a second insulating film 10204 is formed over the
entire surface by sputtering, a printing method, a coating method,
or the like. The second insulating film 10204 has a function of
preventing change in characteristics of the transistor due to an
impurity from the substrate which affects the semiconductor
layer.
[0749] Next, a first semiconductor layer 10205 and a second
semiconductor layer 10206 are formed. Note that the first
semiconductor layer 10205 and the second semiconductor layer 10206
are formed sequentially and shapes thereof are processed at the
same time.
[0750] Next, a second conductive layer 10207 is formed by
photolithography, a laser direct writing method, an inkjet method,
or the like. Note that as a method for etching which processes a
shape of the second conductive layer 10207, dry etching is
preferable. Note that as the second conductive layer 10207, a
light-transmitting material may be used or a reflective material
may be used.
[0751] Next, a channel region of the transistor is formed. Here, an
example of a step thereof is described. The second semiconductor
layer 10206 is etched by using the second conductive layer 10207 as
a mask. Alternatively, the second semiconductor layer 10206 is
etched by using a mask for processing the shape of the second
conductive layer 10207. Then, the first conductive layer 10203 at a
position where the second semiconductor layer 10206 is removed
serves as the channel region of the transistor Thus, the number of
masks can be reduced, so that manufacturing cost can be
reduced.
[0752] Next, a third insulating film 10208 is formed and a contact
hole is selectively formed in the third insulating film 10208. Note
that a contact hole may be formed also in the second insulating
film 10204 at the same time as forming the contact hole in the
third insulating film 10208.
[0753] Next, a third conductive layer 10209 is formed by
photolithography, a laser direct writing method, an inkjet method,
or the like.
[0754] Next, a first alignment film 10210 is formed. Note that
after the first alignment film 10210 is formed, rubbing may be
performed so as to control the alignment of the liquid crystal
molecules. Rubbing is a step of forming stripes on an alignment
film by rubbing the alignment film with a cloth. By performing
rubbing, the alignment film can have alignment properties.
[0755] The first substrate 10201 which is manufactured as described
above and the second substrate 10216 on which the light-shielding
film 10214, the color filter 10215, the fourth conductive layer
10213, the spacer 10217, and the second alignment film 10212 are
manufactured are attached to each other by a sealant with a gap of
several .mu.m therebetween. Then, liquid crystals 10211 which
include the liquid crystal molecules 10218 are injected into a
space between the two substrates. Note that in the MVA mode, the
fourth conductive layer 10213 is formed over the entire surface of
the second substrate 10216. In addition, the alignment control
projection 10219 is formed so as to be in contact with the fourth
conductive layer 10213. The alignment control projection 10219
preferably has a shape with a smooth curved surface. Thus, since
alignment of the adjacent liquid crystal molecules 10218 is
extremely similar, an alignment defect can be reduced. Further, a
defect of the alignment film caused by breaking of the alignment
film can be reduced.
[0756] FIG. 38B is an example of a cross-sectional view of a pixel
in the case where a PVA (Patterned Vertical Alignment) mode and a
transistor are combined. By applying the pixel structure shown in
FIG. 38B to a liquid crystal display device, a liquid crystal
display device having a wide viewing angle, high response speed,
and high contrast can be obtained.
[0757] Features of the pixel structure shown in FIG. 38B are
described. Liquid crystal molecules 10248 shown in FIG. 38B are
long and narrow molecules each having a major axis and a minor
axis. In FIG. 38B, direction of each of the liquid crystal
molecules 10248 is expressed by the length thereof. That is, the
direction of the major axis of the liquid crystal molecule 10248,
which is expressed as long, is parallel to the page, and as the
liquid crystal molecule 10248 is expressed to be shorter, the
direction of the major axis becomes closer to a normal direction of
the page. That is, the liquid crystal molecules 10248 shown in FIG.
38B are aligned such that the direction of the major axis is normal
to the alignment film. Thus, the liquid crystal molecules 10248 at
a position where an electrode cutout portion 10249 is formed are
aligned radially with a boundary of the electrode cutout portion
10249 and the fourth conductive layer 10243 as a center. With this
state, a liquid crystal display device having a wide viewing angle
can be obtained.
[0758] Here, the case is described in which a bottom-gate
transistor using an amorphous semiconductor is used as the
transistor. In the case where a transistor using an amorphous
semiconductor is used, a liquid crystal display device can be
formed at low cost by using a large substrate.
[0759] A liquid crystal display device includes a basic portion
displaying images, which is called a liquid crystal panel. The
liquid crystal panel is manufactured as follows: two processed
substrates are attached to each other with a gap of several .mu.m
therebetween, and a liquid crystal material is injected into a
space between the two substrates. In FIG. 38B, the two substrates
correspond to the first substrate 10231 and the second substrate
10246. A transistor and a pixel electrode are formed over the first
substrate. A light-shielding film 10244, a color filter 10245, a
fourth conductive layer 10243, a spacer 10247, and a second
alignment film 10242 are formed on the second substrate.
[0760] The light-shielding film 10244 is not necessarily formed on
the second substrate 10246. When the light-shielding film 10244 is
not formed, the number of steps is reduced, so that manufacturing
cost can be reduced. In addition, since a structure is simple,
yield can be improved. On the other hand, when the light-shielding
film 10244 is formed, a display device with little light leakage at
the time of black display can be obtained.
[0761] The color filter 10245 is not necessarily formed on the
second substrate 10246. When the color filter 10245 is not formed,
the number of steps is reduced, so that manufacturing cost can be
reduced. In addition, since a structure is simple, yield can be
improved. Note that even when the color filter 10245 is not formed,
a display device which can perform color display can be obtained by
field sequential driving. On the other hand, needless to say, when
the color filter 10245 is formed, a display device which can
perform color display can be obtained.
[0762] Spherical spacers may be dispersed on the second substrate
10246 instead of forming the spacer 10247. When the spherical
spacers are dispersed, the number of steps is reduced, so that
manufacturing cost can be reduced. In addition, since a structure
is simple, yield can be improved. On the other hand, when the
spacer 10247 is formed, a distance between the two substrates can
be uniform because a position of the spacer is not varied, so that
a display device with little display unevenness can be
obtained.
[0763] Subsequently, a process to be performed to the first
substrate 10231 is described.
[0764] First, a first insulating film 10232 is formed over the
first substrate 10231 by sputtering, a printing method, a coating
method, or the like. The first insulating film 10232 has a function
of preventing change in characteristics of the transistor due to an
impurity from the substrate which affects a semiconductor layer
Note that the first insulating film 10232 is not necessarily
formed.
[0765] Next, a first conductive layer 10233 is formed over the
first insulating film 10232 by photolithography, a laser direct
writing method, an inkjet method, or the like.
[0766] Next, a second insulating film 10234 is formed over the
entire surface by sputtering, a printing method, a coating method,
or the like. The second insulating film 10234 has a function of
preventing change in characteristics of the transistor due to an
impurity from the substrate which affects the semiconductor
layer.
[0767] Next, a first semiconductor layer 10235 and a second
semiconductor layer 10236 are formed. Note that the first
semiconductor layer 10235 and the second semiconductor layer 10236
are formed sequentially and shapes thereof are processed at the
same time.
[0768] Next, a second conductive layer 10237 is formed by
photolithography, a laser direct writing method, an inkjet method,
or the like. Note that as a method for etching which processes a
shape of the second conductive layer 10237, dry etching is
preferable. Note that as the second conductive layer 10237, a
light-transmitting material may be used or a reflective material
may be used.
[0769] Next, a channel region of the transistor is formed. Here, an
example of a step thereof is described. The second semiconductor
layer 10236 is etched by using the second conductive layer 10237 as
a mask. Alternatively, the second semiconductor layer 10236 is
etched by using a mask for processing the shape of the second
conductive layer 10237. Then, the first conductive layer 10233 at a
position where the second semiconductor layer 10236 is removed
serves as the channel region of the transistor. Thus, the number of
masks can be reduced, so that manufacturing cost can be
reduced.
[0770] Next, a third insulating film 10238 is formed and a contact
hole is selectively formed in the third insulating film 10238. Note
that a contact hole may be formed also in the second insulating
film 10234 at the same time as forming the contact hole in the
third insulating film 10238. Note also that a surface of the third
insulating film 10238 is preferably as even as possible. This is
because alignment of the liquid crystal molecules are affected by
unevenness of a surface with which the liquid crystal is in
contact.
[0771] Next, a third conductive layer 10239 is formed by
photolithography, a laser direct writing method, an inkjet method,
or the like.
[0772] Next, a first alignment film 10240 is formed. Note that
after the first alignment film 10240 is formed, rubbing may be
performed so as to control the alignment of the liquid crystal
molecules. Rubbing is a step of forming stripes on an alignment
film by rubbing the alignment film with a cloth. By performing
rubbing, the alignment film can have alignment properties.
[0773] The first substrate 10231 which is manufactured as described
above and the second substrate 10246 on which the light-shielding
film 10244, the color filter 10245, the fourth conductive layer
10243, the spacer 10247, and the second alignment film 10242 are
manufactured are attached to each other by a sealant with a gap of
several .mu.m therebetween. Then, liquid crystals 10241 which
include the liquid crystal molecules 10248 are injected into a
space between the two substrates. Note that in the PVA mode, the
fourth conductive layer 10243 is patterned and is provided with the
electrode cutout portion 10249. Although a shape of the electrode
cutout portion 10249 is not particularly limited, the electrode
cutout portion 10249 preferably has a shape in which a plurality of
rectangles having different directions are combined. Thus, since a
plurality of regions having different alignment can be formed, a
liquid crystal display device having a wide viewing angle can be
obtained. Note that the fourth conductive layer 10243 at the
boundary between the electrode cutout portion 10249 and the fourth
conductive layer 10243 preferably has a shape with a smooth curved
surface. Thus, since alignment of the adjacent liquid crystal
molecules 10248 is extremely similar, an alignment defect is
reduced. Further, a defect of the alignment film caused by breaking
of the second alignment film 10242 by the electrode cutout portion
10249 can be prevented.
[0774] FIG. 39A is an example of a cross-sectional view of a pixel
in the case where an IPS (In-Plane-Switching) mode and a transistor
are combined. By applying the pixel structure shown in FIG. 39A to
a liquid crystal display device, a liquid crystal display device
theoretically having a wide viewing angle and response speed which
has low dependency on a gray scale can be obtained.
[0775] Features of the pixel structure shown in FIG. 39A are
described. Liquid crystal molecules 10318 shown in FIG. 39A are
long and narrow molecules each having a major axis and a minor
axis. In FIG. 39A, a direction of each of the liquid crystal
molecules 10318 is expressed by the length thereof. That is, the
direction of the major axis of the liquid crystal molecule 10318,
which is expressed as long, is parallel to the page, and as the
liquid crystal molecule 10318 is expressed to be shorter, the
direction of the major axis becomes closer to a normal direction of
the page. Each of the liquid crystal molecules 10318 shown in FIG.
39A is aligned so that the direction of the major axis thereof is
always horizontal to the substrate. Although FIG. 39A shows
alignment with no electric field, when an electric field is applied
to each of the liquid crystal molecules 10318, each of the liquid
crystal molecules 10318 rotates in a horizontal plane as the
direction of the major axis thereof is always horizontal to the
substrate. With this state, a liquid crystal display device having
a wide viewing angle can be obtained.
[0776] Here, the case is described in which a bottom-gate
transistor using an amorphous semiconductor is used as the
transistor. In the case where a transistor using an amorphous
semiconductor is used, a liquid crystal display device can be
formed at low cost by using a large substrate.
[0777] A liquid crystal display device includes a basic portion
displaying images, which is called a liquid crystal panel. The
liquid crystal panel is manufactured as follows: two processed
substrates are attached to each other with a gap of several .mu.m
therebetween, and a liquid crystal material is injected into a
space between the two substrates. In FIG. 39A, the two substrates
correspond to the first substrate 10301 and the second substrate
10316. A transistor and a pixel electrode are formed over the first
substrate. A light-shielding film 10314, a color filter 10315, a
fourth conductive layer 10313, a spacer 10317, and a second
alignment film 10312 are formed on the second substrate.
[0778] The light-shielding film 10314 is not necessarily formed on
the second substrate 10316. When the light-shielding film 10314 is
not formed, the number of steps is reduced, so that manufacturing
cost can be reduced. In addition, since a structure is simple,
yield can be improved. On the other hand, when the light-shielding
film 10314 is formed, a display device with little light leakage at
the time of black display can be obtained.
[0779] The color filter 10315 is not necessarily formed on the
second substrate 10316. When the color filter 10315 is not formed,
the number of steps is reduced, so that manufacturing cost can be
reduced. In addition, since a structure is simple, yield can be
improved. Note that even when the color filter 10315 is not formed,
a display device which can perform color display can be obtained by
field sequential driving. On the other hand, needless to say, when
the color filter 10315 is formed, a display device which can
perform color display can be obtained.
[0780] Spherical spacers may be dispersed on the second substrate
10316 instead of forming the spacer 10317. When the spherical
spacers are dispersed, the number of steps is reduced, so that
manufacturing cost can be reduced. In addition, since a structure
is simple, yield can be improved. On the other hand, when the
spacer 10317 is formed, a distance between the two substrates can
be uniform because a position of the spacer is not varied, so that
a display device with little display unevenness can be
obtained.
[0781] Subsequently, a process to be performed to the first
substrate 10301 is described.
[0782] First, a first insulating film 10302 is formed over the
first substrate 10301 by sputtering, a printing method, a coating
method, or the like. The first insulating film 10302 has a function
of preventing change in characteristics of the transistor due to an
impurity from the substrate which affects a semiconductor layer.
Note that the first insulating film 10302 is not necessarily
formed.
[0783] Next, a first conductive layer 10303 is formed over the
first insulating film 10302 by photolithography, a laser direct
writing method, an inkjet method, or the like.
[0784] Next, a second insulating film 10304 is formed over the
entire surface by sputtering, a printing method, a coating method,
or the like. The second insulating film 10304 has a function of
preventing change in characteristics of the transistor due to an
impurity from the substrate which affects the semiconductor
layer.
[0785] Next, a first semiconductor layer 10305 and a second
semiconductor layer 10306 are formed. Note that the first
semiconductor layer 10305 and the second semiconductor layer 10306
are formed sequentially and shapes thereof are processed at the
same time.
[0786] Next, a second conductive layer 10307 is formed by
photolithography, a laser direct writing method, an inkjet method,
or the like. Note that as a method for etching which processes a
shape of the second conductive layer 10307, dry etching is
preferable. Note that as the second conductive layer 10307, a
light-transmitting material may be used or a reflective material
may be used.
[0787] Next, a channel region of the transistor is formed. Here, an
example of a step thereof is described. The second semiconductor
layer 10306 is etched by using the second conductive layer 10307 as
a mask. Alternatively, the second semiconductor layer 10306 is
etched by using a mask for processing the shape of the second
conductive layer 10307. Then, the first conductive layer 10303 at a
position where the second semiconductor layer 10306 is removed
serves as the channel region of the transistor. Thus, the number of
masks can be reduced, so that manufacturing cost can be
reduced.
[0788] Next, a third insulating film 10308 is formed and a contact
hole is selectively formed in the third insulating film 10308. Note
that a contact hole may be formed also in the second insulating
film 10304 at the same time as forming the contact hole in the
third insulating film 10308.
[0789] Next, a third conductive layer 10309 is formed by
photolithography, a laser direct writing method, an inkjet method,
or the like. Here, the third conductive layer 10309 has a shape in
which two comb-shaped electrodes engage with each other. One of the
comb-shaped electrodes is electrically connected to one of a source
electrode and a drain electrode of the transistor, and the other of
the comb-shaped electrodes is electrically connected to a common
electrode. Thus, a lateral electric field can be effectively
applied to the liquid crystal molecules 10318.
[0790] Next, a first alignment film 10310 is formed. Note that
after the first alignment film 10310 is formed, rubbing may be
performed so as to control the alignment of the liquid crystal
molecules. Rubbing is a step of forming stripes on an alignment
film by rubbing the alignment film with a cloth. By performing
rubbing, the alignment film can have alignment properties.
[0791] The first substrate 10301 which is manufactured as described
above and the second substrate 10316 on which the light-shielding
film 10314, the color filter 10315, the spacer 10317, and the
second alignment film 10312 are formed are attached to each other
by a sealant with a gap of several tm therebetween. Then, liquid
crystals 10311 which include the liquid crystal molecules 10318 are
injected into a space between the two substrates.
[0792] FIG. 39B is an example of a cross-sectional view of a pixel
in the case where an FFS (Fringe Field Switching) mode and a
transistor are combined. By applying the pixel structure shown in
FIG. 39B to a liquid crystal display device, a liquid crystal
display device theoretically having a wide viewing angle and
response speed which has low dependency on a gray scale can be
obtained.
[0793] Features of the pixel structure shown in FIG. 39B are
described. Liquid crystal molecules 10348 shown in FIG. 39B are
long and narrow molecules each having a major axis and a minor
axis. In FIG. 39B, direction of each of the liquid crystal
molecules 10348 is expressed by the length thereof. That is, the
direction of the major axis of the liquid crystal molecule 10348,
which is expressed as long, is parallel to the page, and as the
liquid crystal molecule 10348 is expressed to be shorter, the
direction of the major axis becomes closer to a normal direction of
the page. Each of the liquid crystal molecules 10348 shown in FIG.
39B is aligned so that the direction of the major axis thereof is
always horizontal to the substrate. Although FIG. 39B shows
alignment with no electric field, when an electric field is applied
to each of the liquid crystal molecules 10348, each of the liquid
crystal molecules 10348 rotates in a horizontal plane as the
direction of the major axis thereof is always horizontal to the
substrate. With this state, a liquid crystal display device having
a wide viewing angle can be obtained.
[0794] Here, the case is described in which a bottom-gate
transistor using an amorphous semiconductor is used as the
transistor. In the case where a transistor using an amorphous
semiconductor is used, a liquid crystal display device can be
formed at low cost by using a large substrate.
[0795] A liquid crystal display device includes a basic portion
displaying images, which is called a liquid crystal panel. The
liquid crystal panel is manufactured as follows: two processed
substrates are attached to each other with a gap of several .mu.m
therebetween, and a liquid crystal material is injected into a
space between the two substrates. In FIG. 39B, the two substrates
correspond to the first substrate 10331 and the second substrate
10346. A transistor and a pixel electrode are formed over the first
substrate. A light-shielding film 10344, a color filter 10345, a
spacer 10347, and a second alignment film 10342 are formed on the
second substrate.
[0796] The light-shielding film 10344 is not necessarily formed on
the second substrate 10346. When the light-shielding film 10344 is
not formed, the number of steps is reduced, so that manufacturing
cost can be reduced. In addition, since a structure is simple,
yield can be improved. On the other hand, when the light-shielding
film 10344 is formed, a display device with little light leakage at
the time of black display can be obtained.
[0797] The color filter 10345 is not necessarily formed on the
second substrate 10346. When the color filter 10345 is not formed,
the number of steps is reduced, so that manufacturing cost can be
reduced. In addition, since a structure is simple, yield can be
improved. Note that even when the color filter 10345 is not formed,
a display device which can perform color display can be obtained by
field sequential driving. On the other hand, needless to say, when
the color filter 10345 is formed, a display device which can
perform color display can be obtained.
[0798] Spherical spacers may be dispersed on the second substrate
10346 instead of forming the spacer 10347. When the spherical
spacers are dispersed, the number of steps is reduced, so that
manufacturing cost can be reduced. In addition, since a structure
is simple, yield can be improved. On the other hand, when the
spacer 10347 is formed, a distance between the two substrates can
be uniform because a position of the spacer is not varied, so that
a display device with little display unevenness can be
obtained.
[0799] Subsequently, a process to be performed to the first
substrate 10331 is described.
[0800] First, a first insulating film 10332 is formed over the
first substrate 10331 by sputtering, a printing method, a coating
method, or the like. The first insulating film 10332 has a function
of preventing change in characteristics of the transistor due to an
impurity from the substrate which affects a semiconductor layer.
Note that the first insulating film 10332 is not necessarily
formed.
[0801] Next, a first conductive layer 10333 is formed over the
first insulating film 10332 by photolithography, a laser direct
writing method, an inkjet method, or the like.
[0802] Next, a second insulating film 10334 is formed over the
entire surface by sputtering, a printing method, a coating method,
or the like. The second insulating film 10334 has a function of
preventing change in characteristics of the transistor due to an
impurity from the substrate which affects the semiconductor
layer.
[0803] Next, a first semiconductor layer 10335 and a second
semiconductor layer 10336 are formed. Note that the first
semiconductor layer 10335 and the second semiconductor layer 10336
are formed sequentially and shapes thereof are processed at the
same time.
[0804] Next, a second conductive layer 10337 is formed by
photolithography, a laser direct writing method, an inkjet method,
or the like. Note that as a method for etching which processes a
shape of the second conductive layer 10337, dry etching is
preferable. Note that as the second conductive layer 10337, a
light-transmitting material may be used or a reflective material
may be used.
[0805] Next, a channel region of the transistor is formed. Here, an
example of a step thereof is described. The second semiconductor
layer 10336 is etched by using the second conductive layer 10337 as
a mask. Alternatively, the second semiconductor layer 10336 is
etched by using a mask for processing the shape of the second
conductive layer 10337. Then, the first conductive layer 10333 at a
position where the second semiconductor layer 10336 is removed
serves as the channel region of the transistor. Thus, the number of
masks can be reduced, so that manufacturing cost can be
reduced.
[0806] Next, a third insulating film 10338 is formed and a contact
hole is selectively formed in the third insulating film 10338.
[0807] Next, a fourth conductive layer 10343 is formed by
photolithography, a laser direct writing method, an inkjet method,
or the like.
[0808] Next, a fourth insulating film 10349 is formed and a contact
hole is selectively formed in the fourth insulating film 10349.
Note that a surface of the fourth insulating film 10349 is
preferably as even as possible. This is because alignment of the
liquid crystal molecules are affected by unevenness of a surface
with which the liquid crystal is in contact.
[0809] Next, a third conductive layer 10339 is formed by
photolithography, a laser direct writing method, an inkjet method,
or the like. Here, the third conductive layer 10339 is
comb-shaped.
[0810] Next, a first alignment film 10340 is formed. Note that
after the first alignment film 10340 is formed, rubbing may be
performed so as to control the alignment of the liquid crystal
molecules. Rubbing is a step of forming stripes on an alignment
film by rubbing the alignment film with a cloth. By performing
rubbing, the alignment film can have alignment properties.
[0811] The first substrate 10331 which is manufactured as described
above and the second substrate 10346 on which the light-shielding
film 10344, the color filter 10345, the spacer 10347, and the
second alignment film 10342 are formed are attached to each other
by a sealant with a gap of several .mu.m therebetween. Then, liquid
crystals 10341 which include the liquid crystal molecules 10348 are
injected into a space between the two substrates. Therefore, a
liquid crystal panel can be manufactured.
[0812] Here, materials which can be used for conductive layers or
insulating films are described.
[0813] As the first insulating film 10102 in FIG. 37, the first
insulating film 10202 in FIG. 38A, the first insulating film 10232
in FIG. 38B, the first insulating film 10302 in FIG. 39A, or the
first insulating film 10332 in FIG. 39B, an insulating film such as
a silicon oxide film, a silicon nitride film, or a silicon
oxynitride film can be used. Alternatively, an insulating film
having a stacked-layer structure in which two or more of a silicon
oxide film, a silicon nitride film, a silicon oxynitride film, and
the like are combined can be used.
[0814] As the first conductive layer 10103 in FIG. 37, the first
conductive layer 10203 in FIG. 38A, the first conductive layer
10233 in FIG. 38B, the first conductive layer 10303 in FIG. 39A, or
the first conductive layer 10333 in FIG. 39B, Mo, Ti, Al, Nd, Cr,
or the like can be used. Alternatively, a stacked-layer structure
in which two or more of Mo, Ti, Al, Nd, Cr, and the like are
combined can be used.
[0815] As the second insulating film 10104 in FIG. 37, the second
insulating film 10204 in FIG. 38A, the second insulating film 10234
in FIG. 38B, the second insulating film 10304 in FIG. 39A, or the
second insulating film 10334 in FIG. 39B, a thermal oxide film, a
silicon oxide film, a silicon nitride film, a silicon oxynitride
film, or the like can be used. Alternatively, a stacked-layer
structure in which two or more of a thermal oxide film, a silicon
oxide film, a silicon nitride film, a silicon oxynitride film, and
the like are combined can be used. Note that a silicon oxide film
is preferable in a portion which is in contact with a semiconductor
layer. This is because a trap level at an interface with the
semiconductor layer decreases when a silicon oxide film is used.
Note also that a silicon nitride film is preferable in a portion
which is in contact with Mo. This is because a silicon nitride film
does not oxidize Mo.
[0816] As the first semiconductor layer 10105 in FIG. 37, the first
semiconductor layer 10205 in FIG. 38A, the first semiconductor
layer 10235 in FIG. 38B, the first semiconductor layer 10305 in
FIG. 39A, or the first semiconductor layer 10335 in FIG. 39B,
silicon, silicon germanium, or the like can be used.
[0817] As the second semiconductor layer 10106 in FIG. 37, the
second semiconductor layer 10206 in FIG. 38A, the second
semiconductor layer 10236 in FIG. 38B, the second semiconductor
layer 10306 in FIG. 39A, or the second semiconductor layer 10336 in
FIG. 39B, silicon or the like including phosphorus can be used, for
example.
[0818] As a light-transmitting material of the second conductive
layer 10107 and the third conductive layer 10109 in FIG. 37; the
second conductive layer 10207 and the third conductive layer 10209
in FIG. 38A; the second conductive layer 10237 and a third
conductive layer 10239 in FIG. 38B; the second conductive layer
10307 and a third conductive layer 10309 in FIG. 39A; or the second
conductive layer 10337, the third conductive layer 10339, and the
fourth conductive layer 10343 in FIG. 39B, an indium tin oxide film
formed by mixing tin oxide into indium oxide, an indium tin silicon
oxide film formed by mixing silicon oxide into indium tin oxide, an
indium zinc oxide film formed by mixing zinc oxide into indium
oxide, a zinc oxide film, a tin oxide film, or the like can be
used. Note that indium zinc oxide is a light-transmitting
conductive material formed by sputtering using a target in which
zinc oxide is mixed into indium tin oxide at 2 to 20 wt %.
[0819] As a reflective material of the second conductive layer
10107 and the third conductive layer 10109 in FIG. 37; the second
conductive layer 10207 and the third conductive layer 10209 in FIG.
38A; the second conductive layer 10237 and the third conductive
layer 10239 in FIG. 38B; the second conductive layer 10307 and the
third conductive layer 10309 in FIG. 39A; or the second conductive
layer 10337, the third conductive layer 10339, and the fourth
conductive layer 10343 in FIG. 39B, Ti, Mo, Ta, Cr, W, Al, or the
like can be used. Alternatively, a two-layer structure in which Al
and Ti, Mo, Ta, Cr, or W are stacked, or a three-layer structure in
which Al is interposed between metals such as Ti, Mo, Ta, Cr, and W
may be used.
[0820] As the third insulating film 10108 in FIG. 37, the third
insulating film 10208 in FIG. 38A, the third insulating film 10238
in FIG. 38B, the third insulating film 10308 in FIG. 39A, or the
third insulating film 10338 and the fourth insulating film 10349 in
FIG. 39B, an inorganic material (e.g., silicon oxide, silicon
nitride, or silicon oxynitride), an organic compound material
having a low dielectric constant (e.g., a photosensitive or
nonphotosensitive organic resin material), or the like can be used.
Alternatively, a material including siloxane can be used. Note that
siloxane is a material in which a basic structure is formed by a
bond of silicon (Si) and oxygen (O). As a substituent, an organic
group including at least hydrogen (e.g., an alkyl group or an aryl
group) is used. Alternatively, a fluoro group may be used as the
substituent. Further alternatively, the organic group including at
least hydrogen and the fluoro group may be used as the
substituent.
[0821] As the first alignment film 10110 in FIG. 37, the first
alignment film 10210 in FIG. 38A, the first alignment film 10240 in
FIG. 38B, the first alignment film 10310 in FIG. 39A, or the first
alignment film 10340 in FIG. 39B, a film of a high molecular
compound such as polyimide can be used.
[0822] Next, the pixel structure in the case where each liquid
crystal mode and the transistor are combined is described with
reference to a top plan view (a layout diagram) of the pixel.
[0823] Note that as a liquid crystal mode, a TN (twisted nematic)
mode, an IPS (in-plane-switching) mode, an FFS (fringe field
switching) mode, an MVA (multi-domain vertical alignment) mode, a
PVA (patterned vertical alignment) mode, an ASM (axially symmetric
aligned micro-cell) mode, an OCB (optical compensated
birefringence) mode, an FLC (ferroelectric liquid crystal) mode, an
AFLC (antiferroelectric liquid crystal) mode, or the like can be
used.
[0824] As the transistor, a thin film transistor (a TFT) including
a non-single crystalline semiconductor film typified by amorphous
silicon, polycrystalline silicon, microcrystalline (also referred
to as semi-amorphous) silicon, or the like can be used.
[0825] As a structure of the transistor, a top-gate structure, a
bottom-gate structure, or the like can be used. A channel-etched
transistor, a channel-protective transistor, or the like can be
used as a bottom-gate transistor.
[0826] FIG. 40 is an example of a top plan view of a pixel in the
case where a TN mode and a transistor are combined. By applying the
pixel structure shown in FIG. 40 to a liquid crystal display
device, a liquid crystal display device can be formed at low
cost.
[0827] The pixel shown in FIG. 40 includes a scan line 10401, an
image signal line 10402, a capacitor line 10403, a transistor
10404, a pixel electrode 10405, and a pixel capacitor 10406.
[0828] The scan line 10401 has a function of transmitting a signal
(a scan signal) to the pixel. The image signal line 10402 has a
function for transmitting a signal (an image signal) to the pixel.
Note that since the scan line 10401 and the image signal line 10402
are arranged in matrix, they are formed of conductive layers in
different layers. Note also that a semiconductor layer may be
provided at an intersection of the scan line 10401 and the image
signal line 10402. Thus, intersection capacitance between the scan
line 10401 and the image signal line 10402 can be reduced.
[0829] The capacitor line 10403 is provided in parallel to the
pixel electrode 10405. A portion where the capacitor line 10403 and
the pixel electrode 10405 overlap with each other corresponds to
the pixel capacitor 10406. Note that part of the capacitor line
10403 is extended along the image signal line 10402 so as to
surround the image signal line 10402. Thus, crosstalk can be
reduced. Crosstalk is a phenomenon in which a potential of an
electrode, which should hold the potential, is changed in
accordance with change in potential of the image signal line 10402.
Note also that intersection capacitance can be reduced by providing
a semiconductor layer between the capacitor line 10403 and the
image signal line 10402. Note also that the capacitor line 10403 is
formed of a material which is similar to that of the scan line
10401.
[0830] The transistor 10404 has a function as a switch which turns
on the image signal line 10402 and the pixel electrode 10405. Note
that one of a source region and a drain region of the transistor
10404 is provided so as to be surrounded by the other of the source
region and the drain region of the transistor 10404. Thus, the
channel width of the transistor 10404 increases, so that switching
capability can be improved. Note also that a gate electrode of the
transistor 10404 is provided so as to surround the semiconductor
layer.
[0831] The pixel electrode 10405 is electrically connected to one
of a source electrode and a drain electrode of the transistor
10404. The pixel electrode 10405 is an electrode for applying
signal voltage which is transmitted by the image signal line 10402
to a liquid crystal element. Note that the pixel electrode 10405 is
rectangular. Thus, an aperture ratio can be improved. Note also
that as the pixel electrode 10405, a light-transmitting material
may be used or a reflective material may be used. Alternatively,
the pixel electrode 10405 may be formed by combining a
light-transmitting material and a reflective material.
[0832] FIG. 41A is an example of a top plan view of a pixel in the
case where an MVA mode and a transistor are combined. By applying
the pixel structure shown in FIG. 41A to a liquid crystal display
device, a liquid crystal display device having a wide viewing
angle, high response speed, and high contrast can be obtained.
[0833] The pixel shown in FIG. 41A includes a scan line 10501, a
video signal line 10502, a capacitor line 10503, a transistor
10504, a pixel electrode 10505, a pixel capacitor 10506, and an
alignment control projection 10507.
[0834] The scan line 10501 has a function of transmitting a signal
(a scan signal) to the pixel. The image signal line 10502 has a
function for transmitting a signal (an image signal) to the pixel.
Note that since the scan line 10501 and the image signal line 10502
are arranged in matrix, they are formed of conductive layers in
different layers. Note also that a semiconductor layer may be
provided at an intersection of the scan line 10501 and the image
signal line 10502. Thus, intersection capacitance between the scan
line 10501 and the image signal line 10502 can be reduced.
[0835] The capacitor line 10503 is provided in parallel to the
pixel electrode 10505. A portion where the capacitor line 10503 and
the pixel electrode 10505 overlap with each other corresponds to
the pixel capacitor 10506. Note that part of the capacitor line
10503 is extended along the image signal line 10502 so as to
surround the image signal line 10502. Thus, crosstalk can be
reduced. Crosstalk is a phenomenon in which a potential of an
electrode, which should hold the potential, is changed in
accordance with change in potential of the image signal line 10502.
Note also that intersection capacitance can be reduced by providing
a semiconductor layer between the capacitor line 10503 and the
image signal line 10502. Note also that the capacitor line 10503 is
formed of a material which is similar to that of the scan line
10501.
[0836] The transistor 10504 has a function as a switch which turns
on the image signal line 10502 and the pixel electrode 10505. Note
that one of a source region and a drain region of the transistor
10504 is provided so as to be surrounded by the other of the source
region and the drain region of the transistor 10504. Thus, the
channel width of the transistor 10504 increases, so that switching
capability can be improved. Note also that a gate electrode of the
transistor 10504 is provided so as to surround the semiconductor
layer.
[0837] The pixel electrode 10505 is electrically connected to one
of a source electrode and a drain electrode of the transistor
10504. The pixel electrode 10505 is an electrode for applying
signal voltage which is transmitted by the image signal line 10502
to a liquid crystal element. Note that the pixel electrode 10505 is
rectangular. Thus, an aperture ratio can be improved. Note also
that as the pixel electrode 10505, a light-transmitting material
may be used or a reflective material may be used. Alternatively,
the pixel electrode 10505 may be formed by combining a
light-transmitting material and a reflective material.
[0838] The alignment control projection 10507 is formed on a
counter substrate. The alignment control projection 10507 has a
function of aligning liquid crystal molecules radially. Note that a
shape of the alignment control projection 10507 is not particularly
limited. For example, the alignment control projection 10507 may be
a dogleg shape. Thus, a plurality of regions having different
alignment of the liquid crystal molecules can be formed, so that a
viewing angle can be improved.
[0839] FIG. 41B is an example of a top plan view of a pixel in the
case where a PVA mode and a transistor are combined. By applying
the pixel structure shown in FIG. 41B to a liquid crystal display
device, a liquid crystal display device having a wide viewing
angle, high response speed, and high contrast can be obtained.
[0840] The pixel shown in FIG. 41B includes a scan line 10511, a
video signal line 10512, a capacitor line 10513, a transistor
10514, a pixel electrode 10515, a pixel capacitor 10516, and an
electrode cutout portion 10517.
[0841] The scan line 10511 has a function of transmitting a signal
(a scan signal) to the pixel. The image signal line 10512 has a
function for transmitting a signal (an image signal) to the pixel.
Note that since the scan line 10511 and the image signal line 10512
are arranged in matrix, they are formed of conductive layers in
different layers. Note also that a semiconductor layer may be
provided at an intersection of the scan line 10511 and the image
signal line 10512. Thus, intersection capacitance between the scan
line 10511 and the image signal line 10512 can be reduced.
[0842] The capacitor line 10513 is provided in parallel to the
pixel electrode 10515. A portion where the capacitor line 10513 and
the pixel electrode 10515 overlap with each other corresponds to
the pixel capacitor 10516. Note that part of the capacitor line
10513 is extended along the image signal line 10512 so as to
surround the image signal line 10512. Thus, crosstalk can be
reduced. Crosstalk is a phenomenon in which a potential of an
electrode, which should hold the potential, is changed in
accordance with change in potential of the image signal line 10512.
Note also that intersection capacitance can be reduced by providing
a semiconductor layer between the capacitor line 10513 and the
image signal line 10512. Note also that the capacitor line 10513 is
formed of a material which is similar to that of the scan line
10511.
[0843] The transistor 10514 has a function as a switch which turns
on the image signal line 10512 and the pixel electrode 10515. Note
that one of a source region and a drain region of the transistor
10514 is provided so as to be surrounded by the other of the source
region and the drain region of the transistor 10514. Thus, the
channel width of the transistor 10514 increases, so that switching
capability can be improved. Note also that a gate electrode of the
transistor 10514 is provided so as to surround the semiconductor
layer.
[0844] The pixel electrode 10515 is electrically connected to one
of a source electrode and a drain electrode of the transistor
10514. The pixel electrode 10515 is an electrode for applying
signal voltage which is transmitted by the image signal line 10512
to a liquid crystal element. Note that the pixel electrode 10515
has a shape which is formed in accordance with a shape of the
electrode cutout portion 10517. Specifically, the pixel electrode
10515 has a shape in which a portion where the pixel electrode
10515 is cut is formed in a portion where the electrode cutout
portion 10517 is not formed. Thus, since a plurality of regions
having different alignment of the liquid crystal molecules can be
formed, a viewing angle can be improved. Note also that as the
pixel electrode 10515, a light-transmitting material or a
reflective material may be used. Alternatively, the pixel electrode
10515 may be formed by combining a light-transmitting material and
a reflective material.
[0845] FIG. 42A is an example of a top plan view of a pixel in the
case where an IPS mode and a transistor are combined. By applying
the pixel structure shown in FIG. 42A to a liquid crystal display
device, a liquid crystal display device theoretically having a wide
viewing angle and response speed which has low dependency on a gray
scale can be obtained.
[0846] The pixel shown in FIG. 42A includes a scan line 10601, a
video signal line 10602, a common electrode 10603, a transistor
10604, and a pixel electrode 10605.
[0847] The scan line 10601 has a function of transmitting a signal
(a scan signal) to the pixel. The image signal line 10602 has a
function of transmitting a signal (an image signal) to the pixel.
Note that since the scan line 10601 and the image signal line 10602
are arranged in matrix, they are formed of conductive layers in
different layers. Note also that a semiconductor layer may be
provided at an intersection of the scan line 10601 and the image
signal line 10602. Thus, intersection capacitance between the scan
line 10601 and the image signal line 10602 can be reduced. Note
also that the image signal line 10602 is formed in accordance with
a shape of the pixel electrode 10605.
[0848] The common electrode 10603 is provided in parallel to the
pixel electrode 10605. The common electrode 10603 is an electrode
for generating a lateral electric field. Note that the common
electrode 10603 is bent comb-shaped. Note also that part of the
common electrode 10603 is extended along the image signal line
10602 so as to surround the image signal line 10602. Thus,
crosstalk can be reduced. Crosstalk is a phenomenon in which a
potential of an electrode, which should hold the potential, is
changed in accordance with change in potential of the image signal
line 10602. Note also that intersection capacitance can be reduced
by providing a semiconductor layer between the common electrode
10603 and the image signal line 10602. Part of the common electrode
10603, which is provided in parallel to the scan line 10601, is
formed of a material which is similar to that of the scan line
10601. Part of the common electrode 10603, which is provided in
parallel to the pixel electrode 10605, is formed of a material
which is similar to that of the pixel electrode 10605.
[0849] The transistor 10604 has a function as a switch which turns
on the image signal line 10602 and the pixel electrode 10605. Note
that one of a source region and a drain region of the transistor
10604 is provided so as to be surrounded by the other of the source
region and the drain region of the transistor 10604. Thus, the
channel width of the transistor 10604 increases, so that switching
capability can be improved. Note also that a gate electrode of the
transistor 10604 is provided so as to surround the semiconductor
layer.
[0850] The pixel electrode 10605 is electrically connected to one
of a source electrode and a drain electrode of the transistor
10604. The pixel electrode 10605 is an electrode for applying
signal voltage which is transmitted by the image signal line 10602
to a liquid crystal element. Note that the pixel electrode 10605 is
bent comb-shaped. Thus, a lateral electric field can be applied to
liquid crystal molecules. In addition, since a plurality of regions
having different alignment of the liquid crystal molecules can be
formed, a viewing angle can be improved. Note also that as the
pixel electrode 10605, a light-transmitting material or a
reflective material may be used. Alternatively, the pixel electrode
10605 may be formed by combining a light-transmitting material and
a reflective material.
[0851] Note that a comb-shaped portion in the common electrode
10603 and the pixel electrode 10605 may be formed of different
conductive layers. For example, the comb-shaped portion in the
common electrode 10603 may be formed of a conductive layer which is
the same as that of the scan line 10601 or the image signal line
10602. Similarly, the pixel electrode 10605 may be formed of a
conductive layer which is the same as that of the scan line 10601
or the image signal line 10602.
[0852] FIG. 42B is an example of a top plan view of a pixel in the
case where an FFS mode and a transistor are combined. By applying
the pixel structure shown in FIG. 42B to a liquid crystal display
device, a liquid crystal display device theoretically having a wide
viewing angle and response speed which has low dependency on a gray
scale can be obtained.
[0853] The pixel shown in FIG. 42B includes a scan line 10611, a
video signal line 10612, a common electrode 10613, a transistor
10614, and a pixel electrode 10615.
[0854] The scan line 10611 has a function of transmitting a signal
(a scan signal) to the pixel. The image signal line 10612 has a
function of transmitting a signal (an image signal) to the pixel.
Note that since the scan line 10611 and the image signal line 10612
are arranged in matrix, they are formed of conductive layers in
different layers. Note also that a semiconductor layer may be
provided at an intersection of the scan line 10611 and the image
signal line 10612. Thus, intersection capacitance between the scan
line 10611 and the image signal line 10612 can be reduced. Note
also that the image signal line 10612 is formed in accordance with
a shape of the pixel electrode 10615.
[0855] The common electrode 10613 is formed uniformly below the
pixel electrode 10615 and below and between the pixel electrodes
10615. Note that as the common electrode 10613, a
light-transmitting material or a reflective material may be used.
Alternatively, the common electrode 10613 may be formed by
combining a material in which a light-transmitting material and a
reflective material.
[0856] The transistor 10614 has a function as a switch which turns
on the image signal line 10612 and the pixel electrode 10615. Note
that one of a source region and a drain region of the transistor
10614 is provided so as to be surrounded by the other of the source
region and the drain region of the transistor 10614. Thus, the
channel width of the transistor 10614 increases, so that switching
capability can be improved. Note also that a gate electrode of the
transistor 10614 is provided so as to surround the semiconductor
layer.
[0857] The pixel electrode 10615 is electrically connected to one
of a source electrode and a drain electrode of the transistor
10614. The pixel electrode 10615 is an electrode for applying
signal voltage which is transmitted by the image signal line 10612
to a liquid crystal element. Note that the pixel electrode 10615 is
bent comb-shaped. The comb-shaped pixel electrode 10615 is provided
to be closer to a liquid crystal layer than a uniform portion of
the common electrode 10613. Thus, a lateral electric field can be
applied to liquid crystal molecules. In addition, a plurality of
regions having different alignment of the liquid crystal molecules
can be formed, so that a viewing angle can be improved. Note also
that as the pixel electrode 10615, a light-transmitting material or
a reflective material may be used. Alternatively, the pixel
electrode 10615 may be formed by combining a light-transmitting
material and a reflective material.
[0858] Although this embodiment mode is described with reference to
various drawings, the contents (or may be part of the contents)
described in each drawing can be freely applied to, combined with,
or replaced with the contents (or may be part of the contents)
described in another drawing. Further, even more drawings can be
formed by combining each part with another part in the
above-described drawings.
[0859] The contents (or may be part of the contents) described in
each drawing of this embodiment mode can be freely applied to,
combined with, or replaced with the contents (or may be part of the
contents) described in a drawing in another embodiment mode.
Further, even more drawings can be formed by combining each part
with part of another embodiment mode in the drawings of this
embodiment mode.
[0860] This embodiment mode shows an example of an embodied case of
the contents (or may be part of the contents) described in other
embodiment modes, an example of slight transformation thereof, an
example of partial modification thereof, an example of improvement
thereof, an example of detailed description thereof, an application
example thereof, an example of related part thereof, or the like.
Therefore, the contents described in other embodiment modes can be
freely applied to, combined with, or replaced with this embodiment
mode.
Embodiment Mode 6
[0861] In this embodiment mode, a peripheral portion of a liquid
crystal panel is described.
[0862] FIG. 43 is a cross-sectional view showing an example of a
liquid crystal display device including a so-called edge-light type
backlight unit 20101 and a liquid crystal panel 20107. An
edge-light type corresponds to a type in which a light source is
provided at an end of a backlight unit and fluorescence of the
light source is emitted from the entire light-emitting surface. The
edge-light type backlight unit is thin and can save power
[0863] The backlight unit 20101 includes a diffusion plate 20102, a
light guide plate 20103, a reflection plate 20104, a lamp reflector
20105, and a light source 20106.
[0864] The light source 20106 has a function of emitting light as
necessary. For example, as the light source 20106, a cold cathode
fluorescent lamp, a hot cathode fluorescent lamp, a light-emitting
diode, an inorganic EL element, an organic EL element, or the like
is used. The lamp reflector 20105 has a function of efficiently
guiding fluorescence from the light source 20106 to the light guide
plate 20103. The light guide plate 20103 has a function of guiding
light to the entire surface by total reflection of fluorescence.
The diffusion plate 20102 has a function of reducing variations in
brightness. The reflection plate 20104 has a function of reflecting
light which is leaked from the light guide plate 20103 downward (a
direction which is opposite to the liquid crystal panel 20107) to
be reused.
[0865] A control circuit for controlling luminance of the light
source 20106 is connected to the backlight unit 20101. By using
this control circuit, luminance of the light source 20106 can be
controlled.
[0866] FIGS. 44A to 44D are views each showing a detailed structure
of the edge-light type backlight unit. Note that description of a
diffusion plate, a light guide plate, a reflection plate, and the
like is omitted.
[0867] A backlight unit 20201 shown in FIG. 44A has a structure in
which a cold cathode fluorescent lamp 20203 is used as a light
source. In addition, a lamp reflector 20202 is provided to
efficiently reflect light from the cold cathode fluorescent lamp
20203. Such a structure is often used for a large display device
because luminance from the cold cathode fluorescent lamp is
high.
[0868] A backlight unit 20211 shown in FIG. 44B has a structure in
which light-emitting diodes (LEDs) 20213 are used as light sources.
For example, the light-emitting diodes (LEDs) 20213 which emit
white light are provided at a predetermined interval. In addition,
a lamp reflector 20212 is provided to efficiently reflect light
from the light-emitting diodes (LEDs) 20213.
[0869] Since luminance of light-emitting diodes is high, a
structure using light-emitting diodes is suitable for a large
display device. In addition, since light-emitting diodes are
excellent in color reproductivity, an arrangement area can be
reduced. Therefore, a frame of a display device can be
narrowed.
[0870] Note that in the case where light-emitting diodes are
mounted on a large display device, the light-emitting diodes can be
provided on a back side of the substrate. The light-emitting diodes
of R, G, and B are sequentially provided at a predetermined
interval. By providing the light-emitting diodes, color
reproductivity can be improved.
[0871] A backlight unit 20221 shown in FIG. 44C has a structure in
which light-emitting diodes (LEDs) 20223, light-emitting diodes
(LEDs) 20224, and light-emitting diodes (LEDs) 20225 of R, G, and B
are used as light sources. The light-emitting diodes (LEDs) 20223,
the light-emitting diodes (LEDs) 20224, and the light-emitting
diodes (LEDs) 20225 of R, G, and B are each provided at a
predetermined interval. By using the light-emitting diodes (LEDs)
20223, the light-emitting diodes (LEDs) 20224, and the
light-emitting diodes (LEDs) 20225 of R, G, and B, color
reproductivity can be improved. In addition, a lamp reflector 20222
is provided to efficiently reflect light from the light-emitting
diodes.
[0872] Since luminance of light-emitting diodes is high, a
structure using light-emitting diodes is suitable for a large
display device. In addition, since light-emitting diodes are
excellent in color reproductivity, an arrangement area can be
reduced. Therefore, a frame of a display device can be
narrowed.
[0873] By sequentially making the light-emitting diodes of R, G,
and B emit light in accordance with time, color display can be
performed. This is a so-called field sequential mode.
[0874] In addition, a light-emitting diode which emits white light
can be combined with the light-emitting diodes (LEDs) 20223, the
light-emitting diodes (LEDs) 20224, and the light-emitting diodes
(LEDs) 20225 of R, G, and B.
[0875] Note that in the case where light-emitting diodes are
mounted on a large display device, the light-emitting diodes can be
provided on a back side of the substrate. The light-emitting diodes
of R, G, and B are sequentially provided at a predetermined
interval. By providing the light-emitting diodes, color
reproductivity can be improved.
[0876] A backlight unit 20231 shown in FIG. 44D has a structure in
which light-emitting diodes (LEDs) 20233, light-emitting diodes
(LEDs) 20234, and light-emitting diodes (LEDs) 20235 of R, G, and B
are used as light sources. For example, among the light-emitting
diodes (LEDs) 20233, the light-emitting diodes (LEDs) 20234, and
the light-emitting diodes (LEDs) 20235 of R, G, and B, the
light-emitting diodes of a color with low emission intensity (e.g.,
green) are provided more than other light-emitting diodes. By using
the light-emitting diodes (LEDs) 20233, the light-emitting diodes
(LEDs) 20234, and the light-emitting diodes (LEDs) 20235 of R, G,
and B, color reproductivity can be improved. In addition, a lamp
reflector 20232 is provided to efficiently reflect light from the
light-emitting diodes.
[0877] Since luminance of light-emitting diodes is high, a
structure using light-emitting diodes is suitable for a large
display device. In addition, since light-emitting diodes are
excellent in color reproductivity, an arrangement area can be
reduced. Therefore, a frame of a display device can be
narrowed.
[0878] By sequentially making the light-emitting diodes of R, G,
and B emit light in accordance with time, color display can be
performed. This is a so-called field sequential mode.
[0879] In addition, a light-emitting diode which emits white light
can be combined with the light-emitting diodes (LEDs) 20233, the
light-emitting diodes (LEDs) 20234, and the light-emitting diodes
(LEDs) 20235 of R, G, and B.
[0880] Note that in the case where light-emitting diodes are
mounted on a large display device, the light-emitting diodes can be
provided on a back side of the substrate. The light-emitting diodes
of R, G, and B are sequentially provided at a predetermined
interval. By providing the light-emitting diodes, color
reproductivity can be improved.
[0881] FIG. 47A is a cross-sectional view showing an example of a
liquid crystal display device including a so-called direct-type
backlight unit and a liquid crystal panel. A direct type
corresponds to a type in which a light source is provided directly
under a light-emitting surface and fluorescence of the light source
is emitted from the entire light-emitting surface. The direct-type
backlight unit can efficiently utilize the amount of emitted
light.
[0882] A backlight unit 20500 includes a diffusion plate 20501, a
light-shielding plate 20502, a lamp reflector 20503, and a light
source 20504. In addition, a reference numeral 20505 denotes a
liquid crystal panel.
[0883] The light source 20504 has a function of emitting light as
necessary. For example, as the light source 20505, a cold cathode
fluorescent lamp, a hot cathode fluorescent lamp, a light-emitting
diode, an inorganic EL element, an organic EL element, or the like
is used. The lamp reflector 20503 has a function of efficiently
guiding fluorescence from the light source 20504 to the diffusion
plate 20501 and the light-shielding plate 20502. The
light-shielding plate 20502 has a function of reducing variations
in brightness by shielding much light as light becomes intense in
accordance with provision of the light source 20504. The diffusion
plate 20501 also has a function of reducing variations in
brightness.
[0884] A control circuit for controlling luminance of the light
source 20504 is connected to the backlight unit 20501. By using
this control circuit, luminance of the light source 20504 can be
controlled.
[0885] FIG. 47B is also a cross-sectional view showing an example
of a liquid crystal display device including a so-called
direct-type backlight unit and a liquid crystal panel.
[0886] A backlight unit 20510 includes a diffusion plate 20511; a
light-shielding plate 20512; a lamp reflector 20513; and a light
source (R) 20514a, a light source (G) 20514b, and a light source
(B) 20514c of R, G, and B. In addition, a reference numeral 20515
denotes a liquid crystal panel.
[0887] Each of the light source (R) 20514a, the light source (G)
20514b, and the light source (B) 20514c of R, G, and B has a
function of emitting light as necessary. For example, as each of
the light source (R) 20514a, the light source (G) 20514b, and the
light source (B) 20514c, a cold cathode fluorescent lamp, a hot
cathode fluorescent lamp, a light-emitting diode, an inorganic EL
element, an organic EL element, or the like is used. The lamp
reflector 20513 has a function of efficiently guiding fluorescence
from the light sources 20514a to 20514c to the diffusion plate
20511 and the light-shielding plate 20512. The light-shielding
plate 20512 has a function of reducing variations in brightness by
shielding much light as light becomes intense in accordance with
provision of the light sources 20514a to 20514c. The diffusion
plate 20511 also has a function of reducing variations in
brightness.
[0888] A control circuit for controlling luminance of the light
source (R) 20514a, the light source (G) 20514b, and the light
source (B) 20514c of R, G, and B is connected to the backlight unit
20511. By using this control circuit, luminance of the light source
(R) 20514a, the light source (G) 20514b, and the light source (B)
20514c of R, G, and B can be controlled.
[0889] FIG. 45 is a cross-sectional view showing an example of a
structure of a polarizing plate (also referred to as a polarizing
film).
[0890] A polarizing film 20300 includes a protective film 20301, a
substrate film 20302, a PVA polarizing film 20303, a substrate film
20304, an adhesive layer 20305, and a mold release film 20306.
[0891] The PVA polarizing film 20303 has a function of generating
light in only a certain vibration direction (linear polarized
light). Specifically, the PVA polarizing film 20303 includes
molecules (polarizers) in which lengthwise electron density and
widthwise electron density are greatly different from each other.
The PVA polarizing film 20303 can generate linear polarized light
by uniforming directions of the molecules in which lengthwise
electron density and widthwise electron density are greatly
different from each other.
[0892] For example, a high molecular film of polyvinyl alcohol is
doped with an iodine compound and a PVA film is pulled in a certain
direction, so that a film in which iodine molecules are aligned in
a certain direction can be obtained as the PVA polarizing film
20303. In this film, light which is parallel to a major axis of the
iodine molecule is absorbed by the iodine molecule. Note that a
dichroism dye may be used instead of iodine for high durability use
and high heat resistance use. Note also that it is preferable that
the dye be used for a liquid crystal display device which needs to
have durability and heat resistance, such as an in-car LCD or an
LCD for a projector.
[0893] When the PVA polarizing film 20303 is sandwiched by films to
be base materials (the substrate film 20302 and the substrate film
20304) from both sides, reliability can be improved. Note that the
PVA polarizing film 20303 may be sandwiched by triacetylcellulose
(TAC) films with high transparency and high durability. Note also
that each of the substrate films and the TAC films function as
protective films of polarizer included in the PVA polarizing film
20303.
[0894] The adhesive layer 20305 which is to be attached to a glass
substrate of the liquid crystal panel is attached to one of the
substrate films (the substrate film 20304). Note that the adhesive
layer 20305 is formed by applying an adhesive to one of the
substrate films (the substrate film 20304). The mold release film
20306 (a separate film) is provided to the adhesive layer
20305.
[0895] The protective film 20301 is provided to the other one of
the substrates films (the substrate film 20302).
[0896] A hard coating scattering layer (an anti-glare layer) may be
provided on a surface of the polarizing film 20300. Since the
surface of the hard coating scattering layer has minute unevenness
formed by AG treatment and has an anti-glare function which
scatters external light, reflection of external light in the liquid
crystal panel can be prevented. Surface reflection can also be
prevented.
[0897] Note also that a treatment in which plurality of optical
thin film layers having different refractive indexes are layered
(also referred to as anti-reflection treatment or AR treatment) may
be performed on the surface of the polarizing film 20300. The
plurality of layered optical thin film layers having different
refractive indexes can reduce reflectivity on the surface by an
interference effect of light.
[0898] FIGS. 46A to 46C are diagrams each showing an example of a
system block of the liquid crystal display device.
[0899] In a pixel portion 20405, signal lines 20412 which are
extended from a signal line driver circuit 20403 are provided. In
addition, in the pixel portion 20405, scan lines 20410 which are
extended from a scan line driver circuit 20404 are also provided.
In addition, a plurality of pixels are arranged in matrix in cross
regions of the signal lines 20412 and the scan lines 20410. Note
that each of the plurality of pixels includes a switching element.
Therefore, voltage for controlling inclination of liquid crystal
molecules can be separately input to each of the plurality of
pixels. A structure in which a switching element is provided in
each cross region in this manner is referred to as an active matrix
type. Note also that the present invention is not limited to such
an active matrix type and a structure of a passive matrix type may
be used. Since the passive matrix type does not have a switching
element in each pixel, a process is simple.
[0900] A driver circuit portion 20408 includes a control circuit
20402, the signal line driver circuit 20403, and the scan line
driver circuit 20404. An image signal 20401 is input to the control
circuit 20402. The signal line driver circuit 20403 and the scan
line driver circuit 20404 are controlled by the control circuit
20402 in accordance with this image signal 20401. That is, the
control circuit 20402 inputs a control signal to each of the signal
line driver circuit 20403 and the scan line driver circuit 20404.
Then, in accordance with this control signal, the signal line
driver circuit 20403 inputs a video signal to each of the signal
lines 20412 and the scan line driver circuit 20404 inputs a scan
signal to each of the scan lines 20410. Then, the switching element
included in the pixel is selected in accordance with the scan
signal and the video signal is input to a pixel electrode of the
pixel.
[0901] Note that the control circuit 20402 also controls a power
source 20407 in accordance with the image signal 20401. The power
source 20407 includes a unit for supplying power to a lighting unit
20406. As the lighting unit 20406, an edge-light type backlight
unit or a direct-type backlight unit can be used. Note also that a
front light may be used as the lighting unit 20406. A front light
corresponds to a plate-like lighting unit including a luminous body
and a light conducting body, which is attached to the front surface
side of a pixel portion and illuminates the whole area. By using
such a lighting unit, the pixel portion can be uniformly
illuminated at low power consumption.
[0902] As shown in FIG. 46B, the scan line driver circuit 20404
includes a shift register 20441, a level shifter 20442, and a
circuit functioning as a buffer 20443. A signal such as a gate
start pulse (GSP) or a gate clock signal (GCK) is input to the
shift register 20441.
[0903] As shown in FIG. 46C, the signal line driver circuit 20403
includes a shift register 20431, a first latch 20432, a second
latch 20433, a level shifter 20434, and a circuit functioning as a
buffer 20435. The circuit functioning as the buffer 20435
corresponds to a circuit which has a function of amplifying a weak
signal and includes an operational amplifier or the like. A signal
such as a start pulse (SSP) is input to the level shifter 20434 and
data (DATA) such as a video signal is input to the first latch
20432. A latch (LAT) signal can be temporally held in the second
latch 20433 and is simultaneously input to the pixel portion 20405.
This is referred to as line sequential driving. Therefore, when a
pixel is used in which not line sequential driving but dot
sequential driving is performed, the second latch can be
omitted.
[0904] Note that in this embodiment mode, a known liquid crystal
panel can be used for the liquid crystal panel. For example, a
structure in which a liquid crystal layer is sealed between two
substrates can be used as the liquid crystal panel. A transistor, a
capacitor, a pixel electrode, an alignment film, or the like is
formed over one of the substrates. A polarizing plate, a
retardation plate, or a prism sheet may be provided on the surface
opposite to a top surface of the one of the substrates. A color
filter, a black matrix, a counter electrode, an alignment film, or
the like is provided on the other one of the substrates. A
polarizing plate or a retardation plate may be provided on the
surface opposite to a top surface of the other one of the
substrates. The color filter and the black matrix may be formed
over the top surface of the one of the substrates. Note also that
three-dimensional display can be performed by providing a slit (a
grid) on the top surface side of the one of the substrates or the
surface opposite to the top surface side of the one of the
substrates.
[0905] Each of the polarizing plate, the retardation plate, and the
prism sheet can be provided between the two substrates.
Alternatively, each of the polarizing plate, the retardation plate,
and the prism sheet can be integrated with one of the two
substrates.
[0906] Although this embodiment mode is described with reference to
various drawings, the contents (or may be part of the contents)
described in each drawing can be freely applied to, combined with,
or replaced with the contents (or may be part of the contents)
described in another drawing. Further, even more drawings can be
formed by combining each part with another part in the
above-described drawings.
[0907] The contents (or may be part of the contents) described in
each drawing of this embodiment mode can be freely applied to,
combined with, or replaced with the contents (or may be part of the
contents) described in a drawing in another embodiment mode.
Further, even more drawings can be formed by combining each part
with part of another embodiment mode in the drawings of this
embodiment mode.
[0908] This embodiment mode shows an example of an embodied case of
the contents (or may be part of the contents) described in other
embodiment modes, an example of slight transformation thereof, an
example of partial modification thereof, an example of improvement
thereof, an example of detailed description thereof, an application
example thereof, an example of related part thereof, or the like.
Therefore, the contents described in other embodiment modes can be
freely applied to, combined with, or replaced with this embodiment
mode.
Embodiment Mode 7
[0909] In this embodiment mode, a driving method of a display
device is described. In particular, a driving method of a liquid
crystal display device is described.
[0910] First, overdriving is described with reference to FIGS. 48A
to 48C. FIG. 48A shows time change in output luminance of a display
element with respect to input voltage. Time change in output
luminance of the display element with respect to input voltage
30121 represented by a dashed line 30121 is as shown by output
luminance 30123 represented by a dashed line similarly. That is,
although voltage for obtaining intended output luminance Lo is Vi,
time in accordance with response speed of the element is necessary
before output luminance reaches the intended output luminance Lo
when Vi is directly input as input voltage.
[0911] Overdriving is a technique for increasing this response
speed. Specifically, this is a method as follows: first, Vo which
is larger voltage than Vi is applied to the element for a certain
time to increase response speed of the element and output luminance
is made close to the intended output luminance Lo, and then, the
input voltage is returned to Vi. The input voltage and the output
luminance at this time are as shown by input voltage 30122 and
output voltage 30124, respectively. In the graph of the output
luminance 30124, time for reaching the intended output luminance Lo
is shorter than that of the output luminance 30123.
[0912] Note that although the case where output luminance is
changed positively with respect to input voltage is described in
FIG. 48A, this embodiment mode also includes the case where output
luminance is changed negatively with respect to input voltage.
[0913] A circuit for realizing such driving is described with
reference to FIGS. 48B and 48C. First, the case where an input
image signal 30131 is a signal having an analog value (may be a
discrete value) and an output image signal 30132 is also a signal
having an analog value is described with reference to FIG. 48B. An
overdriving circuit shown in FIG. 48B includes an encoding circuit
30101, a frame memory 30102, a correction circuit 30103, and a D/A
converter circuit 30104.
[0914] First, the input image signal 30131 is input to the encoding
circuit 30101 and encoded. That is, the input image signal 30131 is
converted from an analog signal into a digital signal with an
appropriate bit number. After that, the converted digital signal is
input to each of the frame memory 30102 and the correction circuit
30103. An image signal of the previous frame which is held in the
frame memory 30102 is input to the correction circuit 30103 at the
same time. Then, in the correction circuit 30103, an image signal
corrected using an image signal of a frame and the image signal of
the previous frame is output in accordance with a numeric value
table which is prepared in advance. At this time, an output
switching signal 30133 may be input to the correction circuit 30103
and the corrected image signal and the image signal of the frame
may be switched to be output. Next, the corrected image signal or
the image signal of the frame is input to the D/A converter circuit
30104. Then, the output image signal 30132 which is an analog
signal having a value in accordance with the corrected image signal
or the image signal of the frame is output. In this manner,
overdriving is realized.
[0915] Next, the case where the input image signal 30131 is a
signal having a digital value and the output image signal 30132 is
also a signal having a digital value is described with reference to
FIG. 48C. An overdriving circuit shown in FIG. 48C includes a frame
memory 30112 and a correction circuit 30113.
[0916] First, the input image signal 30131 is a digital signal and
is input to each of the frame memory 30112 and the correction
circuit 30113. An image signal of the previous frame which is held
in the frame memory 30112 is input to the correction circuit 30113
at the same time. Then, in the correction circuit 30113, an image
signal corrected using an image signal of a frame and the image
signal of the previous frame is output in accordance with a numeric
value table which is prepared in advance. At this time, the output
switching signal 30133 may be input to the correction circuit 30113
and the corrected image signal and the image signal of the frame
may be switched to be output. In this manner, overdriving is
realized.
[0917] Note that the case where the input image signal 30131 is an
analog signal and the output image signal 30132 is a digital signal
is included in the overdriving circuit in this embodiment mode. At
this time, it is only necessary to omit the D/A converter circuit
30104 from the circuit shown in FIG. 48B. In addition, the case
where the input image signal 30131 is a digital signal and the
output image signal 30132 is an analog signal is included in the
overdriving circuit in this embodiment mode. At this time, it is
only necessary to omit the encoding circuit 30101 from the circuit
shown in FIG. 48B.
[0918] Driving which controls a potential of a common line is
described with reference to FIGS. 49A and 49B. FIG. 49A is a
diagram showing a plurality of pixel circuits in which one common
line is provided with respect to one scan line in a display device
using a display element which has capacitive properties like a
liquid crystal element. Each of the pixel circuits shown in FIG.
49A includes a transistor 30201, an auxiliary capacitor 30202, a
display element 30203, a video signal line 30204, a scan line
30205, and a common line 30206.
[0919] A gate electrode of the transistor 30201 is electrically
connected to the scan line 30205; one of a source electrode and a
drain electrode of the transistor 30201 is electrically connected
to the video signal line 30204; and the other of the source
electrode and the drain electrode of the transistor 30201 is
electrically connected to one of electrodes of the auxiliary
capacitor 30202 and one of electrodes of the display element 30203.
In addition, the other of the electrodes of the auxiliary capacitor
30202 is electrically connected to the common line 30206.
[0920] First, in each of pixels selected by the scan line 30205,
voltage corresponding to an image signal is applied to the display
element 30203 and the auxiliary capacitor 30202 through the video
signal line 30204 because the transistor 30201 is turned on. At
this time, when the image signal is a signal which makes all of
pixels connected to the common line 30206 display a minimum gray
scale or when the image signal is a signal which makes all of the
pixels connected to the common line 30206 display a maximum gray
scale, it is not necessary that the image signal be written in each
of the pixels through the video signal line 30204. Voltage applied
to the display element 30203 can be changed by changing a potential
of the common line 30206 instead of writing the image signal
through the video signal line 30204.
[0921] Next, FIG. 49B is a diagram showing a plurality of pixel
circuits in which two common lines are provided with respect to one
scan line in a display device using a display element which has
capacitive properties like a liquid crystal element. Each of the
pixel circuits shown in FIG. 49B includes a transistor 30211, an
auxiliary capacitor 30212, a display element 30213, a video signal
line 30214, a scan line 30215, a first common line 30216, and a
second common line 30217.
[0922] A gate electrode of the transistor 30211 is electrically
connected to the scan line 30215; one of a source electrode and a
drain electrode of the transistor 30211 is electrically connected
to the video signal line 30214; and the other of the source
electrode and the drain electrode of the transistor 30211 is
electrically connected to one of electrodes of the auxiliary
capacitor 30212 and one of electrodes of the display element 30213.
In addition, the other of the electrodes of the auxiliary capacitor
30212 is electrically connected to the first common line 30216.
Further, in a pixel which is adjacent to the pixel, the other of
the electrodes of the auxiliary capacitor 30212 is electrically
connected to the second common line 30217.
[0923] In the pixel circuits shown in FIG. 49B, the number of
pixels which are electrically connected to one common line is
small. Therefore, by changing a potential of the first common line
30216 or the second common line 30217 instead of writing an image
signal through the video signal line 30214, frequency of changing
voltage applied to the display element 30213 is significantly
increased. In addition, source inversion driving or dot inversion
driving can be performed. By performing source inversion driving or
dot inversion driving, reliability of the element can be improved
and a flicker can be suppressed.
[0924] A scanning backlight is described with reference to FIGS.
50A to 50C. FIG. 50A is a view showing a scanning backlight in
which cold cathode fluorescent lamps are arranged. The scanning
backlight shown in FIG. 50A includes a diffusion plate 30301 and N
pieces of cold cathode fluorescent lamps 30302-1 to 30302-N. The N
pieces of the cold cathode fluorescent lamps 30302-1 to 30302-N are
arranged on the back side of the diffusion plate 30301, so that the
N pieces of the cold cathode fluorescent lamps 30302-1 to 30302-N
can be scanned while luminance thereof is changed.
[0925] Change in luminance of each of the cold cathode fluorescent
lamps in scanning is described with reference to FIG. 50C. First,
luminance of the cold cathode fluorescent lamp 30302-1 is changed
for a certain period. After that, luminance of the cold cathode
fluorescent lamp 30302-2 which is provided adjacent to the cold
cathode fluorescent lamp 30302-1 is changed for the same period. In
this manner, luminance is changed sequentially from the cold
cathode fluorescent lamp 30302-1 to the cold cathode fluorescent
lamp 30302-N. Although luminance which is changed for a certain
period is set to be lower than original luminance in FIG. 50C, it
may also be higher than original luminance. In addition, although
scanning is performed from the cold cathode fluorescent lamps
30302-1 to 30302-N, scanning may also be performed from the cold
cathode fluorescent lamps 30302-N to 30302-1, which is in a
reversed order.
[0926] By performing driving as in FIG. 50C, average luminance of
the backlight can be decreased. Therefore, power consumption of the
backlight, which mainly takes up power consumption of the liquid
crystal display device, can be reduced.
[0927] Note that an LED may be used as a light source of the
scanning backlight. The scanning backlight in that case is as shown
in FIG. 50B. The scanning backlight shown in FIG. 50B includes a
diffusion plate 30311 and light sources 30312-1 to 30312-N, in each
of which LEDs are arranged. When the LED is used as the light
source of the scanning backlight, there is an advantage in that the
backlight can be thin and lightweight. In addition, there is also
an advantage that a color reproduction area can be widened.
Further, since the LEDs which are arranged in each of the light
sources 30312-1 to 30312-N can be similarly scanned, a dot scanning
backlight can also be obtained. By using the dot scanning
backlight, quality of a moving image can be further improved.
[0928] Note that when the LED is used as the light source of the
backlight, driving can be performed by changing luminance as shown
in FIG. 50C.
[0929] Although this embodiment mode is described with reference to
various drawings, the contents (or may be part of the contents)
described in each drawing can be freely applied to, combined with,
or replaced with the contents (or may be part of the contents)
described in another drawing. Further, even more drawings can be
formed by combining each part with another part in the
above-described drawings.
[0930] The contents (or may be part of the contents) described in
each drawing of this embodiment mode can be freely applied to,
combined with, or replaced with the contents (or may be part of the
contents) described in a drawing in another embodiment mode.
Further, even more drawings can be formed by combining each part
with part of another embodiment mode in the drawings of this
embodiment mode.
[0931] This embodiment mode shows an example of an embodied case of
the contents (or may be part of the contents) described in other
embodiment modes, an example of slight transformation thereof, an
example of partial modification thereof, an example of improvement
thereof, an example of detailed description thereof, an application
example thereof, an example of related part thereof, or the like.
Therefore, the contents described in other embodiment modes can be
freely applied to, combined with, or replaced with this embodiment
mode.
Embodiment Mode 8
[0932] In this embodiment mode, a pixel structure and an operation
of a pixel which can be applied to a liquid crystal display device
are described.
[0933] In this embodiment mode, as an operation mode of a liquid
crystal element, a TN (twisted nematic) mode, an IPS
(in-plane-switching) mode, an FFS (fringe field switching) mode, an
MVA (multi-domain vertical alignment) mode, a PVA (patterned
vertical alignment) mode, an ASM (axially symmetric aligned
micro-cell) mode, an OCB (optical compensated birefringence) mode,
an FLC (ferroelectric liquid crystal) mode, an AFLC
(antiferroelectric liquid crystal) mode, or the like can be
used.
[0934] FIG. 51A is a diagram showing an example of a pixel
structure which can be applied to the liquid crystal display
device.
[0935] A pixel 40100 includes a transistor 40101, a liquid crystal
element 40102, and a capacitor 40103. A gate electrode of the
transistor 40101 is connected to a wiring 40105. A first electrode
of the transistor 40101 is connected to a wiring 40104. A second
electrode of the transistor 40101 is connected to a first electrode
of the liquid crystal element 40102 and a first electrode of the
capacitor 40103. A second electrode of the liquid crystal element
40102 corresponds to a counter electrode 40107. A second electrode
of the capacitor 40103 is connected to a wiring 40106.
[0936] The wiring 40104 functions as a signal line. The wiring
40105 functions as a scan line. The wiring 40106 functions as a
capacitor line. The transistor 40101 functions as a switch. The
capacitor 40103 functions as a storage capacitor.
[0937] It is only necessary that the transistor 40101 function as a
switch, and the transistor 40101 may be a P-channel transistor or
an N-channel transistor.
[0938] A video signal is input to the wiring 40104. A scan signal
is input to the wiring 40105. A constant potential is supplied to
the wiring 40106. Note that the scan signal is an H-level or
L-level digital voltage signal. In the case where the transistor
40101 is an N-channel transistor, an H level of the scan signal is
a potential which can turn on the transistor 40101 and an L level
of the scan signal is a potential which can turn off the transistor
40101. Alternatively, in the case where the transistor 40101 is a
P-channel transistor, the H level of the scan signal is a potential
which can turn off the transistor 40101 and the L level of the scan
signal is a potential which can turn on the transistor 40101. Note
that the video signal has analog voltage. The video signal is a
potential which is lower than the H level of the scan signal and
higher than the L level of the scan signal. Note also that the
constant potential supplied to the wiring 40106 is preferably equal
to a potential of the counter electrode 40107.
[0939] Operations of the pixel 40100 are described by diving the
whole operations into the case where the transistor 40101 is on and
the case where the transistor 40101 is off.
[0940] In the case where the transistor 40101 is on, the wiring
40104 is electrically connected to the first electrode (a pixel
electrode) of the liquid crystal element 40102 and the first
electrode of the capacitor 40103. Therefore, the video signal is
input to the first electrode (the pixel electrode) of the liquid
crystal element 40102 and the first electrode of the capacitor
40103 from the wiring 40104 through the transistor 40101. In
addition, the capacitor 40103 holds a potential difference between
a potential of the video signal and the potential supplied to the
wiring 40106.
[0941] In the case where the transistor 40101 is off, the wiring
40104 is not electrically connected to the first electrode (the
pixel electrode) of the liquid crystal element 40102 and the first
electrode of the capacitor 40103. Therefore, each of the first
electrode of the liquid crystal element 40102 and the first
electrode of the capacitor 40103 is set in a floating state. Since
the capacitor 40103 holds the potential difference between the
potential of the video signal and the potential supplied to the
wiring 40106, each of the first electrode of the liquid crystal
element 40102 and the first electrode of the capacitor 40103 holds
a potential which is the same as or corresponds to the video
signal. Note that the liquid crystal element 40102 has
transmittance in accordance with the video signal.
[0942] FIG. 51B is a diagram showing an example of a pixel
structure which can be applied to the liquid crystal display
device. In particular, FIG. 51B is a diagram showing an example of
a pixel structure which can be applied to a liquid crystal display
device suitable for a lateral electric field mode (including an IPS
mode and an FFS mode).
[0943] A pixel 40110 includes a transistor 40111, a liquid crystal
element 40112, and a capacitor 40113. A gate electrode of the
transistor 40111 is connected to a wiring 40115. A first electrode
of the transistor 40111 is connected to a wiring 40114. A second
electrode of the transistor 40111 is connected to a first electrode
of the liquid crystal element 40112 and a first electrode of the
capacitor 40113. A second electrode of the liquid crystal element
40112 is connected to a wiring 40116. A second electrode of the
capacitor 40113 is connected to the wiring 40116.
[0944] The wiring 40114 functions as a signal line. The wiring
40115 functions as a scan line. The wiring 40116 functions as a
capacitor line. The transistor 40111 functions as a switch. The
capacitor 40113 functions as a storage capacitor.
[0945] It is only necessary that the transistor 40111 function as a
switch, and the transistor 40111 may be a P-channel transistor or
an N-channel transistor.
[0946] A video signal is input to the wiring 40114. A scan signal
is input to the wiring 40115. A constant potential is supplied to
the wiring 40116. Note that the scan signal is an H-level or
L-level digital voltage signal. In the case where the transistor
40111 is an N-channel transistor, an H level of the scan signal is
a potential which can turn on the transistor 40111 and an L level
of the scan signal is a potential which can turn off the transistor
40111. Alternatively, in the case where the transistor 40111 is a
P-channel transistor, the H level of the scan signal is a potential
which can turn off the transistor 40111 and the L level of the scan
signal is a potential which can turn on the transistor 40111. Note
that the video signal has analog voltage. The video signal is a
potential which is lower than the H level of the scan signal and
higher than the L level of the scan signal.
[0947] Operations of the pixel 40110 are described by diving the
whole operations into the case where the transistor 40111 is on and
the case where the transistor 40111 is off.
[0948] In the case where the transistor 40111 is on, the wiring
40114 is electrically connected to the first electrode (a pixel
electrode) of the liquid crystal element 40112 and the first
electrode of the capacitor 40113. Therefore, the video signal is
input to the first electrode (the pixel electrode) of the liquid
crystal element 40112 and the first electrode of the capacitor
40113 from the wiring 40114 through the transistor 40111. In
addition, the capacitor 40113 holds a potential difference between
a potential of the video signal and the potential supplied to the
wiring 40116.
[0949] In the case where the transistor 40111 is off, the wiring
40114 is not electrically connected to the first electrode (the
pixel electrode) of the liquid crystal element 40112 and the first
electrode of the capacitor 40113. Therefore, each of the first
electrode of the liquid crystal element 40112 and the first
electrode of the capacitor 40113 is set in a floating state. Since
the capacitor 40113 holds the potential difference between the
potential of the video signal and the potential supplied to the
wiring 40116, each of the first electrode of the liquid crystal
element 40112 and the first electrode of the capacitor 40113 holds
a potential which is the same as or corresponds to the video
signal. Note that the liquid crystal element 40112 has
transmittance in accordance with the video signal.
[0950] FIG. 52 is a diagram showing an example of a pixel structure
which can be applied to the liquid crystal display device. In
particular, FIG. 52 is a diagram showing an example of a pixel
structure in which an aperture ratio of a pixel can be increased by
reducing the number of wirings.
[0951] FIG. 52 shows two pixels which are provided in the same
column direction (a pixel 40200 and a pixel 40210). For example,
when the pixel 40200 is provided in an N-th row, the pixel 40210 is
provided in an (N+1)th row.
[0952] A pixel 40200 includes a transistor 40201, a liquid crystal
element 40202, and a capacitor 40203. A gate electrode of the
transistor 40201 is connected to a wiring 40205. A first electrode
of the transistor 40201 is connected to a wiring 40204. A second
electrode of the transistor 40201 is connected to a first electrode
of the liquid crystal element 40202 and a first electrode of the
capacitor 40203. A second electrode of the liquid crystal element
40202 corresponds to a counter electrode 40207. A second electrode
of the capacitor 40203 is connected to a wiring which is the same
as a wiring connected to a gate electrode of a transistor of the
previous row.
[0953] A pixel 40210 includes a transistor 40211, a liquid crystal
element 40212, and a capacitor 40213. A gate electrode of the
transistor 40211 is connected to a wiring 40215. A first electrode
of the transistor 40211 is connected to the wiring 40204. A second
electrode of the transistor 40211 is connected to a first electrode
of the liquid crystal element 40212 and a first electrode of the
capacitor 40213. A second electrode of the liquid crystal element
40212 corresponds to the counter electrode 40207. A second
electrode of the capacitor 40213 is connected to the wiring which
is the same as the wiring connected to the gate electrode of the
transistor of the previous row (the wiring 40205).
[0954] The wiring 40204 functions as a signal line. The wiring
40205 functions as a scan line of the N-th row. The wiring 40205
also functions as a capacitor line of the (N+1)th row. The
transistor 40201 functions as a switch. The capacitor 40203
functions as a storage capacitor.
[0955] The wiring 40215 functions as a scan line of the (N+1)th
row. The wiring 40215 also functions as a capacitor line of an
(N+2)th row. The transistor 40211 functions as a switch. The
capacitor 40213 functions as a storage capacitor.
[0956] It is only necessary that each of the transistor 40201 and
the transistor 40211 function as a switch, and each of the
transistor 40201 and the transistor 40211 may be a P-channel
transistor or an N-channel transistor.
[0957] A video signal is input to the wiring 40204. A scan signal
(of an N-th row) is input to the wiring 40205. A scan signal (of an
(N+1)th row) is input to the wiring 40215.
[0958] The scan signal is an H-level or L-level digital voltage
signal. In the case where the transistor 40201 (or the transistor
40211) is an N-channel transistor, an H level of the scan signal is
a potential which can turn on the transistor 40201 (or the
transistor 40211) and an L level of the scan signal is a potential
which can turn off the transistor 40201 (or the transistor 40211).
Alternatively, in the case where the transistor 40201 (or the
transistor 40211) is a P-channel transistor, the H level of the
scan signal is a potential which can turn off the transistor 40201
(or the transistor 40211) and the L level of the scan signal is a
potential which can turn on the transistor 40201 (or the transistor
40211). Note that the video signal has analog voltage. The video
signal is a potential which is lower than the H level of the scan
signal and higher than the L level of the scan signal.
[0959] Operations of the pixel 40200 are described by diving the
whole operations into the case where the transistor 40201 is on and
the case where the transistor 40201 is off.
[0960] In the case where the transistor 40201 is on, the wiring
40204 is electrically connected to the first electrode (a pixel
electrode) of the liquid crystal element 40202 and the first
electrode of the capacitor 40203. Therefore, the video signal is
input to the first electrode (the pixel electrode) of the liquid
crystal element 40202 and the first electrode of the capacitor
40203 from the wiring 40204 through the transistor 40201. In
addition, the capacitor 40203 holds a potential difference between
a potential of the video signal and a potential supplied to the
wiring which is the same as the wiring connected to the gate
electrode of the transistor of the previous row.
[0961] In the case where the transistor 40201 is off, the wiring
40204 is not electrically connected to the first electrode (the
pixel electrode) of the liquid crystal element 40202 and the first
electrode of the capacitor 40203. Therefore, each of the first
electrode of the liquid crystal element 40202 and the first
electrode of the capacitor 40203 is set in a floating state. Since
the capacitor 40203 holds the potential difference between the
potential of the video signal and the potential of the wiring which
is the same as the wiring connected to the gate electrode of the
transistor of the previous row, each of the first electrode of the
liquid crystal element 40202 and the first electrode of the
capacitor 40203 holds a potential which is the same as or
corresponds to the video signal. Note that the liquid crystal
element 40202 has transmittance in accordance with the video
signal.
[0962] Operations of the pixel 40210 are described by diving the
whole operations into the case where the transistor 40211 is on and
the case where the transistor 40211 is off.
[0963] In the case where the transistor 40211 is on, the wiring
40204 is electrically connected to the first electrode (a pixel
electrode) of the liquid crystal element 40212 and the first
electrode of the capacitor 40213. Therefore, the video signal is
input to the first electrode (the pixel electrode) of the liquid
crystal element 40212 and the first electrode of the capacitor
40213 from the wiring 40204 through the transistor 40211. In
addition, the capacitor 40213 holds a potential difference between
a potential of the video signal and a potential supplied to a
wiring which is the same as the wiring connected to the gate
electrode of the transistor of the previous row (the wiring
40205).
[0964] In the case where the transistor 40211 is off, the wiring
40214 is not electrically connected to the first electrode (the
pixel electrode) of the liquid crystal element 40212 and the first
electrode of the capacitor 40213. Therefore, each of the first
electrode of the liquid crystal element 40212 and the first
electrode of the capacitor 40213 is set in a floating state. Since
the capacitor 40213holds the potential difference between the
potential of the video signal and the potential of the wiring which
is the same as the wiring connected to the gate electrode of the
transistor of the previous row (the wiring 40215), each of the
first electrode (the pixel electrode) of the liquid crystal element
40212 and the first electrode of the capacitor 40213 holds a
potential which is the same as or corresponds to the video signal.
Note that the liquid crystal element 40212 has transmittance in
accordance with the video signal.
[0965] FIG. 53 is a diagram showing an example of a pixel structure
which can be applied to the liquid crystal display device. In
particular, FIG. 53 is a diagram showing an example of a pixel
structure in which a viewing angle can be improved by using a
subpixel.
[0966] A pixel 40320 includes a subpixel 40300 and a subpixel
40310. Although the case in which the pixel 40320 includes two
subpixels is described below, the pixel 40320 may include three or
more subpixels.
[0967] The subpixel 40300 includes a transistor 40301, a liquid
crystal element 40302, and a capacitor 40303. A gate electrode of
the transistor 40301 is connected to a wiring 40305. A first
electrode of the transistor 40301 is connected to a wiring 40304. A
second electrode of the transistor 40301 is connected to a first
electrode of the liquid crystal element 40302 and a first electrode
of the capacitor 40303. A second electrode of the liquid crystal
element 40302 corresponds to a counter electrode 40307. A second
electrode of the capacitor 40303 is connected to a wiring
40306.
[0968] The subpixel 40310 includes a transistor 40311, a liquid
crystal element 40312, and a capacitor 40313. A gate electrode of
the transistor 40311 is connected to a wiring 40315. A first
electrode of the transistor 40311 is connected to the wiring 40304.
A second electrode of the transistor 40311 is connected to a first
electrode of the liquid crystal element 40312 and a first electrode
of the capacitor 40313. A second electrode of the liquid crystal
element 40312 corresponds to the counter electrode 40307. A second
electrode of the capacitor 40313 is connected to a wiring
40306.
[0969] The wiring 40304 functions as a signal line. Each of the
wiring 40305 and the wiring 40315 functions as a scan line. The
wiring 40306 functions as a capacitor line. Each of the transistor
40301 and the transistor 40311 functions as a switch. Each of the
capacitor 40303 and the capacitor 40313 functions as a storage
capacitor.
[0970] It is only necessary that each of the transistor 40301 and
the transistor 40311 function as a switch, and each of the
transistor 40301 and the transistor 40311 may be a P-channel
transistor or an N-channel transistor.
[0971] A video signal is input to the wiring 40304. A scan signal
is input to the wiring 40305 and the wiring 40315. A constant
potential is supplied to the wiring 40306.
[0972] The scan signal is an H-level or L-level digital voltage
signal. In the case where the transistor 40301 (or the transistor
40311) is an N-channel transistor, an H level of the scan signal is
a potential which can turn on the transistor 40301 (or the
transistor 40311) and an L level of the scan signal is a potential
which can turn off the transistor 40301 (or the transistor 40311).
Alternatively, in the case where the transistor 40301 (or the
transistor 40311) is a P-channel transistor, the H level of the
scan signal is a potential which can turn off the transistor 40301
(or the transistor 40311) and the L level of the scan signal is a
potential which can turn on the transistor 40301 (or the transistor
40311). Note that the video signal has analog voltage. The video
signal is a potential which is lower than the H level of the scan
signal and higher than the L level of the scan signal. Note also
that the constant potential supplied to the wiring 40306 is
preferably equal to a potential of the counter electrode 40307.
[0973] Operations of the pixel 40320 are described by diving the
whole operations into the case where the transistor 40301 is on and
the transistor 40311 is off, the case where the transistor 40301 is
off and the transistor 40311 is on, and the case where the
transistor 40301 and the transistor 40311 are off.
[0974] In the case where the transistor 40301 is on and the
transistor 40311 is off, the wiring 40304 is electrically connected
to the first electrode (a pixel electrode) of the liquid crystal
element 40302 and the first electrode of the capacitor 40303 in the
subpixel 40300. Therefore, the video signal is input to the first
electrode (the pixel electrode) of the liquid crystal element 40302
and the first electrode of the capacitor 40303 from the wiring
40304 through the transistor 40301. In addition, the capacitor
40303 holds a potential difference between a potential of the video
signal and a potential supplied to the wiring 40306. At this time,
the wiring 40304 is not electrically connected to the first
electrode (the pixel electrode) of the liquid crystal element 40312
and the first electrode of the capacitor 40313 in the subpixel
40310. Therefore, the video signal is not input to the subpixel
40310.
[0975] In the case where the transistor 40301 is off and the
transistor 40311 is on, the wiring 40304 is not electrically
connected to the first electrode (the pixel electrode) of the
liquid crystal element 40302 and the first electrode of the
capacitor 40303 in the subpixel 40300. Therefore, each of the first
electrode of the liquid crystal element 40302 and the first
electrode of the capacitor 40303 is set in a floating state. Since
the capacitor 40303 holds the potential difference between the
potential of the video signal and the potential supplied to the
wiring 40306, each of the first electrode of the liquid crystal
element 40302 and the first electrode of the capacitor 40303 holds
a potential which is the same as or corresponds to the video
signal. At this time, the wiring 40304 is electrically connected to
the first electrode (the pixel electrode) of the liquid crystal
element 40312 and the first electrode of the capacitor 40313 in the
subpixel 40310. Therefore, the video signal is input to the first
electrode (the pixel electrode) of the liquid crystal element 40312
and the first electrode of the capacitor 40313 from the wiring
40304 through the transistor 40311. In addition, the capacitor
40313 holds a potential difference between a potential of the video
signal and a potential supplied to the wiring 40306.
[0976] In the case where the transistor 40301 and the transistor
40311 are off, the wiring 40304 is not electrically connected to
the first electrode (the pixel electrode) of the liquid crystal
element 40302 and the first electrode of the capacitor 40303 in the
subpixel 40300. Therefore, each of the first electrode of the
liquid crystal element 40302 and the first electrode of the
capacitor 40303 is set in a floating state. Since the capacitor
40303 holds the potential difference between the potential of the
video signal and the potential supplied to the wiring 40306, each
of the first electrode of the liquid crystal element 40302 and the
first electrode of the capacitor 40303 holds a potential which is
the same as or corresponds to the video signal. Note that the
liquid crystal element 40302 has transmittance in accordance with
the video signal. At this time, the wiring 40304 is not
electrically connected to the first electrode (the pixel electrode)
of the liquid crystal element 40312 and the first electrode of the
capacitor 40313 similarly in the subpixel 40310. Therefore, each of
the first electrode of the liquid crystal element 40312 and the
first electrode of the capacitor 40313 is set in a floating state.
Since the capacitor 40313 holds the potential difference between
the potential of the video signal and the potential of the wiring
40306, each of the first electrode of the liquid crystal element
40312 and the first electrode of the capacitor 40313 holds a
potential which is the same as or corresponds to the video signal.
Note that the liquid crystal element 40312 has transmittance in
accordance with the video signal.
[0977] A video signal input to the subpixel 40300 may be a value
which is different from that of a video signal input to the
subpixel 40310. In this case, the viewing angle can be widened
because alignment of liquid crystal molecules of the liquid crystal
element 40302 and alignment of liquid crystal molecules of the
liquid crystal element 40312 can be varied from each other.
[0978] Although this embodiment mode is described with reference to
various drawings, the contents (or may be part of the contents)
described in each drawing can be freely applied to, combined with,
or replaced with the contents (or may be part of the contents)
described in another drawing. Further, even more drawings can be
formed by combining each part with another part in the
above-described drawings.
[0979] The contents (or may be part of the contents) described in
each drawing of this embodiment mode can be freely applied to,
combined with, or replaced with the contents (or may be part of the
contents) described in a drawing in another embodiment mode.
Further, even more drawings can be formed by combining each part
with part of another embodiment mode in the drawings of this
embodiment mode.
[0980] This embodiment mode shows an example of an embodied case of
the contents (or may be part of the contents) described in other
embodiment modes, an example of slight transformation thereof, an
example of partial modification thereof, an example of improvement
thereof, an example of detailed description thereof, an application
example thereof, an example of related part thereof, or the like.
Therefore, the contents described in other embodiment modes can be
freely applied to, combined with, or replaced with this embodiment
mode.
Embodiment Mode 9
[0981] In this embodiment mode, various liquid crystal modes are
described.
[0982] First, various liquid crystal modes are described with
reference to cross-sectional views.
[0983] FIGS. 54A and 54B are schematic views of cross sections of a
TN mode.
[0984] A liquid crystal layer 50100 is held between a first
substrate 50101 and a second substrate 50102 which are provided so
as to be opposite to each other. A first electrode 50105 is formed
on a top surface of the first substrate 50101. A second electrode
50106 is formed on a top surface of the second substrate 50102. A
first polarizing plate 50103 is provided on a surface of the first
substrate 50101, which does not face the liquid crystal layer. A
second polarizing plate 50104 is provided on a surface of the
second substrate 50102, which does not face the liquid crystal
layer. Note that the first polarizing plate 50103 and the second
polarizing plate 50104 are provided so as to be in a cross nicol
state.
[0985] The first polarizing plate 50103 may be provided on the top
surface of the first substrate 50101. The second polarizing plate
50104 may be provided on the top surface of the second substrate
50102.
[0986] It is only necessary that at least one of the first
electrode 50105 and the second electrode 50106 have transparency (a
transmissive or reflective liquid crystal display device).
Alternatively, both the first electrode 50105 and the second
electrode 50106 may have transparency, and part of one of the
electrodes may have reflectivity (a semi-transmissive liquid
crystal display device).
[0987] FIG. 54A is a schematic view of a cross section in the case
where voltage is applied to the first electrode 50105 and the
second electrode 50106 (referred to as a vertical electric field
mode). Since liquid crystal molecules are aligned longitudinally,
light emitted from a backlight is not affected by birefringence of
the liquid crystal molecules. In addition, since the first
polarizing plate 50103 and the second polarizing plate 50104 are
provided so as to be in a cross nicol state, light emitted from the
backlight cannot pass through the substrate. Therefore, black
display is performed.
[0988] Note that by controlling voltage applied to the first
electrode 50105 and the second electrode 50106, conditions of the
liquid crystal molecules can be controlled. Therefore, since the
amount of light emitted from the backlight passing through the
substrate can be controlled, predetermined image display can be
performed.
[0989] FIG. 54B is a schematic view of a cross section in the case
where voltage is not applied to the first electrode 50105 and the
second electrode 50106. Since the liquid crystal molecules are
aligned laterally and rotated in a plane, light emitted from a
backlight is affected by birefringence of the liquid crystal
molecules. In addition, since the first polarizing plate 50103 and
the second polarizing plate 50104 are provided so as to be in a
cross nicol state, light emitted from the backlight passes through
the substrate. Therefore, white display is performed. This is a
so-called normally white mode.
[0990] A liquid crystal display device having a structure shown in
FIG. 54A or FIG. 54B can perform full-color display by being
provided with a color filter. The color filter can be provided over
a first substrate 50101 side or a second substrate 50102 side.
[0991] It is only necessary that a known material be used for a
liquid crystal material used for a TN mode.
[0992] FIGS. 55A and 55B are schematic views of cross sections of a
VA mode. In the VA mode, liquid crystal molecules are aligned such
that they are vertical to a substrate when there is no electric
field.
[0993] A liquid crystal layer 50200 is held between a first
substrate 50201 and a second substrate 50202 which are provided so
as to be opposite to each other. A first electrode 50205 is formed
on a top surface of the first substrate 50201. A second electrode
50206 is formed on a top surface of the second substrate 50202. A
first polarizing plate 50203 is provided on a surface of the first
substrate 50201, which does not face the liquid crystal layer. A
second polarizing plate 50204 is provided on a surface of the
second substrate 50202, which does not face the liquid crystal
layer. Note that the first polarizing plate 50203 and the second
polarizing plate 50204 are provided so as to be in a cross nicol
state.
[0994] The first polarizing plate 50203 may be provided on the top
surface of the first substrate 50201. The second polarizing plate
50204 may be provided on the top surface of the second substrate
50202.
[0995] It is only necessary that at least one of the first
electrode 50205 and the second electrode 50206 have transparency (a
transmissive or reflective liquid crystal display device).
Alternatively, both the first electrode 50205 and the second
electrode 50206 may have transparency, and part of one of the
electrodes may have reflectivity (a semi-transmissive liquid
crystal display device).
[0996] FIG. 55A is a schematic view of a cross section in the case
where voltage is applied to the first electrode 50205 and the
second electrode 50206 (referred to as a vertical electric field
mode). Since liquid crystal molecules are aligned laterally, light
emitted from a backlight is affected by birefringence of the liquid
crystal molecules. In addition, since the first polarizing plate
50203 and the second polarizing plate 50204 are provided so as to
be in a cross nicol state, light emitted from the backlight passes
through the substrate. Therefore, white display is performed.
[0997] Note that by controlling voltage applied to the first
electrode 50205 and the second electrode 50206, conditions of the
liquid crystal molecules can be controlled. Therefore, since the
amount of light emitted from the backlight passing through the
substrate can be controlled, predetermined image display can be
performed.
[0998] FIG. 55B is a schematic view of a cross section in the case
where voltage is not applied to the first electrode 50205 and the
second electrode 50206. Since liquid crystal molecules are aligned
longitudinally, light emitted from a backlight is not affected by
birefringence of the liquid crystal molecules. In addition, since
the first polarizing plate 50203 and the second polarizing plate
50204 are provided so as to be in a cross nicol state, light
emitted from the backlight does not pass through the substrate.
Therefore, black display is performed. This is a so-called normally
black mode.
[0999] A liquid crystal display device having a structure shown in
FIG. 55A or FIG. 55B can perform full-color display by being
provided with a color filter. The color filter can be provided over
a first substrate 50201 side or a second substrate 50202 side.
[1000] It is only necessary that a known material be used for a
liquid crystal material used for a VA mode.
[1001] FIGS. 55C and 55D are schematic views of cross sections of
an MVA mode. In the MVA mode, viewing angle dependency of each
portion is compensated by each other.
[1002] A liquid crystal layer 50210 is held between a first
substrate 50211 and a second substrate 50212 which are provided so
as to be opposite to each other. A first electrode 50215 is formed
on a top surface of the first substrate 50211. A second electrode
50216 is formed on a top surface of the second substrate 50212. A
first protrusion 50217 for controlling alignment is formed on the
first electrode 50215. A second protrusion 50218 for controlling
alignment is formed over the second electrode 50216. A first
polarizing plate 50213 is provided on a surface of the first
substrate 50211, which does not face the liquid crystal layer A
second polarizing plate 50214 is provided on a surface of the
second substrate 50212, which does not face the liquid crystal
layer. Note that the first polarizing plate 50213 and the second
polarizing plate 50214 are provided so as to be in a cross nicol
state.
[1003] The first polarizing plate 50213 may be provided on the top
surface of the first substrate 50211. The second polarizing plate
50214 may be provided on the top surface of the second substrate
50212.
[1004] It is only necessary that at least one of the first
electrode 50215 and the second electrode 50216 have transparency (a
transmissive or reflective liquid crystal display device).
Alternatively, both the first electrode 50215 and the second
electrode 50216 may have transparency, and part of one of the
electrodes may have reflectivity (a semi-transmissive liquid
crystal display device).
[1005] FIG. 55C is a schematic view of a cross section in the case
where voltage is applied to the first electrode 50215 and the
second electrode 50216 (referred to as a vertical electric field
mode). Since liquid crystal molecules are aligned so as to tilt
toward the first protrusion 50217 and the second protrusion 50218,
light emitted from a backlight is affected by birefringence of the
liquid crystal molecules. In addition, since the first polarizing
plate 50213 and the second polarizing plate 50214 are provided so
as to be in a cross nicol state, light emitted from the backlight
passes through the substrate. Therefore, white display is
performed.
[1006] Note that by controlling voltage applied to the first
electrode 50215 and the second electrode 50216, conditions of the
liquid crystal molecules can be controlled. Therefore, since the
amount of light emitted from the backlight passing through the
substrate can be controlled, predetermined image display can be
performed.
[1007] FIG. 55D is a schematic view of a cross section in the case
where voltage is not applied to the first electrode 50215 and the
second electrode 50216. Since liquid crystal molecules are aligned
longitudinally, light emitted from a backlight is not affected by
birefringence of the liquid crystal molecules. In addition, since
the first polarizing plate 50213 and the second polarizing plate
50214 are provided so as to be in a cross nicol state, light
emitted from the backlight does not pass through the substrate.
Therefore, black display is performed. This is a so-called normally
black mode.
[1008] A liquid crystal display device having a structure shown in
FIG. 55C or FIG. 55D can perform full-color display by being
provided with a color filter. The color filter can be provided over
a first substrate 50211 side or a second substrate 50212 side.
[1009] It is only necessary that a known material be used for a
liquid crystal material used for an MVA mode.
[1010] FIGS. 56A and 56B are schematic views of cross sections of
an OCB mode. In the OCB mode, viewing angle dependency is low
because alignment of liquid crystal molecules in a liquid crystal
layer can be optically compensated. This state of the liquid
crystal molecules is referred to as bend alignment.
[1011] A liquid crystal layer 50300 is held between a first
substrate 50301 and a second substrate 50302 which are provided so
as to be opposite to each other. A first electrode 50305 is formed
on a top surface of the first substrate 50301. A second electrode
50306 is formed on a top surface of the second substrate 50302. A
first polarizing plate 50303 is provided on a surface of the first
substrate 50301, which does not face the liquid crystal layer
50300. A second polarizing plate 50304 is provided on a surface of
the second substrate 50302, which does not face the liquid crystal
layer 50300. Note that the first polarizing plate 50303 and the
second polarizing plate 50304 are provided so as to be in a cross
nicol state.
[1012] The first polarizing plate 50303 may be provided on the top
surface of the first substrate 50301, i.e., may be provided between
the first substrate 50301 and the liquid crystal layer. The second
polarizing plate 50304 may be provided on the top surface of the
second substrate 50302, i.e., may be provided between the second
substrate 50302 and the liquid crystal layer.
[1013] It is only necessary that at least one of the first
electrode 50305 and the second electrode 50306 have transparency (a
transmissive or reflective liquid crystal display device).
Alternatively, both the first electrode 50305 and the second
electrode 50306 may have transparency, and part of one of the
electrodes may have reflectivity (a semi-transmissive liquid
crystal display device).
[1014] FIG. 56A is a schematic view of a cross section in the case
where voltage is applied to the first electrode 50305 and the
second electrode 50306 (referred to as a vertical electric field
mode). Since liquid crystal molecules are aligned longitudinally,
light emitted from a backlight is not affected by birefringence of
the liquid crystal molecules. In addition, since the first
polarizing plate 50303 and the second polarizing plate 50304 are
provided so as to be in a cross nicol state, light emitted from the
backlight does not pass through the substrate. Therefore, black
display is performed.
[1015] Note that by controlling voltage applied to the first
electrode 50305 and the second electrode 50306, conditions of the
liquid crystal molecules can be controlled. Therefore, since the
amount of light emitted from the backlight passing through the
substrate can be controlled, predetermined image display can be
performed.
[1016] FIG. 56B is a schematic view of a cross section in the case
where voltage is not applied to the first electrode 50305 and the
second electrode 50306. Since liquid crystal molecules are in a
bend alignment state, light emitted from a backlight is affected by
birefringence of the liquid crystal molecules. In addition, since
the first polarizing plate 50303 and the second polarizing plate
50304 are provided so as to be in a cross nicol state, light
emitted from the backlight passes through the substrate. Therefore,
white display is performed. This is a so-called normally white
mode.
[1017] A liquid crystal display device having a structure shown in
FIG. 56A or FIG. 56B can perform full-color display by being
provided with a color filter. The color filter can be provided over
a first substrate 50301 side or a second substrate 50302 side.
[1018] It is only necessary that a known material be used for a
liquid crystal material used for an OCB mode.
[1019] FIGS. 56C and 56D are schematic views of cross sections of
an FLC mode or an AFLC mode.
[1020] A liquid crystal layer 50310 is held between a first
substrate 50311 and a second substrate 50312 which are provided so
as to be opposite to each other. A first electrode 50315 is formed
on a top surface of the first substrate 50311. A second electrode
50316 is formed on a top surface of the second substrate 50312. A
first polarizing plate 50313 is provided on a surface of the first
substrate 50311, which does not face the liquid crystal layer. A
second polarizing plate 50314 is provided on a surface of the
second substrate 50312, which does not face the liquid crystal
layer. Note that the first polarizing plate 50313 and the second
polarizing plate 50314 are provided so as to be in a cross nicol
state.
[1021] The first polarizing plate 50313 may be provided on the top
surface of the first substrate 50311. The second polarizing plate
50314 may be provided on the top surface of the second substrate
50312.
[1022] It is only necessary that at least one of the first
electrode 50315 and the second electrode 50316 have transparency (a
transmissive or reflective liquid crystal display device).
Alternatively, both the first electrode 50315 and the second
electrode 50316 may have transparency, and part of one of the
electrodes may have reflectivity (a semi-transmissive liquid
crystal display device).
[1023] FIG. 56C is a schematic view of a cross section in the case
where voltage is applied to the first electrode 50315 and the
second electrode 50316 (referred to as a vertical electric field
mode). Since liquid crystal molecules are aligned laterally in a
direction which is deviated from a rubbing direction, light emitted
from a backlight is affected by birefringence of the liquid crystal
molecules. In addition, since the first polarizing plate 50313 and
the second polarizing plate 50314 are provided so as to be in a
cross nicol state, light emitted from the backlight passes through
the substrate. Therefore, white display is performed.
[1024] Note that by controlling voltage applied to the first
electrode 50315 and the second electrode 50316, conditions of the
liquid crystal molecules can be controlled. Therefore, since the
amount of light emitted from the backlight passing through the
substrate can be controlled, predetermined image display can be
performed.
[1025] FIG. 56D is a schematic view of a cross section in the case
where voltage is not applied to the first electrode 50315 and the
second electrode 50316. Since liquid crystal molecules are aligned
laterally in a rubbing direction, light emitted from a backlight is
not affected by birefringence of the liquid crystal molecules. In
addition, since the first polarizing plate 50313 and the second
polarizing plate 50314 are provided so as to be in a cross nicol
state, light emitted from the backlight does not pass through the
substrate. Therefore, black display is performed. This is a
so-called normally black mode.
[1026] A liquid crystal display device having a structure shown in
FIG. 56C or FIG. 56D can perform full-color display by being
provided with a color filter. The color filter can be provided over
a first substrate 50311 side or a second substrate 50312 side.
[1027] It is only necessary that a known material be used for a
liquid crystal material used for an FLC mode or an AFLC mode.
[1028] FIGS. 57A and 57B are schematic views of cross sections of
an IPS mode. In the IPS mode, alignment of liquid crystal molecules
in a liquid crystal layer can be optically compensated, the liquid
crystal molecules are constantly rotated in a plane parallel to a
substrate, and a horizontal electric field method in which
electrodes are provided only on one substrate side is used.
[1029] A liquid crystal layer 50400 is held between a first
substrate 50401 and a second substrate 50402 which are provided so
as to be opposite to each other. A first electrode 50405 and a
second electrode 50406 are formed on a top surface of the second
substrate 50402. A first polarizing plate 50403 is provided on a
surface of the first substrate 50401, which does not face the
liquid crystal layer. A second polarizing plate 50404 is provided
on a surface of the second substrate 50402, which does not face the
liquid crystal layer. Note that the first polarizing plate 50403
and the second polarizing plate 50404 are provided so as to be in a
cross nicol state.
[1030] The first polarizing plate 50403 may be provided on the top
surface of the first substrate 50401. The second polarizing plate
50404 may be provided on the top surface of the second substrate
50402.
[1031] Both the first electrode 50405 and the second electrode
50406 may have transparency (a transmissive liquid crystal display
device). Alternatively, part of one of the first electrode 50405
and the second electrode 50406 may have reflectivity (a
semi-transmissive liquid crystal display device).
[1032] FIG. 57A is a schematic view of a cross section in the case
where voltage is applied to the first electrode 50405 and the
second electrode 50406 (referred to as a horizontal electric field
mode). Since liquid crystal molecules are aligned along a line of
electric force which is deviated from a rubbing direction, light
emitted from a backlight is affected by birefringence of the liquid
crystal molecules. In addition, since the first polarizing plate
50403 and the second polarizing plate 50404 are provided so as to
be in a cross nicol state, light emitted from the backlight passes
through the substrate. Therefore, white display is performed.
[1033] Note that by controlling voltage applied to the first
electrode 50405 and the second electrode 50406, conditions of the
liquid crystal molecules can be controlled. Therefore, since the
amount of light emitted from the backlight passing through the
substrate can be controlled, predetermined image display can be
performed.
[1034] FIG. 57B is a schematic view of a cross section in the case
where voltage is not applied to the first electrode 50405 and the
second electrode 50406. Since liquid crystal molecules are aligned
laterally in a rubbing direction, light emitted from a backlight is
not affected by birefringence of the liquid crystal molecules. In
addition, since the first polarizing plate 50403 and the second
polarizing plate 50404 are provided so as to be in a cross nicol
state, light emitted from the backlight does not pass through the
substrate. Therefore, black display is performed. This is a
so-called normally black mode.
[1035] A liquid crystal display device having a structure shown in
FIG. 57A or FIG. 57B can perform full-color display by being
provided with a color filter. The color filter can be provided over
a first substrate 50401 side or a second substrate 50402 side.
[1036] It is only necessary that a known material be used for a
liquid crystal material used for an IPS mode.
[1037] FIGS. 57C and 57D are schematic views of cross sections of
an FFS mode. In the FFS mode, alignment of liquid crystal molecules
in a liquid crystal layer can be optically compensated, the liquid
crystal molecules are constantly rotated in a plane parallel to a
substrate, and a horizontal electric field method in which
electrodes are provided only on one substrate side is used.
[1038] A liquid crystal layer 50410 is held between a first
substrate 50411 and a second substrate 50412 which are provided so
as to be opposite to each other. A second electrode 50416 is formed
on a top surface of the second substrate 50412. An insulating film
50417 is formed on a top surface of the second electrode 50416. A
first electrode 50415 is formed over the insulating film 50417. A
first polarizing plate 50413 is provided on a surface of the first
substrate 50411, which does not face the liquid crystal layer
50410. A second polarizing plate 50414 is provided on a surface of
the second substrate 50412, which does not face the liquid crystal
layer 50410. Note that the first polarizing plate 50413 and the
second polarizing plate 50414 are provided so as to be in a cross
nicol state.
[1039] The first polarizing plate 50413 may be provided on the top
surface of the first substrate 50411, i.e., may be provided between
the first substrate 50411 and the liquid crystal layer. The second
polarizing plate 50414 may be provided on the top surface of the
second substrate 50412, i.e., may be provided between the second
substrate 50412 and the liquid crystal layer.
[1040] It is only necessary that at least one of the first
electrode 50415 and the second electrode 50416 have transparency (a
transmissive or reflective liquid crystal display device).
Alternatively, both the first electrode 50415 and the second
electrode 50416 may have transparency, and part of one of the
electrodes may have reflectivity (a semi-transmissive liquid
crystal display device).
[1041] FIG. 57C is a schematic view of a cross section in the case
where voltage is applied to the first electrode 50415 and the
second electrode 50416 (referred to as a horizontal electric field
mode). Since liquid crystal molecules are aligned along a line of
electric force which is deviated from a rubbing direction, light
emitted from a backlight is affected by birefringence of the liquid
crystal molecules. In addition, since the first polarizing plate
50413 and the second polarizing plate 50414 are provided so as to
be in a cross nicol state, light emitted from the backlight passes
through the substrate. Therefore, white display is performed.
[1042] Note that by controlling voltage applied to the first
electrode 50415 and the second electrode 50416, conditions of the
liquid crystal molecules can be controlled. Therefore, since the
amount of light emitted from the backlight passing through the
substrate can be controlled, predetermined image display can be
performed.
[1043] FIG. 57D is a schematic view of a cross section in the case
where voltage is not applied to the first electrode 50415 and the
second electrode 50416. Since liquid crystal molecules are aligned
laterally in a rubbing direction, light emitted from the backlight
is not affected by birefringence of the liquid crystal molecules.
In addition, since the first polarizing plate 50413 and the second
polarizing plate 50414 are provided so as to be in a cross nicol
state, light emitted from the backlight does not pass through the
substrate. Therefore, black display is performed. This is a
so-called normally black mode.
[1044] A liquid crystal display device having a structure shown in
FIG; 57C or FIG. 57D can perform full-color display by being
provided with a color filter. The color filter can be provided over
a first substrate 50411 side or a second substrate 50412 side.
[1045] It is only necessary that a known material be used for a
liquid crystal material used for an FFS mode.
[1046] Next, various liquid crystal modes are described with
reference to top plan views.
[1047] FIG. 58 is a top plan view of a pixel portion to which an
MVA mode is applied. In the MVA mode, viewing angle dependency of
each portion is compensated by each other.
[1048] FIG. 58 shows a first electrode 50501, second electrodes
(50502a, 50502b, and 50502c), and a protrusion 50503. The first
electrode 50501 is formed over the entire surface of a counter
substrate. The protrusion 50503 is formed so as to be a dogleg
shape. In addition, the second electrodes (50502a, 50502b, and
50502c) are formed over the first electrode 50501 so as to have
shapes corresponding to the protrusion 50503.
[1049] Opening portions of the second electrodes (50502a, 50502b,
and 50502c) function like protrusions.
[1050] In the case where voltage is applied to the first pixel
electrode 50501 and the second pixel electrodes (50502a, 50502b,
and 50502c) (referred to as a vertical electric field mode), liquid
crystal molecules are aligned so as to tilt toward the opening
portions of the second pixel electrodes (50502a, 50502b, and
50502c) and the protrusion 50503. Since light emitted from a
backlight passes through a substrate when a pair of polarizing
plates is provided so as to be in a cross nicol state, white
display is performed.
[1051] Note that by controlling voltage applied to the first
electrode 50501 and the second electrodes (50502a, 50502b, and
50502c), conditions of the liquid crystal molecules can be
controlled. Therefore, since the amount of light emitted from the
backlight passing through the substrate can be controlled,
predetermined image display can be performed.
[1052] In the case where voltage is not applied to the first pixel
electrode 50501 and the second pixel electrodes (50502a, 50502b,
and 50502c), the liquid crystal molecules are aligned
longitudinally. Since light emitted from the backlight does not
pass through a panel when the pair of polarizing plates is provided
so as to be in the cross nicol state, black display is performed.
This is a so-called normally black mode.
[1053] It is only necessary that a known material be used for a
liquid crystal material used for an MVA mode.
[1054] FIGS. 59A to 59D are top plan views of a pixel portion to
which an IPS mode is applied. In the IPS mode, alignment of liquid
crystal molecules in a liquid crystal layer can be optically
compensated, the liquid crystal molecules are constantly rotated in
a plane parallel to a substrate, and a horizontal electric field
method in which electrodes are provided only on one substrate side
is used.
[1055] In the IPS mode, a pair of electrodes is formed so as to
have different shapes.
[1056] FIG. 59A shows a first pixel electrode 50601 and a second
pixel electrode 50602. The first pixel electrode 50601 and the
second pixel electrode 50602 are wavy shapes.
[1057] FIG. 59B shows a first pixel electrode 50611 and a second
pixel electrode 50612. The first pixel electrode 50611 and the
second pixel electrode 50612 have shapes having concentric
openings.
[1058] FIG. 59C shows a first pixel electrode 50621 and a second
pixel electrode 50622. The first pixel electrode 50621 and the
second pixel electrode 50622 are comb shapes and partially overlap
with each other.
[1059] FIG. 59D shows a first pixel electrode 50631 and a second
pixel electrode 50632. The first pixel electrode 50631 and the
second pixel electrode 50632 are comb shapes in which electrodes
engage with each other.
[1060] In the case where voltage is applied to the first pixel
electrodes (50601, 50611, 50621, and 50631) and the second pixel
electrodes (50602, 50612, 50622, and 50632) (referred to as a
horizontal electric field mode), liquid crystal molecules are
aligned along a line of electric force which is deviated from a
rubbing direction. Since light emitted from a backlight passes
through a substrate when a pair of polarizing plates is provided so
as to be in a cross nicol state, white display is performed.
[1061] Note that by controlling voltage applied to the first pixel
electrodes (50601, 50611, 50621, and 50631) and the second pixel
electrodes (50602, 50612, 50622, and 50632), conditions of the
liquid crystal molecules can be controlled. Therefore, since the
amount of light emitted from the backlight passing through the
substrate can be controlled, predetermined image display can be
performed.
[1062] In the case where voltage is not applied to the first pixel
electrodes (50601, 50611, 50621, and 50631) and the second pixel
electrodes (50602, 50612, 50622, and 50632), the liquid crystal
molecules are aligned laterally in the rubbing direction. Since
light emitted from the backlight does not pass through the
substrate when the pair of polarizing plates is provided so as to
be in the cross nicol state, black display is performed. This is a
so-called normally black mode.
[1063] It is only necessary that a known material be used for a
liquid crystal material used for an IPS mode.
[1064] FIGS. 60A to 60D are top plan views of a pixel portion to
which an FFS mode is applied. In the FFS mode, alignment of liquid
crystal molecules in a liquid crystal layer can be optically
compensated, the liquid crystal molecules are constantly rotated in
a plane parallel to a substrate, and a horizontal electric field
method in which electrodes are provided only on one substrate side
is used.
[1065] In the FFS mode, a first electrode is formed over a top
surface of a second electrode so as to be various shapes.
[1066] FIG. 60A shows a first pixel electrode 50701 and a second
pixel electrode 50702. The first pixel electrode 50701 is a bent
dogleg shape. The second pixel electrode 50702 is not necessarily
patterned.
[1067] FIG. 60B shows a first pixel electrode 50711 and a second
pixel electrode 50712. The first pixel electrode 50711 is a
concentric shape. The second pixel electrode 50712 is not
necessarily patterned.
[1068] FIG. 60C shows a first pixel electrode 50721 and a second
pixel electrode 50722. The first pixel electrode 50721 is a comb
shape in which electrodes engage with each other. The second pixel
electrode 50722 is not necessarily patterned.
[1069] FIG; 60D shows a first pixel electrode 50731 and a second
pixel electrode 50732. The first pixel electrode 50731 is a comb
shape. The second pixel electrode 50732 is not necessarily
patterned.
[1070] In the case where voltage is applied to the first pixel
electrodes (50701, 50711, 50721, and 50731) and the second pixel
electrodes (50702, 50712, 50722, and 50732) (referred to as a
horizontal electric field mode), liquid crystal molecules are
aligned along a line of electric force which is deviated from a
rubbing direction. Since light emitted from a backlight passes
through a substrate when a pair of polarizing plates is provided so
as to be in a cross nicol state, white display is performed.
[1071] Note that by controlling voltage applied to the first pixel
electrodes (50701, 50711, 50721, and 50731) and the second pixel
electrodes (50702, 50712, 50722, and 50732), conditions of the
liquid crystal molecules can be controlled. Therefore, since the
amount of light emitted from the backlight passing through the
substrate can be controlled, predetermined image display can be
performed.
[1072] In the case where voltage is not applied to the first pixel
electrodes (50701, 50711, 50721, and 50731) and the second pixel
electrodes (50702, 50712, 50722, and 50732), the liquid crystal
molecules are aligned laterally in the rubbing direction. Since
light emitted from the backlight does not pass through the
substrate when the pair of polarizing plates is provided so as to
be in the cross nicol state, black display is performed. This is a
so-called normally black mode.
[1073] It is only necessary that a known material be used for a
liquid crystal material used for an FFS mode.
[1074] Although this embodiment mode is described with reference to
various drawings, the contents (or may be part of the contents)
described in each drawing can be freely applied to, combined with,
or replaced with the contents (or may be part of the contents)
described in another drawing. Further, even more drawings can be
formed by combining each part with another part in the
above-described drawings.
[1075] The contents (or may be part of the contents) described in
each drawing of this embodiment mode can be freely applied to,
combined with, or replaced with the contents (or may be part of the
contents) described in a drawing in another embodiment mode.
Further, even more drawings can be formed by combining each part
with part of another embodiment mode in the drawings of this
embodiment mode.
[1076] This embodiment mode shows an example of an embodied case of
the contents (or may be part of the contents) described in other
embodiment modes, an example of slight transformation thereof, an
example of partial modification thereof, an example of improvement
thereof, an example of detailed description thereof, an application
example thereof, an example of related part thereof, or the like.
Therefore, the contents described in other embodiment modes can be
freely applied to, combined with, or replaced with this embodiment
mode.
Embodiment Mode 10
[1077] In this embodiment mode, a pixel structure of a display
device is described. In particular, a pixel structure of a display
device using an organic EL element is described.
[1078] FIG. 61A shows an example of a top plan view (a layout
diagram) of a pixel including two transistors. FIG. 61B shows an
example of a cross-sectional view along X-X' in FIG. 61A.
[1079] FIG. 61A shows a first transistor 60105, a first wiring
60106, a second wiring 60107, a second transistor 60108, a third
wiring 60111, a counter electrode 60112, a capacitor 60113, a pixel
electrode 60115, a partition wall 60116, an organic conductive film
60117, an organic thin film 60118, and a substrate 60119. Note that
it is preferable that the first transistor 60105 be used as a
switching transistor, the first wiring 60106 as a gate signal line,
the second wiring 60107 as a source signal line, the second
transistor 60108 as a driving transistor, and the third wiring
60111 as a current supply line.
[1080] A gate electrode of the first transistor 60105 is
electrically connected to the first wiring 60106. One of a source
electrode and a drain electrode of the first transistor 60105 is
electrically connected to the second wiring 60107. The other of the
source electrode and the drain electrode of the first transistor
60105 is electrically connected to a gate electrode of the second
transistor 60108 and one electrode of the capacitor 60113. Note
that the gate electrode of the first transistor 60105 includes a
plurality of gate electrodes. Accordingly, leakage current in the
off state of the first transistor 60105 can be reduced.
[1081] One of a source electrode and a drain electrode of the
second transistor 60108 is electrically connected to the third
wiring 60111, and the other of the source electrode and the drain
electrode of the second transistor 60108 is electrically connected
to the pixel electrode 60115. Accordingly, current flowing to the
pixel electrode 60115 can be controlled by the second transistor
60108.
[1082] The organic conductive film 60117 is provided over the pixel
electrode 60115, and the organic thin film 60118 (an organic
compound layer) is further provided thereover. The counter
electrode 60112 is provided over the organic thin film 60118 (the
organic compound layer). Note that the counter electrode 60112 may
be formed over all pixels to be commonly connected to all the
pixels, or may be patterned using a shadow mask or the like.
[1083] Light emitted from the organic thin film 60118 (the organic
compound layer) is transmitted through either the pixel electrode
60115 or the counter electrode 60112.
[1084] In FIG. 61B, the case where light is emitted to the pixel
electrode side, that is, a side on which the transistor and the
like are formed is referred to as bottom emission; and the case
where light is emitted to the counter electrode side is referred to
as top emission.
[1085] In the case of bottom emission, it is preferable that the
pixel electrode 60115 be formed of a transparent conductive film.
On the other hand, in the case of top emission, it is preferable
that the counter electrode 60112 be formed of a transparent
conductive film.
[1086] In a light-emitting device for color display, EL elements
having respective light emission colors of RGB may be separately
formed, or an EL element with a single color may be formed over an
entire surface and light emission of RGB can be obtained by using a
color filter.
[1087] Note that the structures shown in FIGS. 61A and 61B are
examples, and various structures can be employed for a pixel
layout, a cross-sectional structure, a stacking order of electrodes
of an EL element, and the like, as well as the structures shown in
FIGS. 61A and 61B. Further, as a light-emitting element, various
elements such as a crystalline element such as an LED, and an
element formed of an inorganic thin film can be used as well as the
element formed of the organic thin film shown in the drawing.
[1088] FIG. 62A shows an example of a top plan view (a layout
diagram) of a pixel including three transistors. FIG. 62B shows an
example of a cross-sectional view along X-X' in FIG. 62A.
[1089] FIG. 62A shows a substrate 60200, a first wiring 60201, a
second wiring 60202, a third wiring 60203, a fourth wiring 60204, a
first transistor 60205, a second transistor 60206, a third
transistor 60207, a pixel electrode 60208, a partition wall 60211,
an organic conductive film 60212, an organic thin film 60213, and a
counter electrode 60214. Note that it is preferable that the first
wiring 60201 be used as a source signal line, the second wiring
60202 as a gate signal line for writing, the third wiring 60203 as
a gate signal line for erasing, the fourth wiring 60204 as a
current supply line, the first transistor 60205 as a switching
transistor, the second transistor 60206 as an erasing transistor,
and the third transistor 60207 as a driving transistor.
[1090] A gate electrode of the first transistor 60205 is
electrically connected to the second wiring 60202. One of a source
electrode and a drain electrode of the first transistor 60205 is
electrically connected to the first wiring 60201. The other of the
source electrode and the drain electrode of the first transistor
60205 is electrically connected to a gate electrode of the third
transistor 60207. Note that the gate electrode of the first
transistor 60205 includes a plurality of gate electrodes.
Accordingly, leakage current in the off state of the first
transistor 60205 can be reduced.
[1091] A gate electrode of the second transistor 60206 is
electrically connected to the third wiring 60203. One of a source
electrode and a drain electrode of the second transistor 60206 is
electrically connected to the fourth wiring 60204. The other of the
source electrode and the drain electrode of the second transistor
60206 is electrically connected to the gate electrode of the third
transistor 60207. Note that the gate electrode of the second
transistor 60206 includes a plurality of gate electrodes.
Accordingly, leakage current in the off state of the second
transistor 60206 can be reduced.
[1092] One of a source electrode and a drain electrode of the third
transistor 60207 is electrically connected to the fourth wiring
60204, and the other of the source electrode and the drain
electrode of the third transistor 60207 is electrically connected
to the pixel electrode 60208. Accordingly, current flowing to the
pixel electrode 60208 can be controlled by the third transistor
60207.
[1093] The organic conductive film 60212 is provided over the pixel
electrode 60208, and the organic thin film 60213 (an organic
compound layer) is further provided thereover. The counter
electrode 60214 is provided over the organic thin film 60213 (the
organic compound layer). Note that the counter electrode 60214 may
be formed over all pixels to be commonly connected to all the
pixels, or may be patterned using a shadow mask or the like.
[1094] Light emitted from the organic thin film 60213 (the organic
compound layer) is transmitted through either the pixel electrode
60208 or the counter electrode 60214.
[1095] In FIG. 62B, the case where light is emitted to the pixel
electrode side, that is, a side on which the transistor and the
like are formed is referred to as bottom emission; and the case
where light is emitted to the counter electrode side is referred to
as top emission.
[1096] In the case of bottom emission, it is preferable that the
pixel electrode 60208 be formed of a transparent conductive film.
On the other hand, in the case of top emission, it is preferable
that the counter electrode 60214 be formed of a light-transmitting
conductive film.
[1097] In a light-emitting device for color display, EL elements
having respective light emission colors of RGB may be separately
formed, or an EL element with a single color may be formed over an
entire surface and light emission of RGB can be obtained by using a
color filter.
[1098] Note that the structures shown in FIGS. 62A and 62B are
examples, and various structures can be employed for a pixel
layout, a cross-sectional structure, a stacking order of electrodes
of an EL element, and the like, as well as the structures shown in
FIGS. 62A and 62B. Further, as a light-emitting element, various
elements such as a crystalline element such as an LED, and an
element formed of an inorganic thin film can be used as well as the
element formed of the organic thin film shown in the drawings.
[1099] FIG. 63A shows an example of a top plan view (a layout
diagram) of a pixel including four transistors. FIG. 63B shows an
example of a cross-sectional view along X-X' in FIG. 63A.
[1100] FIG. 63A shows a substrate 60300, a first wiring 60301, a
second wiring 60302, a third wiring 60303, a fourth wiring 60304, a
first transistor 60305, a second transistor 60306, a third
transistor 60307, a fourth transistor 60308, a pixel electrode
60309, a fifth wiring 60311, a sixth wiring 60312, a partition wall
60321, an organic conductive film 60322, an organic thin film
60323, and a counter electrode 60324. Note that it is preferable
that the first wiring 60301 be used as a source signal line, the
second wiring 60302 as a gate signal line for writing, the third
wiring 60303 as a gate signal line for erasing, the fourth wiring
60304 as a signal line for reverse bias, the first transistor 60305
as a switching transistor, the second transistor 60306 as an
erasing transistor, the third transistor 60307 as a driving
transistor, the fourth transistor 60308 as a transistor for reverse
bias, the fifth wiring 60311 as a current supply line, and the
sixth wiring 60312 as a power supply line for reverse bias.
[1101] A gate electrode of the first transistor 60305 is
electrically connected to the second wiring 60302. One of a source
electrode and a drain electrode of the first transistor 60305 is
electrically connected to the first wiring 60301. The other of the
source electrode and the drain electrode of the first transistor
60305 is electrically connected to a gate electrode of the third
transistor 60307. Note that the gate electrode of the first
transistor 60305 includes a plurality of gate electrodes.
Accordingly, leakage current in the off state of the first
transistor 60305 can be reduced.
[1102] A gate electrode of the second transistor 60306 is
electrically connected to the third wiring 60303. One of a source
electrode and a drain electrode of the second transistor 60306 is
electrically connected to the fifth wiring 60311. The other of the
source electrode and the drain electrode of the second transistor
60306 is electrically connected to the gate electrode of the third
transistor 60307. Note that the gate electrode of the second
transistor 60306 includes a plurality of gate electrodes.
Accordingly, leakage current in the off state of the second
transistor 60306 can be reduced.
[1103] One of a source electrode and a drain electrode of the third
transistor 60307 is electrically connected to the fifth wiring
60311, and the other of the source electrode and the drain
electrode of the third transistor 60307 is electrically connected
to the pixel electrode 60309. Accordingly, current flowing to the
pixel electrode 60309 can be controlled by the third transistor
60307.
[1104] A gate electrode of the fourth transistor 60308 is
electrically connected to the fourth wiring 60304. One of a source
electrode and a drain electrode of the fourth transistor 60308 is
electrically connected to the sixth wiring 60312. The other of the
source electrode and the drain electrode of the fourth transistor
60308 is electrically connected to the pixel electrode 60309.
Accordingly, a potential of the pixel electrode 60309 can be
controlled by the fourth transistor 60308, so that a reverse bias
can be applied to the organic conductive film 60322 and the organic
thin film 60323. When a reverse bias is applied to a light-emitting
element including the organic conductive film 60322, the organic
thin film 60323, and the like, reliability of the light-emitting
element can be significantly improved.
[1105] The organic conductive film 60322 is provided over the pixel
electrode 60309, and the organic thin film 60323 (an organic
compound layer) is further provided thereover. The counter
electrode 60324 is provided over the organic thin film 60213 (the
organic compound layer). Note that the counter electrode 60324 may
be formed over all pixels to be commonly connected to all the
pixels, or may be patterned using a shadow mask or the like.
[1106] Light emitted from the organic thin film 60323 (the organic
compound layer) is transmitted through either the pixel electrode
60309 or the counter electrode 60324.
[1107] In FIG. 63B, the case where light is emitted to the pixel
electrode side, that is, a side on which the transistor and the
like are formed is referred to as bottom emission; and the case
where light is emitted to the counter electrode side is referred to
as top emission.
[1108] In the case of bottom emission, it is preferable that the
pixel electrode 60309 be formed of a transparent conductive film.
On the other hand, in the case of top emission, it is preferable
that the counter electrode 60324 be formed of a light-transmitting
conductive film.
[1109] In a light-emitting device for color display, EL elements
having respective light emission colors of RGB may be separately
formed, or an EL element with a single color may be formed over an
entire surface and light emission of RGB can be obtained by using a
color filter.
[1110] Note that the structures shown in FIGS. 63A and 63B are
examples, and various structures can be employed for a pixel
layout, a cross-sectional structure, a stacking order of electrodes
of an EL element, and the like, as well as the structures shown in
FIGS. 63A and 63B. Further, as a light-emitting element, various
elements such as a crystalline element such as an LED, and an
element formed of an inorganic thin film can be used as well as the
element formed of the organic thin film shown in the drawings.
[1111] Although this embodiment mode is described with reference to
various drawings, the contents (or part of the contents) described
in each drawing can be freely applied to, combined with, or
replaced with the contents (or part of the contents) described in
another drawing. Further, much more drawings can be formed by
combining each part with another part in the above-described
drawings.
[1112] The contents (or part of the contents) described in each
drawing in this embodiment mode can be freely applied to, combined
with, or replaced with the contents (or part of the contents)
described in a drawing in another embodiment mode. Further, much
more drawings can be formed by combining each part in each drawing
in this embodiment mode with part of another embodiment mode.
[1113] This embodiment mode shows examples of embodying, slightly
transforming, partially modifying, improving, describing in detail,
or applying the contents (or part of the contents) described in
other embodiment modes, an example of related part thereof, or the
like. Therefore, the contents described in other embodiment modes
can be freely applied to, combined with, or replaced with this
embodiment mode.
Embodiment Mode 11
[1114] In this embodiment mode, a structure and an operation of a
pixel in a display device are described.
[1115] FIGS. 64A and 64B are timing charts showing an example of
digital time gray scale drive. The timing chart of FIG. 64A shows a
driving method when a signal writing period (an address period) to
a pixel and a light-emitting period (a sustain period) are
separated.
[1116] One frame period refers to a period for fully displaying an
image for one display region. One frame period includes a plurality
of subframe periods, and one subframe period includes an address
period and a sustain period. Address periods Ta1 to Ta4 indicate
time for writing signals to pixels in all rows, and periods Tb1 to
Tb4 indicate time for writing signals to pixels in one row (or one
pixel). Sustain periods Ts1 to Ts4 indicate time for maintaining a
lighting state or a non-lighting state in accordance with a video
signal written to the pixel, and a ratio of the length of the
sustain periods is set to satisfy Ts1 Ts2: Ts3:
Ts4=2.sup.3:2.sup.2:2.sup.1:2.sup.0=8 : 4: 2: 1. A gray scale is
expressed depending on in which sustain period light emission is
performed.
[1117] Here, the i-th pixel row is described with reference to FIG.
64B. First, in the address period Ta1, a pixel selection signal is
input to a scan line in order from a first row, and in a period
Th1(i) in the address period Ta1, a pixel in the i-th row is
selected. Then, while the pixel in the i-th row is selected, a
video signal is input to the pixel in the i-th row from a signal
line. Then, when the video signal is written to the pixel in the
i-th row, the pixel in the i-th row maintains the signal until a
signal is input again. Lighting and non-lighting of the pixel in
the i-th row in the sustain period Ts1 are controlled by the
written video signal. Similarly, in the address periods Ta2, Ta3,
and Ta4, a video signal is input to the pixel in the i-th row, and
lighting and non-lighting of the pixel in the i-th row in the
sustain periods Ts2, Ts3, and Ts4 are controlled by the video
signal. Then, in each subframe period, a pixel to which a signal
for not lighting in the address period and for lighting when the
sustain period starts after the address period ends is written is
lit.
[1118] Here, the case where a 4-bit gray scale is expressed is
described; however, the number of bits and the number of gray
scales are not limited thereto. Note that lighting is not needed to
be performed in order of Ts1, Ts2, Ts3, and Ts4, and the order may
be random or light emission may be performed in the period divided
into a plurality of periods. A ratio of lighting time of Ts1, Ts2,
Ts3, and Ts4 is not needed to be power-of-two, and may be the same
length or slightly different from a power of two.
[1119] Next, a driving method when a signal writing period (an
address period) to a pixel and a light-emitting period (a sustain
period) are not separated is described. A pixel in a row in which a
writing operation of a video signal is completed maintains the
signal until another signal is written to the pixel (or the signal
is erased). Data holding time refers to a period from the writing
operation is performed until another signal is written to the
pixel. In the data holding time, the pixel is lit or not lit in
accordance with the video signal written to the pixel. The same
operations are performed until the last row, and the address period
ends. Then, an operation proceeds to a signal writing operation in
a next subframe period sequentially from a row in which the data
holding time ends.
[1120] As described above, in the case of a driving method in which
a pixel is immediately lit or not lit in accordance with a video
signal written to the pixel after the signal writing operation is
completed and the data holding time starts, signals cannot be input
to two rows at the same time. Accordingly, address periods need to
be prevented from overlapping, so that the data holding time cannot
be made shorter than the address period. As a result, it becomes
difficult to perform high-level gray scale display.
[1121] Thus, the data holding time is set to be shorter than the
address period by provision of an erasing period. A driving method
when the data holding time is set shorter than the address period
by provision of an erasing period is described with reference to
FIG. 65A.
[1122] First, in the address period Ta1, a pixel scan signal is
input to a scan line in order from a first row, and a pixel is
selected. Then, while the pixel is selected, a video signal is
input to the pixel from a signal line. Then, when the video signal
is written to the pixel, the pixel maintains the signal until a
signal is input again. Lighting and non-lighting of the pixel in
the sustain period Ts1 are controlled by the written video signal.
In a row in which a writing operation of a video signal is
completed, a pixel is immediately lit or not lit in accordance with
the written video signal. The same operations are performed until
the last row, and the address period Ta1 ends. Then, an operation
proceeds to a signal writing operation in a next subframe period
sequentially from a row in which the data holding time ends.
Similarly, in the address periods Ta2, Ta3, and Ta4, a video signal
is input to the pixel, and lighting and non-lighting of the pixel
in the sustain periods Ts2, Ts3, and Ts4 are controlled by the
video signal. The end of the sustain period Ts4 is set by the start
of an erasing operation. This is because when a signal written to a
pixel in an erasing time Te of each row is erased, the pixel is
forced to be not lit regardless of the video signal written to the
pixel in the address period until another signal is written to the
pixel. That is, the data holding time ends from a pixel in which
the erasing time Te starts.
[1123] Here, the i-th pixel row is described with reference to FIG.
65B. In the address period Ta1, a pixel scan signal is input to a
scan line in order from a first row, and a pixel is selected. Then,
in the period Th1(i), while the pixel in the i-th row is selected,
a video signal is input to the pixel in the i-th row. Then, when
the video signal is written to the pixel in the i-th row, the pixel
in the i-th row maintains the signal until a signal is input again.
Lighting and non-lighting of the pixel in the i-th row in a sustain
period Ts1(i) are controlled by the written video signal. That is,
the pixel in the i-th row is immediately lit or not lit in
accordance with the video signal written to the pixel after the
writing operation of the video signal to the i-th row is completed.
Similarly, in the address periods Ta2, Ta3, and Ta4, a video signal
is input to the pixel in the i-th row, and lighting and
non-lighting of the pixel in the i-th row in the sustain periods
Ts2, Ts3, and Ts4 are controlled by the video signal. The end of a
sustain period Ts4(i) is set by the start of an erasing operation.
This is because the pixel is forced to be not lit regardless of the
video signal written to the pixel in the i-th row in an erasing
time Te(i) in the i-th row. That is, the data holding time of the
pixel in the i-th row ends when the erasing time Te(i) starts.
[1124] Thus, a display device with a high-level gray scale and a
high duty ratio (a ratio of a lighting period in one frame period)
can be provided, in which data holding time is shorter than an
address period without separating the address period and a sustain
period. Since instantaneous luminance can be lowered, reliability
of a display element can be improved.
[1125] Here, the case where a 4-bit gray scale is expressed is
described; however, the number of bits and the number of gray
scales are not limited thereto. Note that lighting is not needed to
be performed in order of Ts1, Ts2, Ts3, and Ts4, and the order may
be random or light emission may be performed in the period divided
into a plurality of periods. A ratio of lighting time of Ts1, Ts2,
Ts3, and Ts4 is not needed to be power-of-two, and may be the same
length or slightly different from a power of two.
[1126] Next, a structure and an operation of a pixel to which
digital time gray scale drive can be applied are described.
[1127] FIG. 66 is a diagram showing an example of a pixel structure
to which digital time gray scale drive can be applied.
[1128] A pixel 80300 includes a switching transistor 80301, a
driving transistor 80302, a light-emitting element 80304, and a
capacitor 80303. A gate of the switching transistor 80301 is
connected to a scan line 80306, a first electrode (one of a source
electrode and a drain electrode) of the switching transistor 80301
is connected to a signal line 80305, and a second electrode (the
other of the source electrode and the drain electrode) of the
switching transistor 80301 is connected to a gate of the driving
transistor 80302. The gate of the driving transistor 80302 is
connected to a power supply line 80307 through the capacitor 80303,
a first electrode of the driving transistor 80302 is connected to
the power supply line 80307, and a second electrode of the driving
transistor 80302 is connected to a first electrode (a pixel
electrode) of the light-emitting element 80304. A second electrode
of the light-emitting element 80304 corresponds to a common
electrode 80308.
[1129] Note that the second electrode (the common electrode 80308)
of the light-emitting element 80304 is set to a low power supply
potential. The low power supply potential refers to a potential
satisfying (the low power supply potential)<(a high power supply
potential) based on the high power supply potential set to the
power supply line 80307. As the low power supply potential, GND, 0
V, or the like may be set, for example. A potential difference
between the high power supply potential and the low power supply
potential is applied to the light-emitting element 80304, and
current is supplied to the light-emitting element 80304. Here, in
order to make the light-emitting element 80304 emit light, each
potential is set so that the potential difference between the high
power supply potential and the low power supply potential is
forward threshold voltage or more.
[1130] Note that gate capacitance of the driving transistor 80302
may be used as a substitute for the capacitor 80303, so that the
capacitor 80303 can be omitted. The gate capacitance of the driving
transistor 80302 may be formed in a region where a source region, a
drain region, an LDD region, or the like overlaps with the gate
electrode. Alternatively, capacitance may be formed between a
channel region and the gate electrode.
[1131] When a pixel is selected by the scan line 80306, that is,
when the switching transistor 80301 is turned on, a video signal is
input to the pixel from the signal line 80305. Then, a charge for
voltage corresponding to the video signal is stored in the
capacitor 80303, and the capacitor 80303 maintains the voltage. The
voltage is voltage between the gate electrode and the first
electrode of the driving transistor 80302 and corresponds to
gate-source voltage Vgs of the driving transistor 80302.
[1132] An operation region of a transistor can be generally divided
into a linear region and a saturation region. When drain-source
voltage is denoted by Vds, gate-source voltage is denoted by Vgs,
and threshold voltage is denoted by Vth, a boundary between the
linear region and the saturation region sets so as to satisfy
(Vgs-Vth)=Vds. In the case where (Vgs-Vth)>Vds is satisfied, a
transistor operates in a linear region, and a current value is
determined in accordance with the level of Vds and Vgs. On the
other hand, in the case where (Vgs-Vth)<Vds is satisfied, a
transistor operates in a saturation region and ideally, a current
value hardly changes even when Vds changes. That is, a current
value is determined only by the level of Vgs.
[1133] Here, in the case of voltage-input voltage driving method, a
video signal is input to the gate of the driving transistor 80302
so that the driving transistor 80302 is in either of two states of
being sufficiently turned on and turned off. That is, the driving
transistor 80302 operates in a linear region.
[1134] Thus, when a video signal which makes the driving transistor
80302 turned on is input, a power supply potential VDD set to the
power supply line 80307 without change is ideally set to the first
electrode of the light-emitting element 80304.
[1135] That is, ideally, constant voltage is applied to the
light-emitting element 80304 to obtain constant luminance from the
light-emitting element 80304. Then, a plurality of subframe periods
are provided in one frame period. A video signal is written to a
pixel in each subframe period, lighting and non-lighting of the
pixel are controlled in each subframe period, and a gray scale is
expressed by the sum of lighting subframe periods.
[1136] Note that when the video signal by which the driving
transistor 80302 operates in a saturation region is input, current
can be supplied to the light-emitting element 80304. When the
light-emitting element 80304 is an element luminance of which is
determined in accordance with current, luminance decay due to
deterioration of the light-emitting element 80304 can be
suppressed. Further, when the video signal is an analog signal,
current in accordance with the video signal can be supplied to the
light-emitting element 80304. In this case, analog gray scale drive
can be performed.
[1137] FIG. 67 is a diagram showing another example of a pixel
structure to which digital time gray scale drive can be
applied.
[1138] A pixel 80400 includes a switching transistor 80401, a
driving transistor 80402, a capacitor 80403, a light-emitting
element 80404, and a rectifying element 80409. A gate of the
switching transistor 80401 is connected to a first scan line 80406,
a first electrode (one of a source electrode and a drain electrode)
of the switching transistor 80401 is connected to a signal line
80405, and a second electrode (the other of the source electrode
and the drain electrode) of the switching transistor 80401 is
connected to a gate of the driving transistor 80402. The gate of
the driving transistor 80402 is connected to a power supply line
80407 through the capacitor 80403, and is also connected to a
second scan line 80410 through the rectifying element 80409. A
first electrode of the driving transistor 80402 is connected to the
power supply line 80407, and a second electrode of the driving
transistor 80402 is connected to a first electrode (a pixel
electrode) of the light-emitting element 80404. A second electrode
of the light-emitting element 80404 corresponds to a common
electrode 80408.
[1139] The second electrode (the common electrode 80408) of the
light-emitting element 80404 is set to a low power supply
potential. Note that the low power supply potential refers to a
potential satisfying (the low power supply potential)<(a high
power supply potential) with respect to the high power supply
potential set to the power supply line 80407. As the low power
supply potential, GND, 0 V, or the like may be set, for example. In
order to apply a potential difference between the high power supply
potential and the low power supply potential to the light-emitting
element 80404 and supply current to the light-emitting element
80404 so that the light-emitting element 80404 emits light, each
potential is set so that the potential difference between the high
power supply potential and the low power supply potential is equal
to forward threshold voltage or more.
[1140] Gate capacitance of the driving transistor 80402 may be used
as a substitute for the capacitor 80403, so that the capacitor
80403 can be omitted. The gate capacitance of the driving
transistor 80402 may be formed in a region where a source region, a
drain region, an LDD region, or the like overlaps with the gate
electrode. Alternatively, capacitance may be formed between a
channel region and the gate electrode.
[1141] As the rectifying element 80409, a diode-connected
transistor can be used. A PN junction diode, a PIN junction diode,
a Schottky diode, a diode formed of a carbon nanotube, or the like
may be used other than a diode-connected transistor. A
diode-connected transistor may be an n-channel transistor or a
p-channel transistor.
[1142] The pixel 80400 is such that the rectifying element 80409
and the second scan line 80410 are added to the pixel shown in FIG.
66. Accordingly, the switching transistor 80401, the driving
transistor 80402, the capacitor 80403, the light-emitting element
80404, the signal line 80405, the first scan line 80406, the power
supply line 80407, and the common electrode 80408 shown in FIG. 67
correspond to the switching transistor 80301, the driving
transistor 80302, the capacitor 80303, the light-emitting element
80304, the signal line 80305, the scan line 80306, the power supply
line 80307, and the common electrode 80308 shown in FIG. 66.
Accordingly, a writing operation and a light-emitting operation in
FIG. 67 are similar to those described in FIG. 66, so that
description thereof is omitted.
[1143] An erasing operation of the pixel shown in FIG. 67 is
described. In an erasing operation, an H-level signal is input to
the second scan line 80410. Thus, current is supplied to the
rectifying element 80409, and a gate potential of the driving
transistor 80402 held by the capacitor 80403 can be set to a
certain potential. That is, the potential of the gate electrode of
the driving transistor 80402 is set to a certain value, and the
driving transistor 80402 can be forced to be turned off regardless
of a video signal written to the pixel.
[1144] An L-level signal input to the second scan line 80410 has a
potential such that current is not supplied to the rectifying
element 80409 when a video signal for non-lighting is written to a
pixel. An H-level signal input to the second scan line 80410 has a
potential such that a potential to turn off the driving transistor
80302 can be set to the gate regardless of a video signal written
to a pixel.
[1145] FIG. 68 is a diagram showing another example of a pixel
structure to which digital time gray scale drive can be
applied.
[1146] A pixel 80500 includes a switching transistor 80501, a
driving transistor 80502, a capacitor 80503, a light-emitting
element 80504, and an erasing transistor 80509. A gate of the
switching transistor 80501 is connected to a first scan line 80506,
a first electrode (one of a source electrode and a drain electrode)
of the switching transistor 80501 is connected to a signal line
80505, and a second electrode (the other of the source electrode
and the drain electrode) of the switching transistor 80501 is
connected to a gate of the driving transistor 80502. The gate of
the driving transistor 80502 is connected to a power supply line
80507 through the capacitor 80503, and is also connected to a first
electrode of the erasing transistor 80509. A first electrode of the
driving transistor 80502 is connected to the power supply line
80507, and a second electrode of the driving transistor 80502 is
connected to a first electrode (a pixel electrode) of the
light-emitting element 80504. A gate of the erasing transistor
80509 is connected to a second scan line 80510, and a second
electrode of the erasing transistor 80509 is connected to the power
supply line 80507. A second electrode of the light-emitting element
80504 corresponds to a common electrode 80508.
[1147] The second electrode (the common electrode 80508) of the
light-emitting element 80504 is set to a low power supply
potential. The low power supply potential refers to a potential
satisfying (the low power supply potential)<(a high power supply
potential) with respect to the high power supply potential set to
the power supply line 80507. As the low power supply potential,
GND, 0 V, or the like may be set, for example. In order to apply a
potential difference between the high power supply potential and
the low power supply potential to the light-emitting element 80504
and supply current to the light-emitting element 80504 so that the
light-emitting element 80504 emits light, each potential is set so
that the potential difference between the high power supply
potential and the low power supply potential is equal to forward
threshold voltage or more.
[1148] Gate capacitance of the driving transistor 80502 may be used
as a substitute for the capacitor 80503, so that the capacitor
80503 can be omitted. The gate capacitance of the driving
transistor 80502 may be formed in a region where a source region, a
drain region, an LDD region, or the like overlaps with the gate
electrode. Alternatively, capacitance may be formed between a
channel region and the gate electrode.
[1149] As the erasing transistor 80509, a diode-connected
transistor can be used. Further, a PN junction diode, a PIN
junction diode, a Schottky diode, a diode formed of a carbon
nanotube, or the like may be used other than a diode-connected
transistor. A diode-connected transistor may be an n-channel
transistor or a p-channel transistor.
[1150] The pixel 80500 is such that the erasing transistor 80509
and the second scan line 80510 are added to the pixel shown in FIG.
66. Accordingly, the switching transistor 80501, the driving
transistor 80502, the capacitor 80503, the light-emitting element
80504, the signal line 80505, the first scan line 80506, the power
supply line 80507, and the common electrode 80508 shown in FIG. 68
correspond to the switching transistor 80301, the driving
transistor 80302, the capacitor 80303, the light-emitting element
80304, the signal line 80305, the scan line 80306, the power supply
line 80307, and the common electrode 80308 shown in FIG. 66.
Accordingly, a writing operation and a light-emitting operation in
FIG. 68 are similar to those described in FIG. 66, so that
description thereof is omitted.
[1151] An erasing operation of the pixel shown in FIG. 68 is
described. In an erasing operation, an H-level signal is input to
the second scan line 80510. Thus, the erasing transistor 80509 is
turned on, and the gate electrode and the first electrode of the
driving transistor can be made to have the same potential. That is,
Vgs of the driving transistor 80502 can be 0 V. Accordingly, the
driving transistor 80502 can be forced to be turned off.
[1152] Next, a structure and an operation of a pixel called a
threshold voltage compensation pixel are described. A threshold
voltage compensation pixel can be applied to digital time gray
scale drive and analog gray scale drive.
[1153] FIG. 69 is a diagram showing an example of a structure of a
pixel called a threshold voltage compensation pixel.
[1154] The pixel in FIG. 69 includes a driving transistor 80600, a
first switch 80601, a second switch 80602, a third switch 80603, a
first capacitor 80604, a second capacitor 80605, and a
light-emitting element 80620. A gate electrode of the driving
transistor 80600 is connected to a signal line 80611 through the
first capacitor 80604 and the first switch 80601 in this order.
Further, the gate electrode of the driving transistor 80600 is
connected to a power supply line 80612 through the second capacitor
80605. A first electrode of the driving transistor 80600 is
connected to the power supply line 80612. A second electrode of the
driving transistor 80600 is connected to a first electrode of the
light-emitting element 80620 through the third switch 80603.
Further, the second electrode of the driving transistor 80600 is
connected to the gate electrode of the driving transistor 80600
through the second switch 80602. A second electrode of the
light-emitting element 80620 corresponds to a common electrode
80621.
[1155] The second electrode of the light-emitting element 80620 is
set to a low power supply potential. Note that the low power supply
potential refers to a potential satisfying (the low power supply
potential)<(a high power supply potential) based on the high
power supply potential set to the power supply line 80612. As the
low power supply potential, GND, 0 V, or the like may be set, for
example. In order to apply a potential difference between the high
power supply potential and the low power supply potential to the
light-emitting element 80620 and supply current to the
light-emitting element 80620 so that the light-emitting element
80620 emits light, each potential is set so that the potential
difference between the high power supply potential and the low
power supply potential is equal to forward threshold voltage or
more. Note that gate capacitance of the driving transistor 80600
may be used as a substitute for the second capacitor 80605, so that
the second capacitor 80605 can be omitted. The gate capacitance of
the driving transistor 80600 may be formed in a region where a
source region, a drain region, an LDD region, or the like overlaps
with the gate electrode. Alternatively, capacitance may be formed
between a channel region and the gate electrode. Note that on/off
of the first switch 80601, the second switch 80602, and the third
switch 80603 is controlled by a first scan line 80613, a second
scan line 80615, and a third scan line 80614, respectively.
[1156] A method for driving the pixel shown in FIG. 69 is described
in which an operation period is divided into an initialization
period, a data writing period, a threshold detecting period, and a
light-emitting period.
[1157] In the initialization period, the second switch 80602 and
the third switch 80603 are turned on. Then, a potential of the gate
electrode of the driving transistor 80600 is lower than at least a
potential of the power supply line 80612. At this time, the first
switch 80601 may be in an on state or an off state. Note that the
initialization period is not necessarily required.
[1158] In the threshold detecting period, a pixel is selected by
the first scan line 80613. That is, the first switch 80601 is
turned on, and a certain constant voltage is input from the signal
line 80611. At this time, the second switch 80602 is turned on and
the third switch 80603 is turned off. Accordingly, the driving
transistor 80600 is diode-connected, and the second electrode and
the gate electrode of the driving transistor 80600 are placed in a
floating state. Then, a potential of the gate electrode of the
driving transistor 80600 is a value obtained by subtracting
threshold voltage of the driving transistor 80600 from the
potential of the power supply line 80612. Thus, the threshold
voltage of the driving transistor 80600 is held in the first
capacitor 80604. A potential difference between the potential of
the gate electrode of the driving transistor 80600 and the constant
voltage input from the signal line 80611 is held in the second
capacitor 80605.
[1159] In the data writing period, a video signal (voltage) is
input from the signal line 80611. At this time, the first switch
80601 is kept on, the second switch 80602 is turned off, and the
third switch 80603 is kept off. Since the gate electrode of the
driving transistor 80600 is in a floating state, the potential of
the gate electrode of the driving transistor 80600 changes
depending on a potential difference between the constant voltage
input from the signal line 80611 in the threshold detecting period
and a video signal input from the signal line 80611 in the data
writing period. For example, when (a capacitance value of the first
capacitor 80604)<<(a capacitance value of the second
capacitor 80605) is satisfied, the potential of the gate electrode
of the driving transistor 80600 in the data writing period is
approximately equal to the sum of a potential difference (the
amount of change) between the potential of the signal line 80611 in
the threshold detecting period and the potential of the signal line
80611 in the data writing period; and a value obtained by
subtracting the threshold voltage of the driving transistor 80600
from the potential of the power supply line 80612. That is, the
potential of the gate electrode of the driving transistor 80600
becomes a potential obtained by correcting the threshold voltage of
the driving transistor 80600.
[1160] In the light-emitting period, current in accordance with a
potential difference (Vgs) between the gate electrode of the
driving transistor 80600 and the power supply line 80612 is
supplied to the light-emitting element 80620. At this time, the
first switch 80601 is turned off, the second switch 80602 is kept
off, and the third switch 80603 is turned on. Note that current
flowing to the light-emitting element 80620 is constant regardless
of the threshold voltage of the driving transistor 80600.
[1161] Note that a pixel structure of the invention is not limited
to that shown in FIG. 69. For example, a switch, a resistor, a
capacitor, a transistor, a logic circuit, or the like may be added
to the pixel in FIG. 69. For example, the second switch 80602 may
include a p-channel transistor or an n-channel transistor, the
third switch 80603 may include a transistor with polarity different
from that of the second switch 80602, and the second switch 80602
and the third switch 80603 may be controlled by the same scan
line.
[1162] A structure and an operation of a pixel called a current
input pixel are described. A current input pixel can be applied to
digital gray scale drive and analog gray scale drive.
[1163] FIG. 70 is a diagram showing an example of a structure of a
current input pixel.
[1164] The pixel shown in FIG. 70 includes a driving transistor
80700, a first switch 80701, a second switch 80702, a third switch
80703, a capacitor 80704, and a light-emitting element 80730. A
gate electrode of the driving transistor 80700 is connected to a
signal line 80711 through the second switch 80702 and the first
switch 80701 in this order. Further, the gate electrode of the
driving transistor 80700 is connected to a power supply line 80712
through the capacitor 80704. A first electrode of the driving
transistor 80700 is connected to the power supply line 80712. A
second electrode of the driving transistor 80700 is connected to
the signal line 80711 through the first switch 80701. Further, the
second electrode of the driving transistor 80700 is connected to a
first electrode of the light-emitting element 80730 through the
third switch 80703. A second electrode of the light-emitting
element 80730 corresponds to a common electrode 80731.
[1165] The second electrode of the light-emitting element 80730 is
set to a low power supply potential. Note that the low power supply
potential refers to a potential satisfying (the low power supply
potential)<(a high power supply potential) based on the high
power supply potential set to the power supply line 80712. As the
low power supply potential, GND, 0 V, or the like may be set, for
example. In order to apply potential difference between the high
power supply potential and the low power supply potential to the
light-emitting element 80730 and supply current to the
light-emitting element 80730 so that the light-emitting element
80730 emits light, each potential is set so that the potential
difference between the high power supply potential and the low
power supply potential is equal to forward threshold voltage or
more. Note that gate capacitance of the driving transistor 80700
may be used as a substitute for the capacitor 80704, so that the
capacitor 80704 can be omitted. The gate capacitance of the driving
transistor 80700 may be formed in a region where a source region, a
drain region, an LDD region, or the like overlaps with the gate
electrode. Alternatively, capacitance may be formed between a
channel region and the gate electrode. Note that on/off of the
first switch 80701, the second switch 80702, and the third switch
80703 is controlled by a first scan line 80713, a second scan line
80714, and a third scan line 80715, respectively.
[1166] A method for driving the pixel shown in FIG. 70 is described
in which an operation period is divided into a data writing period
and a light-emitting period.
[1167] In the data writing period, a pixel is selected by the first
scan line 80713. That is, the first switch 80701 is turned on, and
current is input as a video signal from the signal line 80711. At
this time, the second switch 80702 is turned on and the third
switch 80703 is turned off. Accordingly, a potential of the gate
electrode of the driving transistor 80700 becomes a potential in
accordance with the video signal. That is, voltage between the gate
electrode and the source electrode of the driving transistor 80700
which is such that the driving transistor 80700 supplies current
same as the video signal is held in the capacitor 80704.
[1168] Next, in the light-emitting period, the first switch 80701
and the second switch 80702 are turned off, and the third switch
80703 is turned on. Thus, current with the same value as the video
signal is supplied to the light-emitting element 80730.
[1169] Note that the invention is not limited to the pixel
structure shown in FIG. 70. For example, a switch, a resistor, a
capacitor, a transistor, a logic circuit, or the like may be added
to the pixel in FIG. 70. For example, the first switch 80701 may
include a p-channel transistor or an n-channel transistor, the
second switch 80702 may include a transistor with the same polarity
as that of the first switch 80701, and the first switch 80701 and
the second switch 80702 may be controlled by the same scan line.
The second switch 80702 may be provided between the gate electrode
of the driving transistor 80700 and the signal line 80711.
[1170] Although this embodiment mode is described with reference to
various drawings, the contents (or part of the contents) described
in each drawing can be freely applied to, combined with, or
replaced with the contents (or part of the contents) described in
another drawing. Further, much more drawings can be formed by
combining each part with another part in the above-described
drawings.
[1171] The contents (or part of the contents) described in each
drawing in this embodiment mode can be freely applied to, combined
with, or replaced with the contents (or part of the contents)
described in a drawing in another embodiment mode. Further, much
more drawings can be formed by combining each part in each drawing
in this embodiment mode with part of another embodiment mode.
[1172] This embodiment mode shows examples of embodying, slightly
transforming, partially modifying, improving, describing in detail,
or applying the contents (or part of the contents) described in
other embodiment modes, an example of related part thereof, or the
like. Therefore, the contents described in other embodiment modes
can be freely applied to, combined with, or replaced with this
embodiment mode.
Embodiment Mode 12
[1173] In this embodiment mode, a structure and a manufacturing
method of a transistor are described.
[1174] FIGS. 71A to 71G show examples of structures and
manufacturing methods of transistors included in a semiconductor
device to which the invention can be applied. FIG. 71A shows
structure examples of transistors included in a semiconductor
device to which the invention can be applied. FIGS. 71B to 71G show
an example of a manufacturing method of the transistors included in
a semiconductor device to which the invention can be applied.
[1175] Note that the structure and the manufacturing method of the
transistors included in a semiconductor device to which the
invention can be applied are not limited to those shown in FIGS.
71A to 71G, and various structures and manufacturing methods can be
employed.
[1176] First, structure examples of transistors included in a
semiconductor device to which the invention can be applied are
described with reference to FIG. 71A. FIG. 71A is a cross-sectional
view of a plurality of transistors each having a different
structure. Here, in FIG. 71A, the plurality of transistors each
having a different structure are juxtaposed, which is for
describing structures of the transistors. Accordingly, the
transistors are not needed to be actually juxtaposed as shown in
FIG. 71A and can be separately formed as needed.
[1177] Next, characteristics of each layer forming the transistor
included in a semiconductor device to which the invention can be
applied are described.
[1178] A substrate 110111 can be a glass substrate using barium
borosilicate glass, aluminoborosilicate glass, or the like, a
quartz substrate, a ceramic substrate, a metal substrate containing
stainless steel, or the like. In addition, a substrate formed of
plastics typified by polyethylene terephthalate (PET), polyethylene
naphthalate (PEN), or polyethersulfone (PES), or a substrate formed
of a flexible synthetic resin such as acrylic can also be used. By
using a flexible substrate, a semiconductor device capable of being
bent can be formed. A flexible substrate has no strict limitations
on the area or a shape of the substrate. Accordingly, for example,
when a substrate having a rectangular shape, each side of which is
1 meter or more, is used as the substrate 110111, productivity can
be significantly improved. Such an advantage is highly favorable as
compared with the case where a circular silicon substrate is
used.
[1179] An insulating film 110112 functions as a base film and is
provided to prevent alkali metal such as Na or alkaline earth metal
from the substrate 110111 from adversely affecting characteristics
of a semiconductor element. The insulating film 110112 can have a
single-layer structure or a stacked-layer structure of an
insulating film containing oxygen or nitrogen, such as silicon
oxide (SiOx), silicon nitride (SiNx), silicon oxynitride (SiOxNy)
(x>y), or silicon nitride oxide (SiNxOy) (x>y). For example,
when the insulating film 110112 is provided to have a two-layer
structure, it is preferable that a silicon nitride oxide film be
used as a first insulating film and a silicon oxynitride film be
used as a second insulating film. Further, when the insulating film
110112 is provided to have a three-layer structure, it is
preferable that a silicon oxynitride film be used as a first
insulating film, a silicon nitride oxide film be used as a second
insulating film, and a silicon oxynitride film be used as a third
insulating film.
[1180] Semiconductor layers 110113, 110114, and 110115 can be
formed using an amorphous semiconductor or a semi-amorphous
semiconductor (SAS). Alternatively, a polycrystalline semiconductor
layer may be used. SAS is a semiconductor having an intermediate
structure between amorphous and crystalline (including single
crystal and polycrystalline) structures and having a third state
which is stable in free energy. Moreover, SAS includes a
crystalline region with a short-range order and lattice distortion.
A crystalline region of 0.5 to 20 nm can be observed at least in
part of a film. When silicon is contained as a main component,
Raman spectrum shifts to a wave number side lower than 520
cm.sup.-1. The diffraction peaks of (111) and (220) which are
thought to be contributed to a silicon crystalline lattice are
observed by X-ray diffraction. SAS contains hydrogen or halogen of
at least 1 atomic % or more to compensate dangling bonds. SAS is
formed by glow discharge decomposition (plasma CVD) of a material
gas. As the material gas, Si.sub.2H.sub.6, SiH.sub.2Cl.sub.2,
SiHCl.sub.3, SiCl.sub.4, SiF.sub.4, or the like as well as
SiH.sub.4 can be used. Alternatively, GeF.sub.4 may be mixed. The
material gas may be diluted with H.sub.2, or H.sub.2 and one or
more kinds of rare gas elements selected from He, Ar, Kr, and Ne. A
dilution ratio is in the range of 2 to 1000 times. Pressure is in
the range of approximately 0.1 to 133 Pa, and a power supply
frequency is 1 to 120 MHz, preferably 13 to 60 MHz. A substrate
heating temperature may be 300.degree. C. or lower. A concentration
of impurities in atmospheric components such as oxygen, nitrogen,
and carbon is preferably 1.times.10.sup.20 cm.sup.-1 or less as
impurity elements in the film. In particular, an oxygen
concentration is 5.times.10.sup.19/cm.sup.3 or less, preferably
1.times.10.sup.19/cm.sup.3 or less. Here, an amorphous
semiconductor layer is formed using a material containing silicon
(Si) as its main component (e.g., Si.sub.XGe.sub.1-x) by a known
method (such as a sputtering method, an LPCVD method, or a plasma
CVD method). Then, the amorphous semiconductor layer is
crystallized by a known crystallization method such as a laser
crystallization method, a thermal crystallization method using RTA
or an annealing furnace, or a thermal crystallization method using
a metal element which promotes crystallization.
[1181] An insulating film 110116 can have a single-layer structure
or a stacked-layer structure of an insulating film containing
oxygen or nitrogen, such as silicon oxide (SiOx), silicon nitride
(SiNx), silicon oxynitride (SiOxNy) (x>y), or silicon nitride
oxide (SiNxOy) (x>y).
[1182] A gate electrode 110117 can have a single-layer structure of
a conductive film or a stacked-layer structure of two or three
conductive films. As a material for the gate electrode 110117, a
known conductive film can be used. For example, a single film of an
element such as tantalum (Ta), titanium (Ti), molybdenum (Mo),
tungsten (W), chromium (Cr), or silicon (Si); a nitride film
containing the aforementioned element (typically, a tantalum
nitride film, a tungsten nitride film, or a titanium nitride film);
an alloy film in which the aforementioned elements are combined
(typically, a Mo--W alloy or a Mo--Ta alloy); a silicide film
containing the aforementioned element (typically, a tungsten
silicide film or a titanium silicide film); and the like can be
used. Note that the aforementioned single film, nitride film, alloy
film, silicide film, and the like can have a single-layer structure
or a stacked-layer structure.
[1183] An insulating film 110118 can have a single-layer structure
or a stacked-layer structure of an insulating film containing
oxygen or nitrogen, such as silicon oxide (SiOx), silicon nitride
(SiNx), silicon oxynitride (SiOxNy) (x>y), or silicon nitride
oxide (SiNxOy) (x>y); or a film containing carbon, such as a DLC
(diamond-like carbon), by a known method (such as a sputtering
method or a plasma CVD method).
[1184] An insulating film 110119 can have a single-layer structure
or a stacked-layer structure of a siloxane resin; an insulating
film containing oxygen or nitrogen, such as silicon oxide (SiOx),
silicon nitride (SiNx), silicon oxynitride (SiOxNy) (x>y), or
silicon nitride oxide (SiNxOy) (x>y); a film containing carbon,
such as a DLC (diamond-like carbon); or an organic material such as
epoxy, polyimide, polyamide, polyvinyl phenol, benzocyclobutene, or
acrylic. Note that a siloxane resin corresponds to a resin having
Si--O--Si bonds. Siloxane includes a backbone structure of a bond
of silicon (Si) and oxygen (O). As a substituent, an organic group
containing at least hydrogen (such as an alkyl group or an aryl
group) is used. Alternatively, a fluoro, group, or a fluoro group
and an organic group containing at least hydrogen can be used as a
substituent. Note that in a semiconductor device applicable to the
invention, the insulating film 110119 can be directly provided so
as to cover the gate electrode 110117 without provision of the
insulating film 110118.
[1185] As a conductive film 110123, a single film of an element
such as Al, Ni, C, W, Mo, Ti, Pt, Cu, Ta, Au, or Mn, a nitride film
containing the aforementioned element, an alloy film in which the
aforementioned elements are combined, a silicide film containing
the aforementioned element, or the like can be used. For example,
as an alloy containing the plurality of elements, an Al alloy
containing C and Ti, an Al alloy containing Ni, an Al alloy
containing C and Ni, an Al alloy containing C and Mn, or the like
can be used. Further, when the conductive film has a stacked-layer
structure, Al can be interposed between Mo, Ti, or the like; thus,
resistance of Al to heat and chemical reaction can be improved.
[1186] Next, characteristics of each structure are described with
reference to the cross-sectional view of the plurality of
transistors each having a different structure in FIG. 71A.
[1187] Reference numeral 110101 denotes a single drain transistor.
Since the single drain transistor can be formed by a simple method,
it is advantageous in low manufacturing cost and high yield. Here,
the semiconductor layers 110113 and 110115 have different
concentrations of impurities, and the semiconductor layer 110113 is
used as a channel region and the semiconductor layers 110115 are
used as a source region and a drain region. By controlling the
concentration of impurities in this manner, resistivity of the
semiconductor layer can be controlled. Further, an electrical
connection state of the semiconductor layer and the conductive film
110123 can be closer to ohmic contact. Note that as a method of
separately forming the semiconductor layers each having different
amount of impurities, a method can be used in which impurities are
doped in a semiconductor layer using the gate electrode 110117 as a
mask.
[1188] Reference numeral 110102 denotes a transistor in which the
gate electrode 110117 is tapered at an angle of at least certain
degrees. Since the transistor can be formed by a simple method, it
is advantageous in low manufacturing cost and high yield. Here, a
tapered angle is 45.degree. or more and less than 95.degree., and
preferably 60.degree. or more and less than 95.degree.. Note that
the tapered angle may be less than 45.degree.. The semiconductor
layers 110113, 110114, and 110115 have different concentrations of
impurities. The semiconductor layer 110113 is used as a channel
region, the semiconductor layers 110114 as lightly doped drain
(LDD) regions, and the semiconductor layers 110115 as a source
region and a drain region. By controlling the amount of impurities
in this manner, resistivity of the semiconductor layer can be
controlled. Further, an electrical connection state of the
semiconductor layer and the conductive film 110123 can be closer to
ohmic contact. Moreover, since the transistor includes the LDD
regions, a high electric field is hardly applied inside the
transistor, so that deterioration of the element due to hot
carriers can be suppressed. Note that as a method of separately
forming the semiconductor layers having different amount of
impurities, a method can be used in which impurities are doped in a
semiconductor layer using the gate electrode 110117 as a mask. In
the transistor 110102, since the gate electrode 110117 is tapered
at an angle of at least certain degrees, gradient of the
concentration of impurities doped in the semiconductor layer
through the gate electrode 110117 can be provided, and the LDD
region can be easily formed.
[1189] Reference numeral 110103 denotes a transistor in which the
gate electrode 110117 is formed of at least two layers and a lower
gate electrode is longer than an upper gate electrode. In this
specification, a shape of the lower and upper gate electrodes is
called a hat shape. When the gate electrode 110117 has a hat shape,
an LDD region can be formed without addition of a photomask. Note
that a structure where the LDD region overlaps with the gate
electrode 110117, like the transistor 110103, is particularly
called a GOLD (gate overlapped LDD) structure. As a method of
forming the gate electrode 110117 with a hat shape, the following
method may be used.
[1190] First, when the gate electrode 110117 is patterned, the
lower and upper gate electrodes are etched by dry etching so that
side surfaces thereof are inclined (tapered). Then, an inclination
of the upper gate electrode is processed to be almost perpendicular
by anisotropic etching. Thus, the gate electrode a cross section of
which is a hat shape is formed. After that, impurity elements are
doped twice, so that the semiconductor layer 110113 used as the
channel region, the semiconductor layers 110114 used as the LDD
regions, and the semiconductor layers 110115 used as a source
electrode and a drain electrode are formed.
[1191] Note that part of the LDD region, which overlaps with the
gate electrode 110117, is referred to as an Lov region, and part of
the LDD region, which does not overlap with the gate electrode
110117, is referred to as an Loff region. The Loff region is highly
effective in suppressing an off-current value, whereas it is not
very effective in preventing deterioration in an on-current value
due to hot carriers by relieving an electric field in the vicinity
of the drain. On the other hand, the Lov region is effective in
preventing deterioration in the on-current value by relieving the
electric field in the vicinity of the drain, whereas it is not very
effective in suppressing the off-current value. Thus, it is
preferable to form a transistor having a structure appropriate for
characteristics of each of the various circuits. For example, when
a semiconductor device applicable to the invention is used for a
display device, a transistor having an Loff region is preferably
used as a pixel transistor in order to suppress the off-current
value. On the other hand, as a transistor in a peripheral circuit,
a transistor having an Lov region is preferably used in order to
prevent deterioration in the on-current value by relieving the
electric field in the vicinity of the drain.
[1192] Reference numeral 110104 denotes a transistor including a
sidewall 110121 in contact with the side surface of the gate
electrode 110117. When the transistor includes the sidewall 110121,
a region overlapping with the sidewall 110121 can be made to be an
LDD region.
[1193] Reference numeral 110105 denotes a transistor in which an
LDD (Loff) region is formed by doping in the semiconductor layer
with the use of a mask. Thus, the LDD region can surely be formed,
and an off-current value of the transistor can be reduced.
[1194] Reference numeral 110106 denotes a transistor in which an
LDD (Lov) region is formed by doping in the semiconductor layer
with the use of a mask. Thus, the LDD region can surely be formed,
and deterioration in an on-current value can be prevented by
relieving the electric field in the vicinity of the drain of the
transistor.
[1195] Next, an example of a method for manufacturing a transistor
included in a semiconductor device to which the invention can be
applied is described with reference to FIGS. 71B to 71G
[1196] Note that a structure and a manufacturing method of a
transistor included in a semiconductor device to which the
invention can be applied are not limited to those in FIGS. 71A to
71G, and various structures and manufacturing methods can be
used.
[1197] In this embodiment mode, a surface of the substrate 110111,
a surface of the insulating film 110112, a surface of the
semiconductor layer 110113, a surface of the semiconductor layer
110114, a surface of the semiconductor layer 110115, a surface of
the insulating film 110116, a surface of the insulating film
110118, or a surface of the insulating film 110119 is oxidized or
nitrided by using plasma treatment, so that the semiconductor layer
or the insulating film can be oxidized or nitrided. By oxidizing or
nitriding the semiconductor layer or the insulating film by plasma
treatment in such a manner, the surface of the semiconductor layer
or the insulating film is modified, and the insulating film can be
formed to be denser than an insulating film formed by a CVD method
or a sputtering method. Thus, a defect such as a pinhole can be
suppressed, and characteristics and the like of a semiconductor
device can be improved.
[1198] First, the surface of the substrate 110111 is washed using
hydrofluoric acid (HF), alkaline, or pure water. The substrate
110111 can be a glass substrate using barium borosilicate glass,
aluminoborosilicate glass, or the like, a quartz substrate, a
ceramic substrate, a metal substrate containing stainless steel, or
the like. In addition, a substrate formed of plastics typified by
polyethylene terephthalate (PET), polyethylene naphthalate (PEN),
or polyethersulfone (PES), or a substrate formed of a flexible
synthetic resin such as acrylic can also be used. Here, the case
where a glass substrate is used as the substrate 110111 is
shown.
[1199] Here, an oxide film or a nitride film may be formed on the
surface of the substrate 110111 by oxidizing or nitriding the
surface of the substrate 110111 by plasma treatment (FIG. 71B).
Hereinafter, an insulating film such as an oxide film or a nitride
film formed by performing plasma treatment on the surface is also
referred to as a plasma-treated insulating film. In FIG. 71B, an
insulating film 110131 is a plasma-treated insulating film. In
general, when a semiconductor element such as a thin film
transistor is provided over a substrate formed of glass, plastic,
or the like, an impurity element such as alkali metal (e.g., Na) or
alkaline earth metal included in glass, plastic, or the like might
be mixed into the semiconductor element so that the semiconductor
element is contaminated; thus, characteristics of the semiconductor
element may be adversely affected in some cases. Nitridation of a
surface of the substrate formed of glass, plastic, or the like can
prevent an impurity element such as alkali metal (e.g., Na) or
alkaline earth metal included in the substrate from being mixed
into the semiconductor element.
[1200] When the surface is oxidized by plasma treatment, the plasma
treatment is performed in an oxygen atmosphere (e.g., in an
atmosphere of oxygen (O.sub.2) and a rare gas (containing at least
one of He, Ne, Ar, Kr, and Xe), in an atmosphere of oxygen,
hydrogen (H.sub.2), and a rare gas, or in an atmosphere of
dinitrogen monoxide and a rare gas). On the other hand, when the
surface is nitrided by plasma treatment, the plasma treatment is
performed in a nitrogen atmosphere (e.g., in an atmosphere of
nitrogen (N.sub.2) and a rare gas (containing at least one of He,
Ne, Ar, Kr, and Xe), in an atmosphere of nitrogen, hydrogen, and a
rare gas, or in an atmosphere of NH.sub.3 and a rare gas). As a
rare gas, Ar can be used, for example. Alternatively, a gas in
which Ar and Kr are mixed may be used. Accordingly, the
plasma-treated insulating film contains a rare gas (containing at
least one of He, Ne, Ar, Kr, and Xe) used for plasma treatment. For
example, the plasma-treated insulating film contains Ar when Ar is
used.
[1201] In addition, it is preferable to perform plasma treatment in
the atmosphere containing the aforementioned gas, with conditions
of an electron density in the range of 1.times.10.sup.11 to
1.times.10.sup.13 cm.sup.-3 and a plasma electron temperature in
the range of 0.5 to 1.5 eV. Since the plasma electron density is
high and the electron temperature in the vicinity of an object to
be treated is low, damage by plasma to the object to be treated can
be prevented. Further, since the plasma electron density is as high
as 1.times.10.sup.11 cm.sup.-3 or more, an oxide film or a nitride
film formed by oxidizing or nitriding the object to be treated by
plasma treatment is superior in its uniformity of thickness and the
like as well as being dense, as compared with a film formed by a
CVD method, a sputtering method, or the like. Alternatively, since
the plasma electron temperature is as low as 1 eV or less,
oxidation or nitridation can be performed at a lower temperature as
compared with a conventional plasma treatment or thermal oxidation.
For example, oxidation or nitridation can be performed sufficiently
even when plasma treatment is performed at a temperature lower than
a strain point of a glass substrate by 100 degrees or more. Note
that as frequency for generating plasma, high frequency waves such
as microwaves (2.45 GHz) can be used. Note that hereinafter, plasma
treatment is performed using the aforementioned conditions unless
otherwise specified.
[1202] Although FIG. 71B shows the case where the plasma-treated
insulating film is formed by plasma treatment on the surface of the
substrate 110111, this embodiment mode includes the case where a
plasma-treated insulating film is not formed on the surface of the
substrate 110111.
[1203] Although a plasma-treated insulating film formed by plasma
treatment on the surface of the object to be treated is not shown
in FIGS. 71C to 71G, this embodiment mode includes the case where a
plasma-treated insulating film formed by plasma treatment exists on
the surface of the substrate 110111, the insulating film 110112,
the semiconductor layer 110113, the semiconductor layer 110114, the
semiconductor layer 110115, the insulating film 110116, the
insulating film 110118, or the insulating film 110119.
[1204] Next, the insulating film 110112 is formed over the
substrate 110111 by a known method (such as a sputtering method, an
LPCVD method, or a plasma CVD method) (FIG. 71C). For the
insulating film 110112, silicon oxide (SiOx) or silicon oxynitride
(SiOxNy) (x>y) can be used.
[1205] Here, a plasma-treated insulating film may be formed on the
surface of the insulating film 110112 by oxidizing or nitriding the
surface of the insulating film 110112 by plasma treatment. By
oxidizing the surface of the insulating film 110112, the surface of
the insulating film 110112 is modified, and a dense film with fewer
defects such as a pinhole can be obtained. Further, by oxidizing
the surface of the insulating film 110112, the plasma-treated
insulating film containing a little amount of N atoms can be
formed; thus, interface characteristics of the plasma-treated
insulating film and a semiconductor layer are improved when the
semiconductor layer is provided over the plasma-treated insulating
film. The plasma-treated insulating film contains a rare gas
(containing at least one of He, Ne, Ar, Kr, and Xe) used for plasma
treatment. Note that the plasma treatment can be similarly
performed under the aforementioned conditions.
[1206] Next, the island-shaped semiconductor layers 110113 and
110114 are formed over the insulating film 110112 (FIG. 71D). The
island-shaped semiconductor layers 110113 and 110114 can be formed
in such a manner that an amorphous semiconductor layer is formed
over the insulating film 110112 by using a material containing
silicon (Si) as its main component (e.g., Si.sub.xGe.sub.1-x) or
the like by a known method (such as a sputtering method, an LPCVD
method, or a plasma CVD method), the amorphous semiconductor layer
is crystallized, and the semiconductor layer is selectively etched.
Note that crystallization of the amorphous semiconductor layer can
be performed by a known crystallization method such as a laser
crystallization method, a thermal crystallization method using RTA
or an annealing furnace, a thermal crystallization method using a
metal element which promotes crystallization, or a method in which
these methods are combined. Here, end portions of the island-shaped
semiconductor layers are provided with an angle of about 90.degree.
(.theta.=85 to 100.degree.). Alternatively, the semiconductor layer
110114 to be a low concentration drain region may be formed by
doping impurities with the use of a mask.
[1207] Here, a plasma-treated insulating film may be formed on the
surfaces of the semiconductor layers 110113 and 110114 by oxidizing
or nitriding the surfaces of the semiconductor layers 110113 and
110114 by plasma treatment. For example, when Si is used for the
semiconductor layers 110113 and 110114, silicon oxide (SiOx) or
silicon nitride (SiNx) is formed as the plasma-treated insulating
film. Alternatively, after the semiconductor layers 110113 and
110114 are oxidized by plasma treatment, the semiconductor layers
110113 and 110114 may be nitrided by performing plasma treatment
again. In this case, silicon oxide (SiOx) is formed in contact with
the semiconductor layers 110113 and 110114, and silicon nitride
oxide (SiNxOy) (x>y) is formed on the surface of the silicon
oxide. Note that when the semiconductor layer is oxidized by plasma
treatment, the plasma treatment is performed in an oxygen
atmosphere (e.g., in an atmosphere of oxygen (O.sub.2) and a rare
gas (containing at least one of He, Ne, Ar, Kr, and Xe), in an
atmosphere of oxygen, hydrogen (H.sub.2), and a rare gas, or in an
atmosphere of dinitrogen monoxide and a rare gas). On the other
hand, when the semiconductor layer is nitrided by plasma treatment,
the plasma treatment is performed in a nitrogen atmosphere (e.g.,
in an atmosphere of nitrogen (N.sub.2) and a rare gas (containing
at least one of He, Ne, Ar, Kr, and Xe), in an atmosphere of
nitrogen, hydrogen, and a rare gas, or in an atmosphere of NH.sub.3
and a rare gas). As a rare gas, Ar can be used, for example.
Alternatively, a gas in which Ar and Kr are mixed may be used.
Accordingly, the plasma-treated insulating film contains a rare gas
(containing at least one of He, Ne, Ar, Kr, and Xe) used for plasma
treatment. For example, the plasma-treated insulating film contains
Ar when Ar is used.
[1208] Next, the insulating film 110116 is formed (FIG. 71E). The
insulating film 110116 can have a single-layer structure or a
stacked-layer structure of an insulating film containing oxygen or
nitrogen, such as silicon oxide (SiOx), silicon nitride (SiNx),
silicon oxynitride (SiOxNy) (x>y), or silicon nitride oxide
(SiNxOy) (x>y), by a known method (such as a sputtering method,
an LPCVD method, or a plasma CVD method). Note that when the
plasma-treated insulating film is formed on the surfaces of the
semiconductor layers 110113 and 110114 by plasma treatment to the
surfaces of the semiconductor layers 110113 and 110114, the
plasma-treated insulating film can be used as the insulating film
110116.
[1209] Here, the surface of the insulating film 110116 may be
oxidized or nitrided by plasma treatment, so that a plasma-treated
insulating film is formed on the surface of the insulating film
110116. Note that the plasma-treated insulating film contains a
rare gas (containing at least one of He, Ne, Ar, Kr, and Xe) used
for plasma treatment. The plasma treatment can be similarly
performed under the aforementioned conditions.
[1210] Alternatively, after the insulating film 110116 is oxidized
by plasma treatment once in an oxygen atmosphere, the insulating
film 110116 may be nitrided by performing plasma treatment again in
a nitrogen atmosphere. By oxidizing or nitriding the surface of the
insulating film 110116 by plasma treatment in such a manner, the
surface of the insulating film 110116 is modified, and a dense film
can be formed. An insulating film obtained by plasma treatment is
denser and has fewer defects such as a pinhole, as compared with an
insulating film formed by a CVD method, a sputtering method, or the
like. Thus, characteristics of a thin film transistor can be
improved.
[1211] Next, the gate electrode 110117 is formed (FIG. 71F). The
gate electrode 110117 can be formed by a known method (such as a
sputtering method, an LPCVD method, or a plasma CVD method).
[1212] In the transistor 110101, the semiconductor layers 110115
used as the source region and the drain region can be formed by
doping impurities after the gate electrode 110117 is formed.
[1213] In the transistor 110102, the semiconductor layers 110114
used as the LDD regions and the semiconductor layers 110115 used as
the source region and the drain region can be formed by doping
impurities after the gate electrode 110117 is formed.
[1214] In the transistor 110103, the semiconductor layers 110114
used as the LDD regions and the semiconductor layers 110115 used as
the source region and the drain region can be formed by doping
impurities after the gate electrode 110117 is formed.
[1215] In the transistor 110104, the semiconductor layers 110114
used as the LDD regions and the semiconductor layers 110115 used as
the source region and the drain region can be formed by doping
impurities after the sidewall 110121 is formed on the side surface
of the gate electrode 110117.
[1216] Note that silicon oxide (SiOx) or silicon nitride (SiNx) can
be used for the sidewall 110121. As a method of forming the
sidewall 110121 on the side surface of the gate electrode 110117, a
method can be used, for example, in which a silicon oxide (SiOx)
film or a silicon nitride (SiNx) film is formed by a known method
after the gate electrode 110117 is formed, and then, the silicon
oxide (SiOx) film or the silicon nitride (SiNx) film is etched by
anisotropic etching. Thus, the silicon oxide (SiOx) film or the
silicon nitride (SiNx) film remains only on the side surface of the
gate electrode 110117, so that the sidewall 110121 can be formed on
the side surface of the gate electrode 110117.
[1217] In the transistor 110105, the semiconductor layers 110114
used as the LDD (Loff) regions and the semiconductor layer 110115
used as the source region and the drain region can be formed by
doping impurities after a mask 110122 is formed to cover the gate
electrode 110117.
[1218] In the transistor 110106, the semiconductor layers 110114
used as the LDD (Lov) regions and the semiconductor layers 110115
used as the source region and the drain region can be formed by
doping impurities after the gate electrode 110117 is formed.
[1219] Next, the insulating film 110118 is formed (FIG. 71G). The
insulating film 110118 can have a single-layer structure or a
stacked-layer structure of an insulating film containing oxygen or
nitrogen, such as silicon oxide (SiOx), silicon nitride (SiNx),
silicon oxynitride (SiOxNy) (x>y), or silicon nitride oxide
(SiNxOy) (x>y); or a film containing carbon, such as a DLC
(diamond-like carbon), by a known method (such as a sputtering
method or a plasma CVD method).
[1220] Here, the surface of the insulating film 110118 may be
oxidized or nitrided by plasma treatment, so that a plasma-treated
insulating film is formed on the surface of the insulating film
110118. Note that the plasma-treated insulating film contains a
rare gas (containing at least one of He, Ne, Ar, Kr, and Xe) used
for plasma treatment. The plasma treatment can be similarly
performed under the aforementioned conditions.
[1221] Next, the insulating film 110119 is formed. The insulating
film 110119 can have a single-layer structure or a stacked-layer
structure of an organic material such as epoxy, polyimide,
polyamide, polyvinyl phenol, benzocyclobutene, or acrylic; or a
siloxane resin, in addition to an insulating film containing oxygen
or nitrogen, such as silicon oxide (SiOx), silicon nitride (SiNx),
silicon oxynitride (SiOxNy) (x>y), or silicon nitride oxide
(SiNxOy) (x>y); or a film containing carbon, such as a DLC
(diamond-like carbon), by known method (such as a sputtering method
or a plasma CVD method). Note that a siloxane resin corresponds to
a resin having Si--O--Si bonds. Siloxane includes a skeleton
structure of a bond of silicon (Si) and oxygen (O). As a
substituent, an organic group containing at least hydrogen (such as
an alkyl group or an aryl group) is used. Alternatively, a fluoro
group, or a fluoro group and an organic group containing at least
hydrogen can be used as a substituent. In addition, the
plasma-treated insulating film contains a rare gas (containing at
least one of He, Ne, Ar, Kr, and Xe) used for plasma treatment. For
example, the plasma-treated insulating film contains Ar when Ar is
used.
[1222] When an organic material such as polyimide, polyamide,
polyvinyl phenol, benzocyclobutene, or acrylic, or a siloxane resin
is used for the insulating film 110119, the surface of the
insulating film 110119 can be modified by oxidizing or nitriding
the surface of the insulating film by plasma treatment.
Modification of the surface improves strength of the insulating
film 110119, and physical damage such as a crack generated when an
opening is formed, for example, or film reduction in etching can be
reduced. Further, when the conductive film 110123 is formed over
the insulating film 110119, modification of the surface of the
insulating film 110119 improves adhesion to the conductive film.
For example, when a siloxane resin is used for the insulating film
110119 and nitrided by plasma treatment, a plasma-treated
insulating film containing nitrogen or a rare gas is formed by
nitriding a surface of the siloxane resin, and physical strength is
improved.
[1223] Next, a contact hole is formed in the insulating films
110119, 110118, and 110116 in order to form the conductive film
110123 which is electrically connected to the semiconductor layer
110115. Note that the contact hole may have a tapered shape. Thus,
coverage with the conductive film 110123 can be improved.
[1224] FIG. 75 shows cross-sectional structures of a bottom-gate
transistor and a capacitor.
[1225] A first insulating film (an insulating film 110502) is
formed over an entire surface of a substrate 110501. The first
insulating film can prevent impurities from the substrate from
adversely affecting a semiconductor layer and changing properties
of a transistor. That is, the first insulating film functions as a
base film. Thus, a transistor with high reliability can be formed.
As the first insulating film, a single layer or a stacked layer of
a silicon oxide film, a silicon nitride film, a silicon oxynitride
film (SiOxNy), or the like can be used.
[1226] A first conductive layer (conductive layers 110503 and
110504) is formed over the first insulating film. The conductive
layer 110503 includes a portion functioning as a gate electrode of
a transistor 110520. The conductive layer 110504 includes a portion
functioning as a first electrode of a capacitor 110521. As the
first conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al,
Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these
elements can be used. Alternatively, a stacked layer of these
elements (including the alloy thereof) can be used.
[1227] A second insulating film (an insulating film 110514) is
formed to cover at least the first conductive layer. The second
insulating film functions as a gate insulating film. As the second
insulating film, a single layer or a stacked layer of a silicon
oxide film, a silicon nitride film, a silicon oxynitride film
(SiOxNy), or the like can be used.
[1228] For a portion of the second insulating film, which is in
contact with the semiconductor layer, a silicon oxide film is
preferably used. This is because the trap level at the interface
between the semiconductor layer and the second insulating film is
lowered.
[1229] When the second insulating film is in contact with Mo, a
silicon oxide film is preferably used for a portion of the second
insulating film in contact with Mo. This is because the silicon
oxide film does not oxidize Mo.
[1230] A semiconductor layer is formed in part of a portion over
the second insulating film, which overlaps with the first
conductive layer, by a photolithography method, an inkjet method, a
printing method, or the like. Part of the semiconductor layer
extends to a portion over the second insulating film, which does
not overlap with the first conductive layer. The semiconductor
layer includes a channel formation region (a channel formation
region 110510), an LDD region (LDD regions 110508 and 110509), and
an impurity region (impurity regions 110505, 110506, and 110507).
The channel formation region 110510 functions as a channel
formation region of the transistor 110520. The LDD regions 110508
and 110509 function as LDD regions of the transistor 110520. Note
that the LDD regions 110508 and 110509 are not necessarily formed.
The impurity region 110505 includes a portion functioning as one of
a source electrode and a drain electrode of the transistor 110520.
The impurity region 110506 includes a portion functioning as the
other of the source electrode and the drain electrode of the
transistor 110520. The impurity region 110507 includes a portion
functioning as a second electrode of the capacitor 110521.
[1231] A third insulating film (an insulating film 110511) is
entirely formed. A contact hole is selectively formed in part of
the third insulating film. The insulating film 110511 functions as
an interlayer film. As the third insulating film, an inorganic
material (e.g., silicon oxide, silicon nitride, or silicon
oxynitride), an organic compound material having a low dielectric
constant (e.g., a photosensitive or nonphotosensitive organic resin
material), or the like can be used. Alternatively, a material
containing siloxane may be used. Note that siloxane is a material
in which a skeleton structure is formed by a bond of silicon (Si)
and oxygen (O). As a substitute, an organic group containing at
least hydrogen (such as an alkyl group or an aryl group) is used.
Alternatively, a fluoro group, or a fluoro group and an organic
group containing at least hydrogen may be used as a
substituent.
[1232] A second conductive layer (conductive layers 110512 and
110513) is formed over the third insulating film. The conductive
layer 110512 is connected to the other of the source electrode and
the drain electrode of the transistor 110520 through the contact
hole formed in the third insulating film. Thus, the conductive
layer 110512 includes a portion functioning as the other of the
source electrode and the drain electrode of the transistor 110520.
When the conductive layer 110513 is electrically connected to the
conductive layer 110504, the conductive layer 110513 includes a
portion functioning as the first electrode of the capacitor 110521.
As the second conductive layer, an element such as Ti, Mo, Ta, Cr,
W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy
of these elements can be used. Alternatively, a stacked layer of
these elements (including the alloy thereof) can be used.
[1233] Note that in steps after forming the second conductive
layer, various insulating films or various conductive films may be
formed.
[1234] Next, structures of a transistor and a capacitor are
described in the case where an amorphous silicon (a-Si:H) film is
used as a semiconductor layer of the transistor.
[1235] FIG. 72 shows cross-sectional structures of a top-gate
transistor and a capacitor.
[1236] A first insulating film (an insulating film 110202) is
formed over an entire surface of a substrate 110201. The first
insulating film can prevent impurities from the substrate from
adversely affecting a semiconductor layer and changing properties
of a transistor. That is, the first insulating film functions as a
base film. Thus, a transistor with high reliability can be formed.
As the first insulating film, a single layer or a stacked layer of
a silicon oxide film, a silicon nitride film, a silicon oxynitride
film (SiOxNy), or the like can be used.
[1237] Note that the first insulating film is not necessarily
formed. When the first insulating film is not formed, reduction in
the number of steps and manufacturing cost can be realized.
Further, since the structure can be simplified, the yield can be
improved.
[1238] A first conductive layer (conductive layers 110203, 110204,
and 110205) is formed over the first insulating film. The
conductive layer 110203 includes a portion functioning as one of a
source electrode and a drain electrode of a transistor 110220. The
conductive layer 110204 includes a portion functioning as the other
of the source electrode and the drain electrode of the transistor
110220. The conductive layer 110205 includes a portion functioning
as a first electrode of a capacitor 110221. As the first conductive
layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au,
Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements can be
used. Alternatively, a stacked layer of these elements (including
the alloy thereof) can be used.
[1239] A first semiconductor layer (semiconductor layers 110206 and
110207) is formed above the conductive layers 110203 and 110204.
The semiconductor layer 110206 includes a portion functioning as
one of the source electrode and the drain electrode. The
semiconductor layer 110207 includes a portion functioning as the
other of the source electrode and the drain electrode. As the first
semiconductor layer, silicon containing phosphorus or the like can
be used, for example.
[1240] A second semiconductor layer (a semiconductor layer 110208)
is formed over the first insulating film and between the conductive
layer 110203 and the conductive layer 110204. Part of the
semiconductor layer 110208 extends over the conductive layers
110203 and 110204. The semiconductor layer 110208 includes a
portion functioning as a channel region of the transistor 110220.
As the second semiconductor layer, a semiconductor layer having no
crystallinity such as an amorphous silicon (a-Si:H) layer, a
semiconductor layer such as a microcrystalline semiconductor
(.mu.-Si:H) layer, or the like can be used.
[1241] A second insulating film (insulating films 110209 and
110210) is formed to cover at least the semiconductor layer 110208
and the conductive layer 110205. The second insulating film
functions as a gate insulating film. As the second insulating film,
a single layer or a stacked layer of a silicon oxide film, a
silicon nitride film, a silicon oxynitride film (SiOxNy), or the
like can be used.
[1242] For a portion of the second insulating film, which is in
contact with the second semiconductor layer, a silicon oxide film
is preferably used. This is because the trap level at the interface
between the second semiconductor layer and the second insulating
film is lowered.
[1243] When the second insulating film is in contact with Mo, a
silicon oxide film is preferably used for a portion of the second
insulating film in contact with Mo. This is because the silicon
oxide film does not oxidize Mo.
[1244] A second conductive layer (conductive layers 110211 and
110212) is formed over the second insulating film. The conductive
layer 110211 includes a portion functioning as a gate electrode of
the transistor 110220. The conductive layer 110212 functions as a
second electrode of the capacitor 110221 or a wiring. As the second
conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu,
Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these
elements can be used. Alternatively, a stacked layer of these
elements (including the alloy thereof) can be used.
[1245] Note that in steps after forming the second conductive
layer, various insulating films or various conductive films may be
formed.
[1246] FIG. 73 shows cross-sectional structures of an inversely
staggered (bottom gate) transistor and a capacitor. In particular,
the transistor shown in FIG. 73 has a channel etch structure.
[1247] A first insulating film (an insulating film 110302) is
formed over an entire surface of a substrate 110301. The first
insulating film can prevent impurities from the substrate from
adversely affecting a semiconductor layer and changing properties
of a transistor. That is, the first insulating film functions as a
base film. Thus, a transistor with high reliability can be formed.
As the first insulating film, a single layer or a stacked layer of
a silicon oxide film, a silicon nitride film, a silicon oxynitride
film (SiOxNy), or the like can be used.
[1248] Note that the first insulating film is not necessarily
formed. When the first insulating film is not formed, reduction in
the number of steps and manufacturing cost can be realized.
Further, since the structure can be simplified, the yield can be
improved.
[1249] A first conductive layer (conductive layers 110303 and
110304) is formed over the first insulating film. The conductive
layer 110303 includes a portion functioning as a gate electrode of
a transistor 110320. The conductive layer 110304 includes a portion
functioning as a first electrode of a capacitor 110321. As the
first conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al,
Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these
elements can be used. Alternatively, a stacked layer of these
elements (including the alloy thereof) can be used.
[1250] A second insulating film (an insulating film 110305) is
formed to cover at least the first conductive layer. The second
insulating film functions as a gate insulating film. As the second
insulating film, a single layer or a stacked layer of a silicon
oxide film, a silicon nitride film, a silicon oxynitride film
(SiOxNy), or the like can be used.
[1251] For a portion of the second insulating film, which is in
contact with the semiconductor layer, a silicon oxide film is
preferably used. This is because the trap level at the interface
between the semiconductor layer and the second insulating film is
lowered.
[1252] When the second insulating film is in contact with Mo, a
silicon oxide film is preferably used for a portion of the second
insulating film in contact with Mo. This is because the silicon
oxide film does not oxidize Mo.
[1253] A first semiconductor layer (a semiconductor layer 110306)
is formed in part of a portion over the second insulating film,
which overlaps with the first conductive layer, by a
photolithography method, an inkjet method, a printing method, or
the like. Part of the semiconductor layer 110306 extends to a
portion over the second insulating film, which does not overlap
with the first conductive layer. The semiconductor layer 110306
includes a portion functioning as a channel region of the
transistor 110320. As the semiconductor layer 110306, a
semiconductor layer having no crystallinity such as an amorphous
silicon (a-Si:H) layer, a semiconductor layer such as a
microcrystalline semiconductor (.mu.-Si:H) layer, or the like can
be used.
[1254] A second semiconductor layer (semiconductor layers 110307
and 110308) is formed over part of the first semiconductor layer.
The semiconductor layer 110307 includes a portion functioning as
one of a source electrode and a drain electrode. The semiconductor
layer 110308 includes a portion functioning as the other of the
source electrode and the drain electrode. As the second
semiconductor layer, silicon containing phosphorus or the like can
be used, for example.
[1255] A second conductive layer (conductive layers 110309, 110310,
and 110311) is formed over the second semiconductor layer and the
second insulating film. The conductive layer 110309 includes a
portion functioning as one of the source electrode and the drain
electrode of the transistor 110320. The conductive layer 110310
includes a portion functioning as the other of the source electrode
and the drain electrode of the transistor 110320. The conductive
layer 110311 includes a portion functioning as a second electrode
of the capacitor 110321. As the second conductive layer, an element
such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe,
Ba, or Ge, or an alloy of these elements can be used.
Alternatively, a stacked layer of these elements (including the
alloy thereof) can be used.
[1256] Note that in steps after forming the second conductive
layer, various insulating films or various conductive films may be
formed.
[1257] Here, an example of a step characteristic of a channel etch
type transistor is described. The first semiconductor layer and the
second semiconductor layer can be formed using the same mask.
Specifically, the first semiconductor layer and the second
semiconductor layer are continuously formed. Further, the first
semiconductor layer and the second semiconductor layer are formed
using the same mask.
[1258] Another example of a step characteristic of a channel etch
type transistor is described. The channel region of the transistor
can be formed without using an additional mask. Specifically, after
the second conductive layer is formed, part of the second
semiconductor layer is removed using the second conductive layer as
a mask. Alternatively, part of the second semiconductor layer is
removed by using the same mask as the second conductive layer. The
first semiconductor layer below the removed second semiconductor
layer serves as the channel region of the transistor.
[1259] FIG. 74 shows cross-sectional structures of an inversely
staggered (bottom gate) transistor and a capacitor. In particular,
the transistor shown in FIG. 74 has a channel protection (channel
stop) structure.
[1260] A first insulating film (an insulating film 110402) is
formed over an entire surface of a substrate 110401. The first
insulating film can prevent impurities from the substrate from
adversely affecting a semiconductor layer and changing properties
of a transistor. That is, the first insulating film functions as a
base film. Thus, a transistor with high reliability can be formed.
As the first insulating film, a single layer or a stacked layer of
a silicon oxide film, a silicon nitride film, a silicon oxynitride
film (SiOxNy), or the like can be used.
[1261] Note that the first insulating film is not necessarily
formed. When the first insulating film is not formed, reduction in
the number of steps and manufacturing cost can be realized.
Further, since the structure can be simplified, the yield can be
improved.
[1262] A first conductive layer (conductive layers 110403 and
110404) is formed over the first insulating film. The conductive
layer 110403 includes a portion functioning as a gate electrode of
a transistor 110420. The conductive layer 110404 includes a portion
functioning as a first electrode of a capacitor 110421. As the
first conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al,
Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these
elements can be used. Alternately, a stacked layer of these
elements (including the alloy thereof) can be used.
[1263] A second insulating film (an insulating film 110405) is
formed to cover at least the first conductive layer. The second
insulating film functions as a gate insulating film. As the second
insulating film, a single layer or a stacked layer of a silicon
oxide film, a silicon nitride film, a silicon oxynitride film
(SiOxNy), or the like can be used.
[1264] For a portion of the second insulating film, which is in
contact with the semiconductor layer, a silicon oxide film is
preferably used. This is because the trap level at the interface
between the semiconductor layer and the second insulating film is
lowered.
[1265] When the second insulating film is in contact with Mo, a
silicon oxide film is preferably used for a portion of the second
insulating film in contact with Mo. This is because the silicon
oxide film does not oxidize Mo.
[1266] A first semiconductor layer (a semiconductor layer 110406)
is formed in part of a portion over the second insulating film,
which overlaps with the first conductive layer, by a
photolithography method, an inkjet method, a printing method, or
the like. Part of the semiconductor layer 110406 extends to a
portion over the second insulating film, which does not overlap
with the first conductive layer. The semiconductor layer 110406
includes a portion functioning as a channel region of the
transistor 110420. As the semiconductor layer 110406, a
semiconductor layer having no crystallinity such as an amorphous
silicon (a-Si:H) layer, a semiconductor layer such as a
microcrystalline semiconductor (.mu.-Si:H) layer, or the like can
be used.
[1267] A third insulating film (an insulating film 110412) is
formed over part of the first semiconductor layer. The insulating
film 110412 prevents the channel region of the transistor 110420
from being removed by etching. That is, the insulating film 110412
functions as a channel protection film (a channel stop film). As
the third insulating film, a single layer or a stacked layer of a
silicon oxide film, a silicon nitride film, a silicon oxynitride
film (SiOxNy), or the like can be used.
[1268] A second semiconductor layer (semiconductor layers 110407
and 110408) is formed over part of the first semiconductor layer
and part of the third insulating film. The semiconductor layer
110407 includes a portion functioning as one of a source electrode
and a drain electrode. The semiconductor layer 110408 includes a
portion functioning as the other of the source electrode and the
drain electrode. As the second semiconductor layer, silicon
containing phosphorus or the like can be used, for example.
[1269] A second conductive layer (conductive layers 110409, 110410,
and 110411) is formed over the second semiconductor layer. The
conductive layer 110409 includes a portion functioning as one of
the source electrode and the drain electrode of the transistor
110420. The conductive layer 110410 includes a portion functioning
as the other of the source electrode and the drain electrode of the
transistor 110420. The conductive layer 110411 includes a portion
functioning as a second electrode of the capacitor 110421. As the
second conductive layer, an element such as Ti, Mo, Ta, Cr, W. Al,
Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these
elements can be used. Alternately, a stacked layer of these
elements (including the alloy thereof) can be used.
[1270] Note that in steps after forming the second conductive
layer, various insulating films or various conductive films may be
formed.
[1271] Here, an example of a step characteristic of a channel
protection transistor is described. The first semiconductor layer,
the second semiconductor layer, and the second conductive layer can
be formed using the same mask. At the same time, the channel region
can be formed. Specifically, the first semiconductor layer is
formed, and next, the third insulating film (i.e., the channel
protection film or the channel stop film) is patterned using a
mask. Next, the second semiconductor layer and the second
conductive layer are continuously formed. Then, after the second
conductive layer is formed, the first semiconductor layer, the
second semiconductor layer, and the second conductive film are
patterned using the same mask. Note that part of the first
semiconductor layer below the third insulating film is protected by
the third insulating film, and thus is not removed by etching. This
part (a part of the first semiconductor layer over which the third
insulating film is formed) serves as the channel region.
[1272] Although this embodiment mode is described with reference to
various drawings, the contents (or part of the contents) described
in each drawing can be freely applied to, combined with, or
replaced with the contents (or part of the contents) described in
another drawing. Further, much more drawings can be formed by
combining each part with another part in the above-described
drawings.
[1273] The contents (or part of the contents) described in each
drawing in this embodiment mode can be freely applied to, combined
with, or replaced with the contents (or part of the contents)
described in a drawing in another embodiment mode. Further, much
more drawings can be formed by combining each part in each drawing
in this embodiment mode with part of another embodiment mode.
[1274] This embodiment mode shows examples of embodying, slightly
transforming, partially modifying, improving, describing in detail,
or applying the contents (or part of the contents) described in
other embodiment modes, an example of related part thereof, or the
like. Therefore, the contents described in other embodiment modes
can be freely applied to, combined with, or replaced with this
embodiment mode.
Embodiment Mode 13
[1275] In this embodiment mode, a structure of an EL element is
described. In particular, a structure of an inorganic EL element is
described.
[1276] An inorganic EL element is classified as either a dispersion
type inorganic EL element or a thin-film type inorganic EL element,
depending on its element structure. These elements differ in that
the former includes an electroluminescent layer in which particles
of a light-emitting material are dispersed in a binder, whereas the
latter includes an electroluminescent layer formed of a thin film
of a light-emitting material. However, the former and the latter
have in common in that they need electrons accelerated by a high
electric field. Note that mechanisms for obtaining light emission
are donor-acceptor recombination light emission which utilizes a
donor level and an acceptor level; and localized light emission
which utilizes inner-shell electron transition of a metal ion. In
general, donor-acceptor recombination light emission is employed in
dispersion type inorganic EL elements and localized light emission
is employed in thin-film type inorganic EL elements in many
cases.
[1277] A light-emitting material includes a base material and an
impurity element to be a luminescence center. Light emission of
various colors can be obtained by changing the impurity element to
be included. The light-emitting material can be formed using
various methods, such as a solid phase method or a liquid phase
method (a coprecipitation method). Further, a liquid phase method
such as a spray pyrolysis method, a double decomposition method, a
method employing precursor pyrolysis, a reverse micelle method, a
method in which one or more of these methods are combined with
high-temperature baking, or a freeze-drying method, or the like can
be used.
[1278] A solid phase method is a method in which a base material
and an impurity element or a compound containing an impurity
element are weighed, mixed in a mortar, and heated and baked in an
electric furnace so as to be reacted; thus, the impurity element is
included in the base material. The baking temperature is preferably
700 to 1500.degree. C. This is because a solid-phase reaction does
not proceed when the temperature is too low, and the base material
decomposes when the temperature is too high. Note that although the
materials may be baked in powder form, they are preferably baked in
pellet form. Although a solid phase method needs a comparatively
high temperature, it is a simple method, and thus has high
productivity and is suitable for mass production.
[1279] A liquid phase method (a coprecipitation method) is a method
in which a base material or a compound containing a base material,
and an impurity element or a compound containing an impurity
element are reacted in a solution, dried, and then baked. Particles
of a light-emitting material are uniformly distributed, and the
reaction can progress even when the particles are small and the
baking temperature is low.
[1280] As a base material to be used for a light-emitting material,
sulfide, oxide, or nitride can be used. As sulfide, zinc sulfide
(ZnS), cadmium sulfide (CdS), calcium sulfide (CaS), yttrium
sulfide (Y.sub.2S.sub.3), gallium sulfide (Ga.sub.2S.sub.3),
strontium sulfide (SrS), barium sulfide (BaS), or the like can be
used, for example. As oxide, zinc oxide (ZnO), yttrium oxide
(Y.sub.2O.sub.3), or the like can be used, for example. As nitride,
aluminum nitride (AlN), gallium nitride (GaN), indium nitride
(InN), or the like can be used, for example. Further, zinc selenide
(ZnSe), zinc telluride (ZnTe), or the like; or a ternary mixed
crystal such as calcium gallium sulfide (CaGa.sub.2S.sub.4),
strontium gallium sulfide (SrGa.sub.2S.sub.4), or barium gallium
sulfide (BaGa.sub.2S.sub.4) may be used.
[1281] As a luminescence center for localized light emission,
manganese (Mn), copper (Cu), samarium (Sm), terbium (Tb), erbium
(Er), thulium (Tm), europium (Eu), cerium (Ce), praseodymium (Pr),
or the like can be used. Note that a halogen element such as
fluorine (F) or chlorine (Cl) may be added for charge
compensation.
[1282] On the other hand, as a luminescence center for
donor-acceptor recombination light emission, a light-emitting
material including a first impurity element forming a donor level
and a second impurity element forming an acceptor level can be
used. As the first impurity element, fluorine (F), chlorine (Cl),
aluminum (Al), or the like can be used, for example. As the second
impurity element, copper (Cu), silver (Ag), or the like can be
used, for example.
[1283] When the light-emitting material for donor-acceptor
recombination light emission is synthesized using a solid phase
method, a base material, the first impurity element or a compound
containing the first impurity element, and the second impurity
element or a compound containing the second impurity element are
weighed, mixed in a mortar, and heated and baked in an electric
furnace. As the base material, the aforementioned base material can
be used. As the first impurity element or the compound containing
the first impurity element, fluorine (F), chlorine (Cl), aluminum
sulfide (Al.sub.2S.sub.3), or the like can be used, for example. As
the second impurity element or the compound containing the second
impurity element, copper (Cu), silver (Ag), copper sulfide
(Cu.sub.2S), silver sulfide (Ag.sub.2S), or the like can be used,
for example. The baking temperature is preferably 700 to
1500.degree. C. This is because a solid-phase reaction does not
proceed when the temperature is too low, and the base material
decomposes when the temperature is too high. Note that although the
materials may be baked in powder form, they are preferably baked in
pellet form.
[1284] As the impurity element in the case of using a solid phase
reaction, a compound formed of the first impurity element and the
second impurity element may be used in combination. In this case,
the impurity elements are easily diffused, and the solid phase
reaction proceeds readily, so that a uniform light-emitting
material can be obtained. Further, since an unnecessary impurity
element is not included, a light-emitting material with high purity
can be obtained. As the compound formed of the first impurity
element and the second impurity element, copper chloride (CuCl),
silver chloride (AgCl), or the like can be used, for example.
[1285] Note that the concentration of these impurity elements is in
the range of 0.01 to 10 atomic %, and is preferably in the range of
0.05 to 5 atomic % with respect to the base material.
[1286] In the case of a thin-film type inorganic EL element, an
electroluminescent layer includes the aforementioned light-emitting
material, and can be formed using a vacuum evaporation method such
as a resistance heating evaporation method or an electron beam
evaporation (EB evaporation) method, a physical vapor deposition
(PVD) method such as a sputtering method, a chemical vapor
deposition (CVD) method such as a metal organic CVD method or a
low-pressure hydride transport CVD method, an atomic layer epitaxy
(ALE) method, or the like.
[1287] FIGS. 76A to 76C each show an example of a thin-film type
inorganic EL element which can be used as the light-emitting
element. In FIGS. 76A to 76C, a light-emitting element includes a
first electrode layer 120100, an electroluminescent layer 120102,
and a second electrode layer 120103.
[1288] The light-emitting elements in FIGS. 76B and 76C each have a
structure where an insulating film is provided between the
electrode layer and the electroluminescent layer in the
light-emitting element in FIG. 76A. The light-emitting element in
FIG. 76B includes an insulating film 120104 between the first
electrode layer 120100 and the electroluminescent layer 120102. The
light-emitting element in FIG. 76C includes an insulating film
120105 between the first electrode layer 120100 and the
electroluminescent layer 120102, and an insulating film 120106
between the second electrode layer 120103 and the
electroluminescent layer 120102. Accordingly, the insulating film
may be provided between the electroluminescent layer and one of the
electrode layers interposing the electroluminescent layer, or may
be provided between the electroluminescent layer and each of the
electrode layers interposing the electroluminescent layer. The
insulating film may be a single layer or stacked layers including a
plurality of layers.
[1289] Note that the insulating film 120104 is provided in contact
with the first electrode layer 120100 in FIG. 76B; however, the
insulating film 120104 may be provided in contact with the second
electrode layer 120103 by reversing the order of the insulating
film and the electroluminescent layer.
[1290] In the case of a dispersion type inorganic EL, a film-shaped
electroluminescent layer is formed by dispersing particulate
light-emitting materials in a binder. When particles with a desired
size cannot be sufficiently obtained by a method of forming the
light-emitting material, the light-emitting materials may be
processed into particles by being crushed in a mortar or the like.
The binder is a substance for fixing the particulate light-emitting
material in a dispersed state and maintaining the shape as the
electroluminescent layer. The light-emitting material is uniformly
dispersed in the electroluminescent layer and fixed by the
binder.
[1291] In the case of a dispersion type inorganic EL, as a method
of forming the electroluminescent layer, a droplet discharging
method by which the electroluminescent layer can be selectively
formed, a printing method (such as screen printing or offset
printing), a coating method such as a spin coating method, a
dipping method, a dispenser method, or the like can be used. The
thickness of the electroluminescent layer is not particularly
limited, but preferably in the range of 10 to 1000 nm. In the
electroluminescent layer including the light-emitting material and
the binder, a ratio of the light-emitting material is preferably 50
wt % or more and 80 wt % or less.
[1292] FIGS. 77A to 77C each show an example of a dispersion type
inorganic EL element which can be used as the light-emitting
element. A light-emitting element in FIG. 77A has a stacked-layer
structure of a first electrode layer 120200, an electroluminescent
layer 120202, and a second electrode layer 120203. The
electroluminescent layer 120202 includes a light-emitting material
120201 held by a binder.
[1293] An insulating material is used for the binder. As the
insulating material, an organic material or an inorganic material
can be used. Alternatively, a mixed material containing an organic
material and an inorganic material may be used. As the organic
insulating material, a polymer having a comparatively high
dielectric constant, such as a cyanoethyl cellulose based resin, or
a resin such as polyethylene, polypropylene, a polystyrene based
resin, a silicone resin, an epoxy resin, or vinylidene fluoride can
be used. Alternatively, a heat-resistant polymer such as aromatic
polyamide or polybenzimidazole, or a siloxane resin may be used.
Note that a siloxane resin corresponds to a resin having Si--O--Si
bonds. Siloxane includes a backbone structure of a bond of silicon
(Si) and oxygen (O). As a substituent, an organic group containing
at least hydrogen (such as an alkyl group or an aryl group) is
used. Alternatively, a fluoro group, or a fluoro group and an
organic group containing at least hydrogen may be used as a
substituent. Further alternately, a resin material, for example, a
vinyl resin such as polyvinyl alcohol or polyvinylbutyral, a phenol
resin, a novolac resin, an acrylic resin, a melamine resin, an
urethane resin, an oxazole resin (polybenzoxazole), or the like may
be used. A dielectric constant can be adjusted by appropriately
mixing these resins with fine particles having a high dielectric
constant, such as barium titanate (BaTiO.sub.3) or strontium
titanate (SrTiO.sub.3).
[1294] The inorganic insulating material included in the binder can
be formed using silicon oxide (SiOx), silicon nitride (SiNx),
silicon containing oxygen and nitrogen, aluminum nitride (AlN),
aluminum containing oxygen and nitrogen, aluminum oxide
(Al.sub.2O.sub.3) containing oxygen and nitrogen, titanium oxide
(TiO.sub.2), BaTiO.sub.3, SrTiO.sub.3, lead titanate (PbTiO.sub.3),
potassium niobate (KNbO.sub.3), lead niobate (PbNbO.sub.3),
tantalum oxide (Ta.sub.2O.sub.5), barium tantalite
(BaTa.sub.2O.sub.6), lithium tantalite (LiTaO.sub.3), yttrium oxide
(Y.sub.2O.sub.3), zirconium oxide (ZrO.sub.2), ZnS, or a substance
containing another inorganic insulating material. When an inorganic
material having a high dielectric constant is included in the
organic material (by addition or the like), the dielectric constant
of the electroluminescent layer formed of the light-emitting
material and the binder can be more effectively controlled, and the
dielectric constant can be further increased.
[1295] In a manufacturing step, the light-emitting material is
dispersed in a solution containing the binder. As a solvent for the
solution containing the binder, it is acceptable as long as a
solvent dissolves a binder material and can make a solution having
a viscosity suitable for a method of forming the electroluminescent
layer (various wet processes) and for desired film thickness. For
example, an organic solvent or the like can be used as the solvent.
When a siloxane resin is used as the binder, propylene glycol
monomethyl ether, propylene glycol monomethyl ether acetate (also
referred to as PGMEA), 3-methoxy-3-methyl-1-butanol (also referred
to as MMB), or the like can be used as the solvent.
[1296] The light-emitting elements shown in FIGS. 77B and 77C each
have a structure where an insulating film is provided between the
electrode layer and the electroluminescent layer in the
light-emitting element in FIG. 77A. The light-emitting element in
FIG. 77B includes an insulating film 120204 between the first
electrode layer 120200 and the electroluminescent layer 120202. The
light-emitting element in FIG. 77C includes an insulating film
120205 between the first electrode layer 120200 and the
electroluminescent layer 120202, and an insulating film 120206
between the second electrode layer 120203 and the
electroluminescent layer 120202. Accordingly, the insulating film
may be provided between the electroluminescent layer and one of the
electrode layers interposing the electroluminescent layer, or may
be provided between the electroluminescent layer and each of the
electrode layers interposing the electroluminescent layer. The
insulating film may be a single layer or stacked layers including a
plurality of layers.
[1297] Although the insulating film 120204 is provided in contact
with the first electrode layer 120200 in FIG. 77B, the insulating
film 120204 may be provided in contact with the second electrode
layer 120203 by reversing the order of the insulating film and the
electroluminescent layer.
[1298] A material used for an insulating film such as the
insulating film 120104 in FIG. 76B and the insulating film 120204
in FIG. 77B preferably has high withstand voltage and dense film
quality. Further, the material preferably has a high dielectric
constant. For example, silicon oxide (SiO.sub.2), yttrium oxide
(Y.sub.2O.sub.3), titanium oxide (TiO.sub.2), aluminum oxide
(Al.sub.2O.sub.3), hafnium oxide (HfO.sub.2), tantalum oxide
(Ta.sub.2O.sub.5), barium titanate (BaTiO.sub.3), strontium
titanate (SrTiO.sub.3), lead titanate (PbTiO.sub.3), silicon
nitride (Si.sub.3N.sub.4), zirconium oxide (ZrO.sub.2), or the
like; or a mixed film of those materials or a stacked-layer film
including two or more of those materials can be used. The
insulating film can be formed by sputtering, evaporation, CVD, or
the like. The insulating film may be formed by dispersing particles
of the insulating material in a binder. A binder material may be
formed using a material and a method similar to those of the binder
contained in the electroluminescent layer. The thickness of the
insulating film is not particularly limited, but preferably in the
range of 10 to 1000 nm.
[1299] Note that the light-emitting element can emit light when
voltage is applied between the pair of electrode layers interposing
the electroluminescent layer. The light-emitting element can
operate with DC drive or AC drive.
[1300] Although this embodiment mode is described with reference to
various drawings, the contents (or part of the contents) described
in each drawing can be freely applied to, combined with, or
replaced with the contents (or part of the contents) described in
another drawing. Further, much more drawings can be formed by
combining each part with another part in the above-described
drawings.
[1301] The contents (or part of the contents) described in each
drawing in this embodiment mode can be freely applied to, combined
with, or replaced with the contents (or part of the contents)
described in a drawing in another embodiment mode. Further, much
more drawings can be formed by combining each part in each drawing
in this embodiment mode with part of another embodiment mode.
[1302] This embodiment mode shows examples of embodying, slightly
transforming, partially modifying, improving, describing in detail,
or applying the contents (or part of the contents) described in
other embodiment modes, an example of related part thereof, or the
like. Therefore, the contents described in other embodiment modes
can be freely applied to, combined with, or replaced with this
embodiment mode.
Embodiment Mode 14
[1303] In this embodiment mode, an example of a display device is
described. In particular, the case where a display device is
optically treated is described.
[1304] A rear projection display device 130100 in FIGS. 78A and 78B
is provided with a projector unit 130111, a mirror 130112, and a
screen panel 130101. The rear projection display device 130100 may
also be provided with a speaker 130102 and operation switches
130104. The projector unit 130111 is provided at a lower portion of
a housing 130110 of the rear projection display device 130100, and
projects incident light which projects an image based on a video
signal to the mirror 130112. The rear projection display device
130100 displays an image projected from a rear surface of the
screen panel 130101.
[1305] FIG. 79 shows a front projection display device 130200. The
front projection display device 130200 is provided with the
projector unit 130111 and a projection optical system 130201. The
projection optical system 130201 projects an image to a screen or
the like provided at the front.
[1306] Hereinafter, a structure of the projector unit 130111 which
is applied to the rear projection display device 130100 in FIGS.
78A and 78B and the front projection display device 130200 in FIG.
79 is described.
[1307] FIG. 80 shows a structure example of the projector unit
130111. The projector unit 130111 is provided with a light source
unit 130301 and a modulation unit 130304. The light source unit
130301 is provided with a light source optical system 130303
including lenses and a light source lamp 130302. The light source
lamp 130302 is stored in a housing so that stray light is not
scattered. As the light source lamp 130302, a high-pressure mercury
lamp or a xenon lamp, for example, which can emit a large amount of
light is used. The light source optical system 130303 is provided
with an optical lens, a film having a function of polarizing light,
a film for adjusting phase difference, an IR film, or the like as
appropriate. The light source unit 130301 is provided so that
emitted light is incident on the modulation unit 130304. The
modulation unit 130304 is provided with a plurality of display
panels 130308, a color filter, a dichroic mirror 130305, a total
reflection mirror 130306, a retardation plate 130307, a prism
130309, and a projection optical system 130310. Light emitted from
the light source unit 130301 is split into a plurality of optical
paths by the dichroic mirror 130305.
[1308] Each optical path is provided with the display panel 130308
and a color filter which transmits light with a predetermined
wavelength or wavelength range. The transmissive display panel
130308 modulates transmitted light based on a video signal. Light
of each color transmitted through the display panel 130308 is
incident on the prism 130309, and an image is displayed on a screen
through the projection optical system 130310. Note that a Fresnel
lens may be provided between the mirror and the screen. Projected
light which is projected by the projector unit 130111 and reflected
by the mirror is converted into generally parallel light by the
Fresnel lens to be projected on the screen. Displacement between a
chief ray and an optical axis of the parallel light is preferably
.+-.10.degree. or less, and more preferably, .+-.5.degree. or
less.
[1309] FIG. 81 shows the projector unit 130111 provided with
reflective display panels 130407, 130408, and 130409. Reference
Numeral 130410 is a prism.
[1310] The projector unit 130111 in FIG. 81 is provided with the
light source unit 130101 and a modulation unit 130400. The light
source unit 130101 may have a structure similar to FIG. 80. Light
from the light source unit 130101 is split into a plurality of
optical paths by dichroic mirrors 130401 and 130402 and a total
reflection mirror 130403 to be incident on polarization beam
splitters 130404, 130405, and 130406. The polarization beam
splitters 130404, 130405, and 130406 are provided corresponding to
the reflective display panels 130407, 130408, and 130409 which
correspond to respective colors. The reflective display panels
130407, 130408, and 130409 modulate reflected light based on a
video signal. Light of each color which is reflected by the
reflective display panels 130407, 130408, and 130409 is incident on
the prism 130109 to be synthesized, and projected through a
projection optical system 130411.
[1311] Among light emitted from the light source unit 130101, only
light in a wavelength region of red is transmitted through the
dichroic mirror 130401 and light in wavelength regions of green and
blue is reflected by the dichroic mirror 130401. Further, only the
light in the wavelength region of green is reflected by the
dichroic mirror 130402. The light in the wavelength region of red,
which is transmitted through the dichroic mirror 130401, is
reflected by the total reflection mirror 130403 and incident on the
polarization beam splitter 130404. The light in the wavelength
region of blue is incident on the polarization beam splitter
130405. The light in the wavelength region of green is incident on
the polarization beam splitter 130406. The polarization beam
splitters 130404, 130405, and 130406 have a function of splitting
incident light into p-polarized light and s-polarized light and a
function of transmitting only p-polarized light. The reflective
display panels 130407, 130408, and 130409 polarize incident light
based on a video signal.
[1312] Only s-polarized light corresponding to each color is
incident on the reflective display panels 130407, 130408, and
130409 corresponding to each color. Note that the reflective
display panels 130407, 130408, and 130409 may be liquid crystal
panels. In this case, the liquid crystal panel operates in an
electrically controlled birefringence (ECB) mode. Liquid crystal
molecules are vertically aligned with respect to a substrate at a
certain angle. Accordingly, in the reflective display panels
130407, 130408, and 130409, when a pixel is in an off state,
display molecules are aligned so as not to change a polarization
state of incident light but to reflect the incident light. When the
pixel is in an on state, alignment of the display molecules is
changed, and the polarization state of the incident light is
changed.
[1313] The projector unit 130111 in FIG. 81 can be applied to the
rear projection display device 130100 in FIGS. 78A and 78B and the
front projection display device 130200 in FIG. 79.
[1314] FIGS. 82A to 82C show single-panel type projector units. The
projector unit shown in FIG. 82A is provided with the light source
unit 130301, a display panel 130507, a projection optical system
130511, and a retardation plate 130504. The projection optical
system 130511 includes one or a plurality of lenses. The display
panel 130507 may be provided with a color filter.
[1315] FIG. 82B shows a structure of a projector unit operating in
a field sequential mode. A field sequential mode refers to a mode
in which color display is performed by light of respective colors
such as red, green, and blue sequentially incident on a display
panel with a time lag, without a color filter. High-definition
image can be displayed particularly by combination with a display
panel with high-speed response to change in input signal. In FIG.
82B, a rotating color filter plate 130505 including a plurality of
color filters with red, green, blue, or the like is provided
between the light source unit 130301 and a display panel
130508.
[1316] FIG. 82C shows a structure of a projector unit with a color
separation method using a micro lens, as a color display method.
This method corresponds to a method in which color display is
realized by providing a micro lens array 130506 on a light incident
side of a display panel 130509 and light of each color is lit from
each direction. The projector unit employing this method has little
loss of light due to a color filter, so that light from the light
source unit 130301 can be efficiently utilized. The projector unit
in FIG. 82C is provided with dichroic mirrors 130501, 130502, and
130503 so that light of each color is lit to the display panel
130509 from each direction.
[1317] Although this embodiment mode is described with reference to
various drawings, the contents (or part of the contents) described
in each drawing can be freely applied to, combined with, or
replaced with the contents (or part of the contents) described in
another drawing. Further, much more drawings can be formed by
combining each part with another part in the above-described
drawings.
[1318] The contents (or part of the contents) described in each
drawing in this embodiment mode can be freely applied to, combined
with, or replaced with the contents (or part of the contents)
described in a drawing in another embodiment mode. Further, much
more drawings can be formed by combining each part in each drawing
in this embodiment mode with part of another embodiment mode.
[1319] This embodiment mode shows examples of embodying, slightly
transforming, partially modifying, improving, describing in detail,
or applying the contents (or part of the contents) described in
other embodiment modes, an example of related part thereof, or the
like. Therefore, the contents described in other embodiment modes
can be freely applied to, combined with, or replaced with this
embodiment mode.
Embodiment Mode 15
[1320] In this embodiment mode, an operation of a display device is
described.
[1321] FIG. 83 shows a structure example of a display device.
[1322] A display device 180100 includes a pixel portion 180101, a
signal line driver circuit 180103, and a scan line driver circuit
180104. In the pixel portion 180101, a plurality of signal lines S1
to Sn extend from the signal line driver circuit 180103 in a column
direction. In the pixel portion 180101, a plurality of scan lines
G1 to Gm extend from the scan line driver circuit 180104 in a row
direction. Pixels 180102 are arranged in matrix at each
intersection of the plurality of signal lines S1 to Sn and the
plurality of scan lines G1 to Gm.
[1323] The signal line driver circuit 180103 has a function of
outputting a signal to each of the signal lines S1 to Sn. This
signal may be referred to as a video signal. The scan line driver
circuit 180104 has a function of outputting a signal to each of the
scan lines G1 to Gm. This signal may be referred to as a scan
signal.
[1324] The pixel 180102 includes at least a switching element
connected to the signal line. On/off of the switching element is
controlled by a potential of a scan line (a scan signal). When the
switching element is turned on, the pixel 180102 is selected. On
the other hand, when the switching element is turned off, the pixel
180102 is not selected.
[1325] When the pixel 180102 is selected (a selection state), a
video signal is input to the pixel 180102 from the signal line. A
state (e.g., luminance, transmittance, or voltage of a storage
capacitor) of the pixel 180102 is changed in accordance with the
video signal input.
[1326] When the pixel 180102 is not selected (a non-selection
state), the video signal is not input to the pixel 180102. Note
that the pixel 180102 holds a potential corresponding to the video
signal which is input when selected; thus, the pixel 180102
maintains the state (e.g., luminance, transmittance, or voltage of
a storage capacitor) in accordance with the video signal.
[1327] Note that a structure of the display device is not limited
to that shown in FIG. 83. For example, an additional wiring (such
as a scan line, a signal line, a power supply line, a capacitor
line, or a common line) may be added in accordance with the
structure of the pixel 180102. As another example, a circuit having
various functions may be added.
[1328] FIG. 84 shows an example of a timing chart for describing an
operation of a display device.
[1329] The timing chart of FIG. 84 shows one frame period
corresponding to a period when an image of one screen is displayed.
One frame period is not particularly limited, but one frame period
is preferably 1/60 second or less so that a viewer does not
perceive a flicker.
[1330] The timing chart of FIG. 84 shows timing for selecting the
scan line G1 in the first row, the scan line Gi (one of the scan
lines G1 to Gm) in the i-th row, the scan line Gi+1 in the (i+1)th
row, and the scan line Gm in the m-th row.
[1331] At the same time as the scan line is selected, the pixel
180102 connected to the scan line is also selected. For example,
when the scan line Gi in the i-th row is selected, the pixel 180102
connected to the scan line Gi in the i-th row is also selected.
[1332] The scan lines G1 to Gm are sequentially selected
(hereinafter also referred to as scanned) from the scan line G1 in
the first row to the scan line Gm in the m-th row. For example,
while the scan line Gi in the i-th row is selected, the scan lines
(G1 to Gi-1 and Gi+1 to Gm) other than the scan line Gi in the i-th
row are not selected. Then, during the next period, the scan line
Gi+1 in the (i+1)th row is selected. Note that a period during
which one scan line is selected is referred to as one gate
selection period.
[1333] Accordingly, when a scan line in a certain row is selected,
video signals from the signal lines S1 to Sn are input to a
plurality of pixels 180102 connected to the scan line,
respectively. For example, while the scan line Gi in the i-th row
is selected, given video signals from the signal lines S1 to Sn are
input to the plurality of pixels 180102 connected to the scan line
Gi in the i-th row, respectively. Thus, each of the plurality of
pixels 180102 can be controlled individually by the scan signal and
the video signal.
[1334] Next, the case where one gate selection period is divided
into a plurality of subgate selection periods is described.
[1335] FIG. 85 is a timing chart in the case where one gate
selection period is divided into two subgate selection periods (a
first subgate selection period and a second subgate selection
period).
[1336] Note that one gate selection period may be divided into
three or more subgate selection periods.
[1337] The timing chart of FIG. 85 shows one frame period
corresponding to a period when an image of one screen is displayed.
One frame period is not particularly limited, but one frame period
is preferably 1/60 second or less so that a viewer does not
perceive a flicker.
[1338] One frame is divided into two subframes (a first subframe
and a second subframe).
[1339] The timing chart of FIG. 85 shows timing for selecting the
scan line Gi in the i-th row, the scan line Gi+1 in the (i+1)th
row, the scan line Gj (one of the scan lines Gi+1 to Gm) in the
j-th row, and the scan line Gj+1 in the j+1)th row.
[1340] At the same time as the scan line is selected, the pixel
180102 connected to the scan line is also selected. For example,
when the scan line Gi in the i-th row is selected, the pixel 180102
connected to the scan line Gi in the i-th row is also selected.
[1341] The scan lines G1 to Gm are sequentially scanned in each
subgate selection period. For example, in one gate selection
period, the scan line Gi in the i-th row is selected in the first
subgate selection period, and the scan line Gj in the j-th row is
selected in the second subgate selection period. Thus, in one gate
selection period, an operation can be performed as if the scan
signals of two rows are selected. At this time, different video
signals are input to the signal lines S1 to Sn in the first subgate
selection period and the second subgate selection period.
Accordingly, different video signals can be input to a plurality of
pixels 180102 connected to the i-th row and a plurality of pixels
180102 connected to the j-th row.
[1342] Next, a driving method for displaying images with high
quality is described.
[1343] FIGS. 86A and 86B are views for describing high frequency
driving.
[1344] FIG. 86A shows the case where one image and one intermediate
image are displayed in one frame period 180400. Reference numerals
180401, 180402, 180403, and 180404 denote an image of one frame, an
intermediate image of the frame, an image of the next frame, and an
intermediate image of the next frame, respectively.
[1345] The intermediate image 180402 of the frame may be made based
on image signals of the frame and the next frame. Alternatively,
the intermediate image 180402 of the frame may be formed from the
image 180401 of the frame, or may be a black image. Accordingly,
the quality of a moving image in a hold-type display device can be
improved. Further, when one image and one intermediate image are
displayed in one frame period 180400, there is an advantage in that
consistency with a frame rate of the video signal can be easily
obtained and an image processing circuit is not complicated.
[1346] FIG. 86B shows the case where one image and two intermediate
images are displayed in a period with two successive one frame
periods 180400 (two frame periods). Reference numeral 180411,
180412, 180413, and 180414 denote an image of the frame, an
intermediate image of the frame, an intermediate image of the next
frame, an image of a frame after next, respectively.
[1347] Each of the intermediate image 180412 of the frame and the
intermediate image 180413 of the next frame may be made based on
video signals of the frame, the next frame, and the frame after
next. Alternatively, each of the intermediate image 180412 of the
frame and the intermediate image 180413 of the next frame may be a
black image. When one image and two intermediate images are
displayed in two frame periods, there is an advantage in that
operating frequency of a peripheral driver circuit is not so high
and image quality of a moving image can be effectively
improved.
[1348] Although this embodiment mode is described with reference to
various drawings, the contents (or part of the contents) described
in each drawing can be freely applied to, combined with, or
replaced with the contents (or part of the contents) described in
another drawing. Further, much more drawings can be formed by
combining each part with another part in the above-described
drawings.
[1349] The contents (or part of the contents) described in each
drawing in this embodiment mode can be freely applied to, combined
with, or replaced with the contents (or part of the contents)
described in a drawing in another embodiment mode. Further, much
more drawings can be formed by combining each part in each drawing
in this embodiment mode with part of another embodiment mode.
[1350] This embodiment mode shows examples of embodying, slightly
transforming, partially modifying, improving, describing in detail,
or applying the contents (or part of the contents) described in
other embodiment modes, an example of related part thereof, or the
like. Therefore, the contents described in other embodiment modes
can be freely applied to, combined with, or replaced with this
embodiment mode.
Embodiment Mode 16
[1351] In this embodiment mode, a structure of an EL element is
described. In particular, a structure of an organic EL element is
described.
[1352] A structure of a mixed junction EL element is described. As
an example, a structure is described, which includes a layer (a
mixed layer) in which a plurality of materials among a hole
injecting material, a hole transporting material, a light-emitting
material, an electron transporting material, an electron injecting
material, and the like are mixed (hereinafter referred to as a
mixed junction type EL element), which is different from a
stacked-layer structure where a hole injecting layer formed of a
hole injecting material, a hole transporting layer formed of a hole
transporting material, a light-emitting layer formed of a
light-emitting material, an electron transporting layer formed of
an electron transporting material, an electron injecting layer
formed of an electron injecting material, and the like are clearly
distinguished.
[1353] FIGS. 87A to 87E are schematic views each showing a
structure of a mixed junction type EL element. Note that a layer
interposed between an anode 190101 and a cathode 190102 corresponds
to an EL layer.
[1354] FIG. 87A shows a structure in which an EL layer includes a
hole transporting region 190103 formed of a hole transporting
material and an electron transporting region 190104 formed of an
electron transporting material. The hole transporting region 190103
is closer to the anode than the electron transporting region
190104. A mixed region 190105 including both the hole transporting
material and the electron transporting material is provided between
the hole transporting region 190103 and the electron transporting
region 190104.
[1355] In a direction from the anode 190101 to the cathode 190102,
a concentration of the hole transporting material in the mixed
region 190105 is decreased and a concentration of the electron
transporting material in the mixed region 190105 is increased.
[1356] A concentration gradient can be freely set. For example, a
ratio of concentrations of each functional material may be changed
(a concentration gradient may be formed) in the mixed region 190105
including both the hole transporting material and the electron
transporting material, without including the hole transporting
layer 190103 formed of only the hole transporting material.
Alternatively, a ratio of concentrations of each functional
material may be changed (a concentration gradient may be formed) in
the mixed region 190105 including both the hole transporting
material and the electron transporting material, without including
the hole transporting layer 190103 formed of only the hole
transporting material and the electron transporting layer 190104
formed of only the electron transporting material. Further
alternatively, a ratio of concentrations may be changed depending
on a distance from the anode or the cathode. Note that the ratio of
concentrations may be changed continuously.
[1357] A region 190106 to which a light-emitting material is added
is included in the mixed region 190105. A light emission color of
the EL element can be controlled by the light-emitting material.
Further, carriers can be trapped by the light-emitting material. As
the light-emitting material, various fluorescent dyes as well as a
metal complex having a quinoline backbone, a benzoxazole backbone,
or a benzothiazole backbone can be used. The light emission color
of the EL element can be controlled by adding the light-emitting
material.
[1358] As the anode 190101, an electrode material having a high
work function is preferably used in order to inject holes
efficiently. For example, a transparent electrode formed of indium
tin oxide (ITO), indium zinc oxide (IZO), ZnO, SnO.sub.2,
In.sub.2O.sub.3, or the like can be used. When a transparency is
not needed, the anode 190101 may be formed of an opaque metal
material.
[1359] As the hole transporting material, an aromatic amine
compound or the like can be used.
[1360] As the electron transporting material, a metal complex
having a quinoline derivative, 8-quinolinol, or a derivative
thereof as a ligand (especially tris(8-quinolinolato)aluminum
(Alq.sub.3)), or the like can be used.
[1361] As the cathode 190102, an electrode material having a low
work function is preferably used in order to inject electrons
efficiently. A metal such as aluminum, indium, magnesium, silver,
calcium, barium, or lithium can be used by itself. Alternatively,
an alloy of the aforementioned metal or an alloy of the
aforementioned metal and another metal may be used.
[1362] FIG. 87B is the schematic view of the structure of the EL
element, which is different from that of FIG. 87A. Note that the
same portions as those in FIG. 87A are denoted by the same
reference numerals, and description thereof is omitted.
[1363] In FIG. 87B, a region to which a light-emitting material is
added is not included. However, when a material
(electron-transporting and light-emitting material) having both an
electron transporting property and a light-emitting property, for
example, tris(8-quinolinolato)aluminum (Alq.sub.3) is used as a
material added to the electron transporting region 190104, light
emission can be performed.
[1364] Alternatively, as a material added to the hole transporting
region 190103, a material (a hole-transporting and light-emitting
material) having both a hole transporting property and a
light-emitting property may be used.
[1365] FIG. 87C is the schematic view of the structure of the EL
element, which is different from those of FIGS. 87A and 87B. Note
that the same portions as those in FIGS. 87A and 87B are denoted by
the same reference numerals, and description thereof is
omitted.
[1366] In FIG. 87C, a region 190107 included in the mixed region
190105 is provided, to which a hole blocking material having a
larger energy difference between the highest occupied molecular
orbital and the lowest unoccupied molecular orbital than the hole
transporting material is added. The region 190107 to which the hole
blocking material is added is provided closer to the cathode 190102
than the region 190106 in the mixed region 190105, to which the
light-emitting material is added; thus, a recombination rate of
carriers can be increased, and light emission efficiency can be
increased. The structure provided with the region 190107 to which
the hole blocking material is added is especially effective in an
EL element which utilizes light emission (phosphorescence) by a
triplet exciton.
[1367] FIG. 87D is the schematic view of the structure of the EL
element, which is different from those of FIGS. 87A to 87C. Note
that the same portions as those in FIGS. 87A to 87C are denoted by
the same reference numerals, and description thereof is
omitted.
[1368] In FIG. 87D, a region 190108 included in the mixed region
190105 is provided, to which an electron blocking material having a
larger energy difference between the highest occupied molecular
orbital and the lowest unoccupied molecular orbital than the
electron transporting material is added. The region 190108 to which
the electron blocking material is added is provided closer to the
anode 190101 than the region 190106 in the mixed region 190105, to
which the light-emitting material is added; thus, a recombination
rate of carriers can be increased, and light emission efficiency
can be increased. The structure provided with the region 190108 to
which the electron blocking material is added is especially
effective in an EL element which utilizes light emission
(phosphorescence) by a triplet exciton.
[1369] FIG. 87E is the schematic view of the structure of the mixed
junction type EL element, which is different from those of FIGS.
87A to 87D. FIG. 87E shows an example of a structure where a region
190109 to which a metal material is added is included in part of an
EL layer in contact with an electrode of the EL element. In FIG.
87E, the same portions as those in FIGS. 87A to 87D are denoted by
the same reference numerals, and description thereof is omitted. In
the structure shown in FIG. 87E, MgAg (an Mg--Ag alloy) may be used
as the cathode 190102, and the region 190109 to which an Al
(aluminum) alloy is added may be included in a region of the
electron transporting region 190104 to which the electron
transporting material is added, which is in contact with the
cathode 190102, for example. By the aforementioned structure,
oxidation of the cathode can be prevented, and electron injection
efficiency from the cathode can be increased. Accordingly, the
lifetime of the mixed junction type EL element can be extended.
Further, driving voltage can be lowered.
[1370] As a method of forming the mixed junction type EL element, a
co-evaporation method or the like can be used.
[1371] In the mixed junction type EL elements as shown in FIGS. 87A
to 87E, a clear interface between the layers does not exist, and
charge accumulation can be reduced. Accordingly, the lifetime of
the EL element can be extended, and a driving voltage can be
lowered.
[1372] Note that the structures shown in FIGS. 87A to 87E can be
implemented in free combination with each other.
[1373] A structure of the mixed junction type EL element is not
limited to those described above. A known structure can be freely
used.
[1374] An organic material which forms an EL layer of an EL element
may be a low molecular material or a high molecular material.
Alternatively, both of the materials may be used. When a low
molecular material is used as an organic compound material, a film
can be formed by an evaporation method. When a high molecular
material is used as the EL layer, the high molecular material is
dissolved in a solvent and a film can be formed by a spin coating
method or an inkjet method.
[1375] The EL layer may be formed of a middle molecular material.
In this specification, a middle molecule organic light-emitting
material refers to an organic light-emitting material without a
sublimation property and with a polymerization degree of
approximately 20 or less. When a middle molecular material is used
as the EL layer, a film can be formed by an inkjet method or the
like.
[1376] Note that a low molecular material, a high molecular
material, and a middle molecular material may be used in
combination.
[1377] An EL element may utilize either light emission
(fluorescence) by a singlet exciton or light emission
(phosphorescence) by a triplet exciton.
[1378] Next, an evaporation device for forming a display device
applicable to the invention is described with reference to
drawings.
[1379] A display device applicable to the invention may be
manufactured to include an EL layer. The EL layer is formed
including at least partially a material which exhibits
electroluminescence. The EL layer may be formed of a plurality of
layers having different functions. In this case, the EL layer may
be formed of a combination of layers having different functions,
which are also called a hole injecting and transporting layer, a
light-emitting layer, an electron injecting and transporting layer,
and the like.
[1380] FIG. 88 shows a structure of an evaporation apparatus for
forming an EL layer over an element substrate provided with a
transistor. In the evaporation apparatus, a plurality of treatment
chambers are connected to transfer chambers 190260 and 190261. Each
treatment chamber includes a loading chamber 190262 for supplying a
substrate, an unloading chamber 190263 for collecting the
substrate, a heat treatment chamber 190268, a plasma treatment
chamber 190272, deposition treatment chambers 190269 to 190271,
190273 to 190275 for depositing an EL material, and a deposition
treatment chamber 190276 for forming a conductive film formed of
aluminum or containing aluminum as its main component as one
electrode of an EL element. Gate valves 190277a to 1902771 are
provided between the transfer chambers and the treatment chambers,
so that the pressure in each treatment chamber can be controlled
independently, and cross contamination between the treatment
chambers is prevented.
[1381] A substrate introduced into the transfer chamber 190260 from
the loading chamber 190262 is transferred to a predetermined
treatment chamber by an arm type transfer means 190266 capable of
rotating. The substrate is transferred from a certain treatment
chamber to another treatment chamber by the transfer means 190266.
The transfer chambers 190260 and 190261 are connected by the
deposition treatment chamber 190270 at which the substrate is
delivered by the transfer means 190266 and a transfer means
190267.
[1382] Each treatment chamber connected to the transfer chambers
190260 and 190261 is maintained in a reduced pressure state.
Accordingly, in the evaporation apparatus, deposition treatment of
an EL layer is continuously performed without exposing the
substrate to the room air. A display panel in which formation of
the EL layer is finished is deteriorated due to moisture or the
like in some cases. Accordingly, in the evaporation apparatus, a
sealing treatment chamber 190265 for performing sealing treatment
before exposure to the room air in order to maintain quality is
connected to the transfer chamber 190261. Since the sealing
treatment chamber 190265 is under atmospheric pressure or reduced
pressure near atmosphere pressure, an intermediate treatment
chamber 190264 is also provided between the transfer chamber 190261
and the sealing treatment chamber 190265. The intermediate
treatment chamber 190264 is provided for delivering the substrate
and buffering the pressure between the chambers.
[1383] An exhaust means is provided in the loading chamber, the
unloading chamber, the transfer chamber, and the deposition
treatment chamber in order to maintain reduced pressure in the
chamber. As the exhaust means, various vacuum pumps such as a dry
pump, a turbo-molecular pump, and a diffusion pump can be used.
[1384] In the evaporation apparatus of FIG. 88, the number of
treatment chambers connected to the transfer chambers 190260 and
190261 and structures thereof can be combined as appropriate in
accordance with a stacked-layer structure of the EL element. An
example of a combination is described below.
[1385] In the heat treatment chamber 190268, degasification
treatment is performed by heating a substrate over which a lower
electrode, an insulating partition wall, or the like is formed. In
the plasma treatment chamber 190272, a surface of the lower
electrode is treated with a rare gas or oxygen plasma. This plasma
treatment is performed for cleaning the surface, stabilizing a
surface state, or stabilizing a physical or chemical state (e.g., a
work function) of the surface.
[1386] The deposition treatment chamber 190269 is for forming an
electrode buffer layer which is in contact with one electrode of
the EL element. The electrode buffer layer has a carrier injection
property (hole injection or electron injection) and suppresses
generation of a short-circuit or a black spot defect of the EL
element. Typically, the electrode buffer layer is formed of an
organic-inorganic hybrid material, has a resistivity of
5.times.10.sup.4 to 1.times.10.sup.6 .OMEGA.cm, and is formed
having a thickness of 30 to 300 nm. Note that the deposition
treatment chamber 190271 is for forming a hole transporting
layer.
[1387] A light-emitting layer in an EL element has a different
structure between the case of emitting single color light and the
case of emitting white light. A deposition treatment chamber in the
evaporation apparatus is preferably provided depending on the
structure. For example, when three kinds of EL elements each having
a different light emission color are formed in a display panel, it
is necessary to form light-emitting layers corresponding to
respective light emission colors. In this case, the deposition
treatment chamber 190270 can be used for forming a first
light-emitting layer, the deposition treatment chamber 190273 can
be used for forming a second light-emitting layer, and the
deposition treatment chamber 190274 can be used for forming a third
light-emitting layer. By using different deposition treatment
chambers for respective light-emitting layers, cross contamination
due to different light-emitting materials can be prevented, and
throughput of the deposition treatment can be improved.
[1388] Note that three kinds of EL elements each having a different
light emission color may be sequentially deposited in each of the
deposition treatment chambers 190270, 190273, and 190274. In this
case, evaporation is performed by moving a shadow mask in
accordance with a region to be deposited.
[1389] When an EL element which emits white light is formed, the EL
element is formed by vertically stacking light-emitting layers of
different light emission colors. In this case also, the element
substrate can be transferred through the deposition treatment
chambers sequentially to form each light-emitting layer.
Alternatively, different light-emitting layers can be formed
continuously in the same deposition treatment chamber.
[1390] In the deposition treatment chamber 190276, an electrode is
formed over the EL layer. The electrode can be formed by an
electron beam evaporation method or a sputtering method, and
preferably by a resistance heating evaporation method.
[1391] The element substrate in which formation of the electrode is
finished is transferred to the sealing treatment chamber 190265
through the intermediate treatment chamber 190264. The sealing
treatment chamber 190265 is filled with an inert gas such as
helium, argon, neon, or nitrogen, and a sealing substrate is
attached to a side of the element substrate where the EL layer is
formed under the atmosphere to be sealed. In a sealed state, a
space between the element substrate and the sealing substrate may
be filled with the inert gas or a resin material. The sealing
treatment chamber 190265 is provided with a dispenser which
provides a sealing material, a mechanical element such as an arm or
a fixing stage which fixes the sealing substrate to face the
element substrate, a dispenser or a spin coater which fills the
chamber with a resin material, and the like.
[1392] FIG. 89 shows an internal structure of a deposition
treatment chamber. The deposition treatment chamber is maintained
in a reduced pressure state. In FIG. 89, a space interposed between
a top plate 190391 and a bottom plate 190392 corresponds to an
internal space of the chamber, which is maintained in a reduced
pressure state.
[1393] One or a plurality of evaporation sources are provided in
the treatment chamber. This is because a plurality of evaporation
sources are preferably provided when a plurality of layers having
different compositions are formed or when different materials are
co-evaporated. In FIG. 89, evaporation sources 190381a, 190381b,
and 190381c are attached to an evaporation source holder 190380.
The evaporation source holder 190380 is held by a multi-joint arm
190383. The multi-joint arm 190383 allows the evaporation source
holder 190380 to move within its movable range by stretching the
joint. Alternatively, the evaporation source holder 190380 may be
provided with a distance sensor 190382 to monitor a distance
between the evaporation sources 190381a to 190381c and a substrate
190389 so that an optimal distance for evaporation may be
controlled. In this case, the multijoint arm may be capable of
moving toward upper and lower directions (Z direction) as well.
[1394] The substrate 190389 is fixed by using a substrate stage
190386 and a substrate chuck 190387 together. The substrate stage
190386 may have a structure where a heater is incorporated so that
the substrate 190389 can be heated. The substrate 190389 is
transferred by tightening the substrate chuck 190387 while being
fixed to the substrate stage 190386. At the time of evaporation, a
shadow mask 190390 provided with an opening corresponding to a
deposition pattern can be used when needed. In this case, the
shadow mask 190390 is provided between the substrate 190389 and the
evaporation sources 190381a to 190381c. The shadow mask 190390 is
fixed to the substrate 190389 in close contact with each other or
with a certain interval therebetween by a mask chuck 190388. When
alignment of the shadow mask 190390 is needed, the alignment is
performed by arranging a camera in the treatment chamber and
providing the mask chuck 190388 with a positioning means which
slightly moves in X--Y-.theta. directions.
[1395] The evaporation sources 190381a to 190381c include an
evaporation material supply means which continuously supplies an
evaporation material to the evaporation source. The evaporation
material supply means includes evaporation material supply sources
190385a, 190385b, and 190385c, which are provided apart from the
evaporation sources 190381a, 190381b, and 190381c, and a material
supply pipe 190384 which connects between the evaporation source
and the evaporation material supply source. Typically, the material
supply sources 190385a to 190385c are provided corresponding to the
evaporation sources 190381a to 190381c. In FIG. 89, the material
supply source 190385a corresponds to the evaporation source
190381a, the material supply source 190385b corresponds to the
evaporation source 190381b, and the material supply source 190385c
corresponds to the evaporation source 190381c.
[1396] As a method for supplying an evaporation material, an
airflow transfer method, an aerosol method, or the like can be
employed. In an airflow transfer method, impalpable powder of an
evaporation material is transferred in airflow to the evaporation
sources 190381a to 190381c by using an inert gas or the like. In an
aerosol method, evaporation is performed while material liquid in
which an evaporation material is dissolved or dispersed in a
solvent is transferred and aerosolized by an atomizer, and the
solvent in the aerosol is vaporized. In each case, the evaporation
sources 190381a to 190381c are provided with a heating means, and a
film is formed over the substrate 190389 by vaporizing the
evaporation material transferred thereto. In FIG. 89, the material
supply pipe 190384 can be bent flexibly and is formed of a thin
pipe which has enough rigidity not to be transformed even under
reduced pressure.
[1397] When an airflow transfer method or an aerosol method is
employed, film formation may be performed under atmospheric
pressure or lower pressure in the deposition treatment chamber, and
preferably performed under a reduced pressure of 133 to 13300 Pa.
The pressure can be adjusted while an inert gas such as helium,
argon, neon, krypton, xenon, or nitrogen fills the deposition
treatment chamber or is supplied (and exhausted at the same time)
to the deposition treatment chamber. Note that an oxidizing
atmosphere may be employed by introducing a gas such as oxygen or
nitrous oxide in the deposition treatment chamber where an oxide
film is formed. Alternately, a reducing atmosphere may be employed
by introducing a gas such as hydrogen in the deposition treatment
chamber where an organic material is deposited.
[1398] As another method for supplying an evaporation material, a
screw may be provided in the material supply pipe 190384 to
continuously push the evaporation material toward the evaporation
source.
[1399] With this evaporation apparatus, a film can be formed
continuously with high uniformity even in the case of a large
display panel. Since it is not necessary to supply an evaporation
material to the evaporation source every time the evaporation
material is run out, throughput can be improved.
[1400] Although this embodiment mode is described with reference to
various drawings, the contents (or part of the contents) described
in each drawing can be freely applied to, combined with, or
replaced with the contents (or part of the contents) described in
another drawing. Further, much more drawings can be formed by
combining each part with another part in the above-described
drawings.
[1401] The contents (or part of the contents) described in each
drawing in this embodiment mode can be freely applied to, combined
with, or replaced with the contents (or part of the contents)
described in a drawing in another embodiment mode. Further, much
more drawings can be formed by combining each part in each drawing
in this embodiment mode with part of another embodiment mode.
[1402] This embodiment mode shows examples of embodying, slightly
transforming, partially modifying, improving, describing in detail,
or applying the contents (or part of the contents) described in
other embodiment modes, an example of related part thereof, or the
like. Therefore, the contents described in other embodiment modes
can be freely applied to, combined with, or replaced with this
embodiment mode.
Embodiment Mode 17
[1403] In this embodiment mode, examples of electronic devices
according to the invention are described.
[1404] FIG. 90 shows a display panel module combining a display
panel 900101 and a circuit board 900111. The display panel 900101
includes a pixel portion 900102, a scan line driver circuit 900103,
and a signal line driver circuit 900104. The circuit board 900111
is provided with a control circuit 900112, a signal dividing
circuit 900113, and the like, for example. The display panel 900101
and the circuit board 900111 are connected to each other by a
connection wiring 900114. An FPC or the like can be used as the
connection wiring.
[1405] In the display panel 900101, the pixel portion 900102 and
part of peripheral driver circuits (a driver circuit having a low
operation frequency among a plurality of driver circuits) may be
formed over the same substrate by using transistors, and another
part of the peripheral driver circuits (a driver circuit having a
high operation frequency among the plurality of driver circuits)
may be formed over an IC chip. Then, the IC chip may be mounted on
the display panel 900101 by COG (Chip On Glass) or the like. Thus,
the area of the circuit board 900111 can be reduced, and a small
display device can be obtained. Alternatively, the IC chip may be
mounted on the display panel 900101 by using TAB (Tape Automated
Bonding) or a printed wiring board. Thus, the area of the display
panel 900101 can be reduced, and a display device with a narrower
frame can be obtained.
[1406] For example, in order to reduce power consumption, a pixel
portion may be formed over a glass substrate by using transistors,
and all peripheral circuits may be formed over an IC chip. Then,
the IC chip may be mounted on a display device by COG or TAB.
[1407] By the display panel module shown in FIG. 90, a television
receiver can be completed. FIG. 91 is a block diagram showing a
main structure of a television receiver. A tuner 900201 receives a
video signal and an audio signal. The video signals are processed
by an video signal amplifier circuit 900202; a video signal
processing circuit 900203 which converts a signal output from the
video signal amplifier circuit 900202 into a color signal
corresponding to each color of red, green, and blue; and a control
circuit 900212 which converts the video signal into the input
specification of a driver circuit. The control circuit 900212
outputs a signal to each of a scan line driver circuit 900210 and a
signal line driver circuit 900204. The scan line driver circuit
900210 and the signal line driver circuit 900204 drive a display
panel 900211. When digital drive is performed, a structure may be
employed in which a signal dividing circuit 900213 is provided on
the signal line side and an input digital signal is divided into m
signals (m is a positive integer) to be supplied.
[1408] Among the signals received by the tuner 900201, an audio
signal is transmitted to an audio signal amplifier circuit 900205,
and an output thereof is supplied to a speaker 900207 through an
audio signal processing circuit 900206. A control circuit 900208
receives control information on receiving station (receiving
frequency) and volume from an input portion 900209 and transmits
signals to the tuner 900201 or the audio signal processing circuit
900206.
[1409] FIG. 92A shows a television receiver incorporated with a
display panel module which is different from FIG. 91. In FIG. 92A,
a display screen 900302 incorporated in a housing 900301 is formed
using the display panel module. Note that speakers 900303, an
operation switch 900304, and the like may be provided as
appropriate.
[1410] FIG. 92B shows a television receiver in which only a display
can be carried wirelessly. A battery and a signal receiver are
incorporated in a housing 900312. The battery drives a display
portion 900313 or a speaker portion 900317. The battery can be
repeatedly charged by a charger 900310. The charger 900310 can
transmit and receive a video signal and transmit the video signal
to the signal receiver of the display. The housing 900312 is
controlled by an operation key 900316. Alternatively, the device
shown in FIG. 92B can transmit a signal to the charger 900310 from
the housing 900312 by operating the operation key 900316. That is,
the device may be an image and audio interactive communication
device. Further alternatively, by operating the operation key
900316, a signal is transmitted to the charger 900310 from the
housing 900312, and another electronic device is made to receive a
signal which can be transmitted from the charger 900310; thus, the
device can control communication of another electronic device. That
is, the device may be a general-purpose remote control device. The
invention can be applied to the display portion 900313.
[1411] FIG. 93A shows a module combining a display panel 900401 and
a printed wiring board 900402. The display panel 900401 may include
a pixel portion 900403 provided with a plurality of pixels, a first
scan line driver circuit 900404, a second scan line driver circuit
900405, and a signal line driver circuit 900406 which supplies a
video signal to a selected pixel.
[1412] The printed wiring board 900402 is provided with a
controller 900407, a central processing unit (CPU) 900408, a memory
900409, a power supply circuit 9004010, an audio processing circuit
900411, a transmitting/receiving circuit 900412, and the like. The
printed wiring board 900402 and the display panel 900401 are
connected by a flexible printed circuit (FPC) 900413. The flexible
printed circuit (FPC) 900413 may have a structure where a
capacitor, a buffer circuit, or the like is provided to prevent
noise on power supply voltage or a signal, and increase in rise
time of a signal. Note that the controller 900407, the audio
processing circuit 900411, the memory 900409, the central
processing unit (CPU) 900408, the power supply circuit 900410, or
the like can be mounted to the display panel 900401 by using a COG
(Chip On Glass) method. By using a COG method, the size of the
printed wiring board 900402 can be reduced.
[1413] Various control signals are input and output through an
interface (I/F) portion 900414 provided in the printed wiring board
900402. An antenna port 900415 for transmitting and receiving a
signal to/from an antenna is provided in the printed wiring board
900402.
[1414] FIG. 93B is a block diagram of the module shown in FIG. 93A.
The module includes a VRAM 900416, a DRAM 900417, a flash memory
900418, and the like as the memory 900409. The VRAM 900416 stores
data on an image displayed on a panel, the DRAM 900417 stores video
data or audio data, and the flash memory 900418 stores various
programs.
[1415] The power supply circuit 900410 supplies electric power for
operating the display panel 900401, the controller 900407, the
central processing unit (CPU) 900408, the audio processing circuit
900411, the memory 900409, and the transmitting/receiving circuit
900412. Note that the power supply circuit 900410 may be provided
with a current source depending on a panel specification.
[1416] The central processing unit (CPU) 900408 includes a control
signal generation circuit 900420, a decoder 900421, a register
900422, an arithmetic circuit 900423, a RAM 900424, an interface
(I/F) portion 900419 for the central processing unit (CPU) 900408,
and the like. Various signals input to the central processing unit
(CPU) 900408 via an interface (VIF) portion 900414 are once stored
in the register 900422, and subsequently input to the arithmetic
circuit 900423, the decoder 900421, or the like. The arithmetic
circuit 900423 performs operation based on the signal input thereto
so as to designate a location to which various instructions are
sent. On the other hand, the signal input to the decoder 900421 is
decoded and input to the control signal generation circuit 900420.
The control signal generation circuit 900420 generates a signal
including various instructions based on the signal input thereto,
and transmits the signal to the location designated by the
arithmetic circuit 900423, specifically the memory 900409, the
transmitting/receiving circuit 900412, the audio processing circuit
900411, and the controller 900407, for example.
[1417] The memory 900409, the transmitting/receiving circuit
900412, the audio processing circuit 900411, and the controller
900407 operate in accordance with respective instructions.
Hereinafter, the operation is briefly described.
[1418] A signal input from an input means 900425 is transmitted to
the central processing unit (CPU) 900408 mounted to the printed
wiring board via the interface (I/F) portion 900414. The control
signal generation circuit 900420 converts image data stored in the
VRAM 900416 into a predetermined format depending on the signal
transmitted from the input means 900425 such as a pointing device
or a keyboard, and transmits the converted data to the controller
900407.
[1419] The controller 900407 performs data processing of the signal
including the image data transmitted from the central processing
unit (CPU) 900408 in accordance with the panel specification, and
supplies the signal to the display panel 900401. The controller
900407 generates an Hsync signal, a Vsync signal, a clock signal
CLK, alternating voltage (AC Cont), and a switching signal hR based
on power supply voltage input from the power supply circuit 900410
or various signals input from the central processing unit (CPU)
900408, and supplies the signals to the display panel 900401.
[1420] The transmitting/receiving circuit 900412 processes a signal
which is to be transmitted and received as an electric wave by an
antenna 900428. Specifically, the transmitting/receiving circuit
900412 may include a high-frequency circuit such as an isolator, a
band pass filter, a VCO (voltage controlled oscillator), an LPF
(low pass filter), a coupler, or a balun. A signal including audio
information among signals transmitted and received by the
transmitting/receiving circuit 900412 is transmitted to the audio
processing circuit 900411 in accordance with an instruction from
the central processing unit (CPU) 900408.
[1421] The signal including the audio information which is
transmitted in accordance with the instruction from the central
processing unit (CPU) 900408 is demodulated into an audio signal by
the audio processing circuit 900411 and transmitted to a speaker
900427. An audio signal transmitted from a microphone 900426 is
modulated by the audio processing circuit 900411 and transmitted to
the transmitting/receiving circuit 900412 in accordance with an
instruction from the central processing unit (CPU) 900408.
[1422] The controller 900407, the central processing unit (CPU)
900408, the power supply circuit 900410, the audio processing
circuit 900411, and the memory 900409 can be mounted as a package
of this embodiment mode.
[1423] It is needless to say that this embodiment mode is not
limited to a television receiver and can be applied to various
uses, such as a monitor of a personal computer, and especially as a
large display medium such as an information display board at the
train station, the airport, or the like, or an advertisement
display board on the street.
[1424] Next, a structure example of a mobile phone according to the
invention is described with reference to FIG. 94.
[1425] A display panel 900501 is detachably incorporated in a
housing 900530. The shape or the size of the housing 900530 can be
changed as appropriate in accordance with the size of the display
panel 900501. The housing 900530 which fixes the display panel
900501 is fitted in a printed wiring board 900531 to be assembled
as a module.
[1426] The display panel 900501 is connected to the printed wiring
board 900531 through an FPC 900513. The printed wiring board 900531
is provided with a speaker 900532, a microphone 900533, a
transmitting/receiving circuit 900534, and a signal processing
circuit 900535 including a CPU, a controller, and the like. Such a
module, an input means 900536, and a battery 900537 are combined
and stored in a housing 900539. A pixel portion of the display
panel 900501 is provided to be seen from an opening window formed
in the housing 900539.
[1427] In the display panel 900501, the pixel portion and part of
peripheral driver circuits (a driver circuit having a low operation
frequency among a plurality of driver circuits) may be formed over
the same substrate by using transistors, and another part of the
peripheral driver circuits (a driver circuit having a high
operation frequency among the plurality of driver circuits) may be
formed over an IC chip. Then, the IC chip may be mounted on the
display panel 900501 by COG (Chip On Glass). Alternatively, the IC
chip may be connected to a glass substrate by using TAB (Tape
Automated Bonding) or a printed wiring board. With such a
structure, power consumption of a display device can be reduced,
and operation time of the mobile phone per charge can be extended.
Further, reduction in cost of the mobile phone can be realized.
[1428] In a mobile phone shown in FIG. 95, a main body (A) 900601
provided with operation switches 900604, a microphone 900605, and
the like is connected to a main body (B) 900602 provided with a
display panel (A) 900608, a display panel (B) 900609, a speaker
900606, and the like by using a hinge 900610 so that the mobile
phone can be opened and closed. The display panel (A) 900608 and
the display panel (B) 900609 are placed in a housing 900603 of the
main body (B) 900602 together with a circuit board 900607. Pixel
portions of the display panel (A) 900608 and the display panel (B)
900609 are arranged to be seen from an opening window formed in the
housing 900603.
[1429] Specifications of the display panel (A) 900608 and the
display panel (B) 900609, such as the number of pixels, can be set
as appropriate in accordance with functions of a mobile phone
900600. For example, the display panel (A) 900608 used as a main
screen and the display panel (B) 900609 used as a sub-screen can be
combined.
[1430] A mobile phone according to this embodiment mode can be
changed in various modes depending on functions or applications
thereof. For example, it may be a camera-equipped mobile phone by
incorporating an imaging element in the hinge 900610. When the
operation switches 900604, the display panel (A) 900608, and the
display panel (B) 900609 are placed in one housing, the
aforementioned effects can be obtained. Further, a similar effect
can be obtained when the structure of this embodiment mode is
applied to an information display terminal equipped with a
plurality of display portions.
[1431] The invention can be applied to various electronic devices.
Specifically, the invention can be applied to a display portion of
an electronic device. Examples of such electronic devices include
cameras such as a video camera and a digital camera, a goggle-type
display, a navigation system, an audio reproducing device (such as
car audio components and audio components), a computer, a game
machine, a portable information terminal (such as a mobile
computer, a mobile phone, a mobile game machine, and an electronic
book), and an image reproducing device provided with a recording
medium (specifically, a device which reproduces a recording medium
such as a digital versatile disc (DVD) and has a display for
displaying the reproduced image).
[1432] FIG. 96A shows a display, which includes a housing 900711, a
support base 900712, a display portion 900713, and the like.
[1433] FIG. 96B shows a camera, which includes a main body 900721,
a display portion 900722, an image receiving portion 900723,
operation keys 900724, an external connection port 900725, a
shutter button 900726, and the like.
[1434] FIG. 96C shows a computer, which includes a main body
900731, a housing 900732, a display portion 900733, a keyboard
900734, an external connection port 900735, a pointing device
900736, and the like.
[1435] FIG. 96D shows a mobile computer, which includes a main body
900741, a display portion 900742, a switch 900743, operation keys
900744, an infrared port 900745, and the like.
[1436] FIG. 96E shows a portable image reproducing device having a
recording medium (e.g., a DVD reproducing device), which includes a
main body 900751, a housing 900752, a display portion A 900753, a
display portion B 900754, a recording medium (e.g., DVD) reading
portion 900755, operation keys 900756, a speaker portion 900757,
and the like. The display portion A 900753 can mainly display image
information, and the display portion B 900754 can mainly display
text information.
[1437] FIG. 96F shows a goggle-type display, which includes a main
body 900761, a display portion 900762, an earphone 900763, a
support portion 900764, and the like.
[1438] FIG. 96G shows a portable game machine, which includes a
housing 900771, a display portion 900772, a speaker portion 900773,
operation keys 900774, a recording medium insert portion 900775,
and the like. The portable game machine in which the display device
in the invention is used for the display portion 900772 can express
bright colors.
[1439] FIG. 96H shows a digital camera having a television
reception function, which includes a housing 900781, a display
portion 900782, operation keys 900783, a speaker 900784, a shutter
button 900785, an image receiving portion 900786, an antenna
900787, and the like.
[1440] As shown in FIGS. 96A to 96H, the electronic device
according to the invention includes a display portion for
displaying some kind of information. The electronic device
according to the invention has low power consumption, and can drive
with a battery for a long time. Further, a moving image without
motion blur can be displayed. Moreover, a manufacturing method is
simple, and manufacturing cost can be reduced.
[1441] Next, application examples of a semiconductor device
according to the invention are described.
[1442] FIG. 97 shows an example where the semiconductor device
according to the invention is incorporated in a constructed object.
FIG. 97 shows a housing 900810, a display portion 900811, a remote
control device 900812 which is an operation portion, a speaker
portion 900813, and the like. The semiconductor device according to
the invention is incorporated in the constructed object as a
wall-hanging type and can be provided without requiring a large
space.
[1443] FIG. 98 shows another example where the semiconductor device
according to the invention is incorporated in a constructed object.
A display panel 900901 is incorporated with a prefabricated bath
900902, and a person who takes a bath can view the display panel
900901. The display panel 900901 has a function of displaying
information by an operation by the person who takes a bath; and a
function of being used as an advertisement or an entertainment
means.
[1444] Note that the semiconductor device according to the
invention can be provided not only to a side wall of the
prefabricated bath 900902 as shown in FIG. 98, but also to various
places. For example, the semiconductor device can be incorporated
with part of a mirror, a bathtub itself, or the like. At this time,
a shape of the display panel 900901 may be changed in accordance
with a shape of the mirror or the bathtub.
[1445] FIG. 99 shows another example where the semiconductor device
according to the invention is incorporated in a constructed object.
A display panel 901002 is bent and attached to a curved surface of
a column-shaped object 901001. Note that here, a utility pole is
described as the column-shaped object 901001.
[1446] The display panel 901002 shown in FIG. 99 is provided at a
position higher than a human viewpoint. When the same images are
displayed on the display panels 901002 provided in constructed
objects which stand together in large numbers outdoors, such as
utility poles, advertisement can be performed to an unspecified
number of viewers. Since it is easy for the display panel 901002 to
display the same images and instantly switch images by external
control, highly efficient information display and advertisement
effect can be realized. When provided with self-luminous display
elements, the display panel 901002 can be useful as a highly
visible display medium even at night. When the display panel 901002
is provided in the utility pole, a power supply means for the
display panel 901002 can be easily obtained. In an emergency such
as disaster, the display panel 901002 can also be used as a means
to transmit correct information to victims rapidly.
[1447] Note that an example of the display panel 901002 includes a
display panel in which a switching element such as an organic
transistor is provided over a film-shaped substrate and a display
element is driven so as to display an image.
[1448] Note that in this embodiment mode, a wall, a column-shaped
object, and a prefabricated bath are shown as examples of
constructed objects; however, this embodiment mode is not limited
thereto, and various constructed objects can be provided with the
semiconductor device according to the invention.
[1449] Next, examples where the semiconductor device according to
the invention is incorporated with a moving object are
described.
[1450] FIG. 100 shows an example where the semiconductor device
according to the invention is incorporated with a car. A display
panel 901101 is incorporated with a car body 901102, and can
display an operation of the car body or information input from
inside or outside the car body on demand. Note that a navigation
function may be provided.
[1451] The semiconductor device according to the invention can be
provided not only to the car body 901102 as shown in FIG. 100, but
also to various places. For example, the semiconductor device can
be incorporated with a glass window, a door, a steering wheel, a
gear shift, a seat, a rear-view mirror, and the like. At this time,
a shape of the display panel 901101 may be changed in accordance
with a shape of an object provided with the semiconductor
device.
[1452] FIGS. 101A and 101B show examples where the semiconductor
device according to the invention is incorporated with a train
car.
[1453] FIG. 101A shows an example where a display panel 901202 is
provided in glass of a door 901201 in a train car, which has an
advantage compared with a conventional advertisement using paper in
that labor cost for changing an advertisement is not necessary.
Since the display panel 901202 can instantly switch images
displaying on a display portion by an external signal, images on
the display panel can be switched in every time period when types
of passengers on the train are changed, for example. Thus, more
effective advertisement effect can be realized.
[1454] FIG. 101B shows an example where the display panels 901202
are provided to a glass window 901203 and a ceiling 901204 as well
as the glass of the door 901201 in the train car. In this manner,
the semiconductor device according to the invention can be easily
provided to a place where a semiconductor device has been difficult
to be provided conventionally; thus, effective advertisement effect
can be obtained. Further, the semiconductor device according to the
invention can instantly switch images displayed on a display
portion by an external signal; thus, cost and time for changing an
advertisement can be reduced, and more flexible advertisement
management and information transmission can be realized.
[1455] Note that the semiconductor device according to the
invention can be provided not only to the door 901201, the glass
window 901203, and the ceiling 901204 as shown in FIGS. 101A and
101B, but also to various places. For example, the semiconductor
device can be incorporated with a strap, a seat, a handrail, a
floor, and the like. At this time, a shape of the display panel
901202 may be changed in accordance with a shape of an object
provided with the semiconductor device.
[1456] FIGS. 102A and 102B show an example where the semiconductor
device according to the invention is incorporated with a passenger
airplane.
[1457] FIG. 102A shows a shape of a display panel 901302 attached
to a ceiling 901301 above a seat of the passenger airplane when the
display panel 901302 is used. The display panel 901302 is
incorporated with the ceiling 901301 with a hinge portion 901303,
and a passenger can view the display panel 901302 by stretching of
the hinge portion 901303. The display panel 901302 has a function
of displaying information by an operation by the passenger and a
function of being used as an advertisement or an entertainment
means. As shown in FIG. 102B, the hinge portion is bent and the
display panel is stored in the ceiling 901301, so that safety in
taking-off and landing can be assured. Note that in an emergency,
the display panel can also be used as an information transmission
means and an evacuation light by lighting a display element in the
display panel.
[1458] The semiconductor device according to the invention can be
provided not only to the ceiling 901301 as shown in FIGS. 102A and
102B, but also to various places. For example, the semiconductor
device can be incorporated with a seat, a table attached to a seat,
an armrest, a window, and the like. A large display panel which a
plurality of people can view may be provided at a wall of an
airframe. At this time, a shape of the display panel 901302 may be
changed in accordance with a shape of an object provided with the
semiconductor device.
[1459] Note that in this embodiment mode, bodies of a train car, a
car, and an airplane are shown as moving objects; however, the
invention is not limited thereto, and the semiconductor device
according to the invention can be provided to various objects such
as a motorcycle, an four-wheel drive car (including a car, a bus,
and the like), a train (including a monorail, a railroad car, and
the like), and a vessel. Since the semiconductor device according
to the invention can instantly switch images displayed on a display
panel in a moving object by an external signal, the moving object
provided with the semiconductor device according to the invention
can be used as an advertisement display board for an unspecified
number of customers, an information display board in disaster, and
the like.
[1460] Although this embodiment mode is described with reference to
various drawings, the contents (or part of the contents) described
in each drawing can be freely applied to, combined with, or
replaced with the contents (or part of the contents) described in
another drawing. Further, much more drawings can be formed by
combining each part with another part in the above-described
drawings.
[1461] The contents (or part of the contents) described in each
drawing in this embodiment mode can be freely applied to, combined
with, or replaced with the contents (or part of the contents)
described in a drawing in another embodiment mode. Further, much
more drawings can be formed by combining each part in each drawing
in this embodiment mode with part of another embodiment mode.
[1462] This embodiment mode shows examples of embodying, slightly
transforming, partially modifying, improving, describing in detail,
or applying the contents (or part of the contents) described in
other embodiment modes, an example of related part thereof, or the
like. Therefore, the contents described in other embodiment modes
can be freely applied to, combined with, or replaced with this
embodiment mode.
[1463] This application is based on Japanese Patent Application
serial No. 2006-328670 filed with Japan Patent Office on Dec. 5,
2006, the entire contents of which are hereby incorporated by
reference.
* * * * *