U.S. patent application number 12/190366 was filed with the patent office on 2009-02-19 for display device and electronic equipment.
This patent application is currently assigned to SONY CORPORATION. Invention is credited to Yukihito Iida, Tetsuo Minami, Takao Tanikame, Katsuhide Uchino.
Application Number | 20090046040 12/190366 |
Document ID | / |
Family ID | 40362581 |
Filed Date | 2009-02-19 |
United States Patent
Application |
20090046040 |
Kind Code |
A1 |
Iida; Yukihito ; et
al. |
February 19, 2009 |
DISPLAY DEVICE AND ELECTRONIC EQUIPMENT
Abstract
Disclosed herein is a display device including a pixel array
section; power supply lines; and auxiliary electrodes, wherein the
pixels each have an auxiliary capacitance, and one of electrodes of
the auxiliary capacitance is connected to the source electrode of
the drive transistor, and an other electrode connected to the
auxiliary electrode for each pixel.
Inventors: |
Iida; Yukihito; (Kanagawa,
JP) ; Minami; Tetsuo; (Tokyo, JP) ; Tanikame;
Takao; (Kanagawa, JP) ; Uchino; Katsuhide;
(Kanagawa, JP) |
Correspondence
Address: |
SONNENSCHEIN NATH & ROSENTHAL LLP
P.O. BOX 061080, WACKER DRIVE STATION, SEARS TOWER
CHICAGO
IL
60606-1080
US
|
Assignee: |
SONY CORPORATION
Tokyo
JP
|
Family ID: |
40362581 |
Appl. No.: |
12/190366 |
Filed: |
August 12, 2008 |
Current U.S.
Class: |
345/76 |
Current CPC
Class: |
G09G 2300/0842 20130101;
G09G 2320/0257 20130101; G09G 3/3225 20130101; G09G 2300/0866
20130101; G09G 3/3258 20130101; G09G 2320/045 20130101; G09G 3/3266
20130101; G09G 2320/0233 20130101; G09G 3/3233 20130101; G09G
2320/0252 20130101; G09G 2300/0819 20130101 |
Class at
Publication: |
345/76 |
International
Class: |
G09G 3/30 20060101
G09G003/30 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 15, 2007 |
JP |
2007-211623 |
Claims
1. A display device comprising: a pixel array section, the pixel
array section having pixels arranged in a matrix form, each of the
pixels including an electro-optical element, a write transistor
adapted to write a video signal, a holding capacitance adapted to
hold the video signal written by the write transistor, and a drive
transistor adapted to drive the electro-optical element based on
the video signal held by the holding capacitance; power supply
lines disposed one for each of the pixel rows of the pixel array
section and in the proximity of the scan line which belongs to the
adjacent pixel row, the power supply lines adapted to selectively
apply a first potential and a second potential lower than the first
potential to the drain electrode of the drive transistor; and
auxiliary electrodes disposed in rows, in columns or in a grid form
for the pixels of the pixel array section arranged in a matrix
form, the auxiliary electrodes being applied with a fixed
potential, wherein the pixels each have an auxiliary capacitance,
and one of electrodes of the auxiliary capacitance is connected to
the source electrode of the drive transistor, and an other
electrode connected to the auxiliary electrode for each pixel.
2. The display device of claim 1, wherein one of the electrodes of
the auxiliary capacitance is formed with a semiconductor layer
which forms source and drain regions of the drive transistor, and
the other electrode of the auxiliary capacitance is formed with a
metallic material so as to be opposed to the semiconductor
layer.
3. The display device of claim 2, wherein the other electrode is
formed in a same wiring layer as for the power supply lines, and
the other electrode is opposed to the one of the electrodes via an
interlayer insulating film which mediates between the wiring layer
and semiconductor layer.
4. The display device of claim 2, wherein the other electrode is
formed in a same wiring layer as for the gate electrode of the
drive transistor, and the other electrode is opposed to the one of
the electrodes via a gate insulating film which mediates between
the wiring layer and gate electrode.
5. The display device of claim 2, wherein the other electrode
includes first and second electrodes electrically connected to each
other, the first electrode is formed in a same wiring layer as for
the gate electrode of the drive transistor so that the first
electrode is opposed to the one of the electrodes via a gate
insulating film which mediates between the wiring layer and gate
electrode, and the second electrode is formed in a same wiring
layer as for the power supply lines so that the second electrode is
opposed to the one of the electrodes via an interlayer insulating
film which mediates between the wiring layer and semiconductor
layer.
6. Electronic equipment having a display device, the display device
comprising: a pixel array section, the pixel array section having
pixels arranged in a matrix form, each of the pixels including an
electro-optical element, a write transistor adapted to write a
video signal, a holding capacitance adapted to hold the video
signal written by the write transistor, and a drive transistor
adapted to drive the electro-optical element based on the video
signal held by the holding capacitance; power supply lines disposed
one for each of the pixel rows of the pixel array section and in
the proximity of the scan line which belongs to the adjacent pixel
row, the power supply lines adapted to selectively apply a first
potential and a second potential lower than the first potential to
the drain electrode of the drive transistor; and auxiliary
electrodes disposed in rows, in columns or in a grid form for the
pixels of the pixel array section arranged in a matrix form, the
auxiliary electrodes being applied with a fixed potential, wherein
the pixels each have an auxiliary capacitance, and one of
electrodes of the auxiliary capacitance is connected to the source
electrode of the drive transistor, and an other electrode connected
to the auxiliary electrode for each pixel.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] The present invention contains subject matter related to
Japanese Patent Application JP 2007-211623 filed in the Japan
Patent Office on Aug. 15, 2007, the entire contents of which being
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a display device and
electronic equipment, and more particularly to a flat panel display
device and electronic equipment having the same in which pixels,
each incorporating an electro-optical element, are disposed in a
matrix form.
[0004] 2. Description of the Related Art
[0005] In the field of image display device, flat panel display
devices having pixels (pixel circuits), each incorporating an
electro-optical element, disposed in a matrix form, are rapidly
becoming widespread. Among flat panel display devices, the
development and commercialization of organic EL display devices
using organic EL (Electro Luminescence) elements have been
continuing at a steady pace. An organic EL element is a type of
current-driven electro-optical element whose light emission
brightness changes according to the current flowing through the
element. This type of element relies on the phenomenon that an
organic thin film emits light when applied with an electric
field.
[0006] An organic EL display device has the following features.
That is, it is low in power consumption because organic EL elements
can be driven by a voltage of 10V or less. Besides, organic EL
elements are self-luminous. Therefore, an organic EL display device
offers higher image visibility as compared to a liquid crystal
display device designed to display an image by controlling the
light intensity from the light source (backlight) for each of the
pixels containing liquid crystal cells. Further, an organic EL
display device desires no lighting members such as backlight as
desired for a liquid crystal display device, thus making it easier
to reduce weight and thickness. Still further, organic EL elements
are extremely fast in response speed or several .mu. seconds or so.
This provides a moving image free from afterimage.
[0007] An organic EL display device can be either simple
(passive)-matrix or active-matrix driven as with a liquid crystal
display device. It should be noted, however, that a simple matrix
display device has some problems although simple in construction.
Such problems include difficulty in implementing a large
high-definition display device because the light emission period of
the electro-optical elements diminishes with increase in the number
of scan lines (i.e., number of pixels).
[0008] For this reason, the development of active matrix display
devices has been going on at a brisk pace in recent years. Such
display devices control the current flowing through the
electro-optical element with an active element such as insulating
gate field effect transistor (typically, thin film transistor or
TFT) provided in the same pixel circuit as the electro-optical
element. In an active matrix display device, the electro-optical
elements maintain light emission over a frame interval. As a
result, a large high-definition display device can be implemented
with ease.
[0009] Incidentally, the I-V characteristic (current-voltage
characteristic) of the organic EL element is typically known to
deteriorate over time (so-called deterioration over time). In a
pixel circuit using an N-channel TFT as a transistor adapted to
current-drive the organic EL element (hereinafter written as "drive
transistor"), the organic EL element is connected to the source of
the drive transistor. Therefore, if the I-V characteristic of the
organic EL element deteriorates over time, a gate-to-source voltage
Vgs of the drive transistor changes, thus changing the light
emission brightness of the same element.
[0010] This will be described more specifically below. The source
potential of the drive transistor is determined by the operating
point between the drive transistor and organic EL element. If the
I-V characteristic of the organic EL element deteriorates, the
operating point between the drive transistor and organic EL element
will change. As a result, the same voltage applied to the gate of
the drive transistor changes the source potential of the drive
transistor. This changes the gate-to-source voltage Vgs of the
drive transistor, thus changing the current level flowing through
the drive transistor. Therefore, the current level flowing through
the organic EL element also changes. As a result, the light
emission brightness of the organic EL element changes.
[0011] In a pixel circuit using a polysilicon TFT, on the other
hand, a threshold voltage Vth of the drive transistor or a mobility
.mu. of a semiconductor thin film making up the channel of the
drive transistor (hereinafter written as "mobility of the drive
transistor") changes over time or is different from one pixel to
another due to the manufacturing process variation (the transistors
have different characteristics), in addition to the deterioration
of the I-V characteristic over time.
[0012] If the threshold voltage Vth or mobility .mu. of the drive
transistor is different from one pixel to another, the current
level flowing through the drive transistor varies from one pixel to
another. Therefore, the same voltage applied to the gates of the
drive transistors leads to a difference in light emission
brightness of the organic EL element between the pixels, thus
impairing the screen uniformity.
[0013] Therefore, the compensation and correction functions are
provided in each of the pixels to ensure immunity to deterioration
of the I-V characteristic of the organic EL element over time and
variation in the threshold voltage Vth or mobility .mu. of the
drive transistor over time, thus maintaining the light emission
brightness of the organic EL element constant (refer, for example,
to Japanese Patent Laid-Open No. 2006-133542 (hereinafter referred
to as Patent Document 1)). The compensation function compensates
for the variation in characteristic of the organic EL element. One
of the correction functions corrects the variation in the threshold
voltage Vth of the drive transistor (hereinafter written as
"threshold correction"). Another correction function corrects the
variation in the mobility .mu. of the drive transistor (hereinafter
written as "mobility correction").
SUMMARY OF THE INVENTION
[0014] In the related art described in Patent Document 1, the
compensation function adapted to compensate for the variation in
the characteristic of the organic EL element and the correction
functions adapted to correct the variation in the threshold voltage
Vth and mobility .mu. are provided in each of the pixels. This
ensures immunity to deterioration of the I-V characteristic of the
organic EL element over time and variation in the threshold voltage
Vth or mobility .mu. of the drive transistor over time, thus
maintaining the light emission brightness of the organic EL element
constant. However, the related art desires a number of elements to
make up each pixel, thus causing an impediment to reducing the
pixel size and, by extension, providing a higher-definition display
device.
[0015] On the other hand, a write gain for writing a video signal
to the pixel is determined by factors such as the capacitance value
of a holding capacitance adapted to hold the written video signal
and the capacitive component of the organic EL element (the details
thereof will be described later). As display devices grow in
definition, the pixel size becomes finer. As a result, the
electrodes making up the organic EL element become smaller.
Accordingly, the capacitance value of the capacitive component of
the organic EL element is smaller, thus resulting in a lower video
signal write gain. If the write gain declines, a signal potential
appropriate to the video signal may not be held in the holding
capacitance. As a result, the light emission brightness appropriate
to the video signal level may not be achieved.
[0016] In light of the foregoing, it is a purpose of the embodiment
of the present invention to provide a display device and electronic
equipment having the same, each of whose pixels is made up of fewer
components and which can secure a sufficient video signal write
gain.
[0017] In order to achieve the above desire, the display device
according to the embodiment of the present invention is defined in
that it includes a pixel array section, power supply lines and
auxiliary electrodes. The pixel array section includes pixels
arranged in a matrix form. Each of the pixels includes an
electro-optical element and write transistor adapted to write a
video signal and holding capacitance adapted to hold the video
signal written by the write transistor. Each of the pixels further
includes a drive transistor adapted to drive the electro-optical
element based on the video signal held by the holding capacitance.
The power supply lines are disposed one for each of the pixel rows
of the pixel array section and in the proximity of the scan line
which belongs to the adjacent pixel row. The power supply lines
selectively apply a first potential and a second potential lower
than the first potential to the drain electrode of the drive
transistor. The auxiliary electrodes are disposed in rows, in
columns or in a grid form for the pixels of the pixel array section
arranged in a matrix form. The auxiliary electrodes are applied
with a fixed potential. The pixels each have an auxiliary
capacitance. One of the electrodes of the auxiliary capacitance is
connected to the source electrode of the drive transistor. The
other electrode thereof is connected to the auxiliary electrode for
each pixel.
[0018] In the display device configured as described above and
electronic equipment having the same, the first and second
potentials are selectively applied to the drain electrode of the
drive transistor via the power supply line. The drive transistor
supplied with a current from the power supply line drives the
electro-optical element to emit light when supplied with the first
potential. The same transistor does not drive the electro-optical
element to emit light when supplied with the second potential. As a
result, the drive transistor has the capabilities to control the
light emission and non-light emission of the same element as well
as current-drive the electro-optical element. This eliminates the
need for a transistor adapted specifically to control the light
emission and non-light emission.
[0019] Further, the auxiliary capacitance, one of whose ends is
connected to the source electrode of the drive transistor, makes it
possible to increase the video signal write gain by the capacitance
value of the auxiliary capacitance because the gain is determined
by the capacitance values of the capacitive component of the
electro-optical element and the holding and auxiliary capacitances.
Here, the auxiliary electrodes, which are disposed in rows, in
columns or in a grid form for the pixels of the pixel array section
arranged in a matrix form and which are applied with a fixed
potential, are each connected to one of the electrodes of the
auxiliary capacitance for each pixel. This makes it possible to
apply a fixed potential to the other electrode of the auxiliary
capacitance without providing any cathode wiring in a TFT layer,
thus allowing to form the auxiliary capacitance for the fixed
potential.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a system configuration diagram illustrating the
schematic configuration of an active matrix organic EL display
device which is a prerequisite for the embodiment of the present
invention;
[0021] FIG. 2 is a circuit diagram illustrating a specific example
of the configuration of a pixel (pixel circuit);
[0022] FIG. 3 is a timing waveform diagram used for the description
of the operation of the active matrix organic EL display device
which is a prerequisite for the embodiment of the present
invention;
[0023] FIGS. 4A to 4D are explanatory diagrams (1) illustrating the
circuit operation of the active matrix organic EL display device
which is a prerequisite for the embodiment of the present
invention;
[0024] FIGS. 5A to 5D are explanatory diagrams (2) illustrating the
circuit operation of the active matrix organic EL display device
which is a prerequisite for the embodiment of the present
invention;
[0025] FIGS. 6A to 6C are explanatory diagrams (3) illustrating the
circuit operation of the active matrix organic EL display device
which is a prerequisite for the embodiment of the present
invention;
[0026] FIG. 7 is a characteristic diagram used for the description
of the problem caused by the variation of a threshold voltage Vth
of a drive transistor;
[0027] FIG. 8 is a characteristic diagram used for the description
of the problem caused by the variation of a mobility .mu. of a
drive transistor;
[0028] FIGS. 9A to 9C are characteristic diagrams used for the
description of the relationship between a video signal voltage Vsig
and a drain-to-source current Ids of the drive transistor with and
without the threshold and mobility corrections;
[0029] FIG. 10 is a circuit diagram illustrating the pixel
configuration having an auxiliary capacitance;
[0030] FIG. 11 is an equivalent circuit diagram illustrating a
wiring resistance R resulting from a cathode wiring run in a TFT
layer;
[0031] FIG. 12 is a timing waveform diagram illustrating the
variation of a cathode potential caused by the wiring resistance
R;
[0032] FIG. 13 is a view illustrating horizontal crosstalk caused
by the wiring resistance R;
[0033] FIG. 14 is a plan view illustrating a layout example of
auxiliary electrodes for the pixel arrangement in a matrix
form;
[0034] FIG. 15 is a plan view schematically illustrating a pixel
layout structure having the auxiliary capacitance;
[0035] FIG. 16 is a sectional view illustrating the sectional
structure of the pixel according to example 1;
[0036] FIG. 17 is a sectional view illustrating the sectional
structure of the pixel according to example 2;
[0037] FIG. 18 is a sectional view illustrating the sectional
structure of the pixel according to example 3;
[0038] FIG. 19 is a perspective view illustrating the appearance of
a television set to which the embodiment of the present invention
is applied;
[0039] FIGS. 20A and 20B are perspective views illustrating the
appearance of a digital camera to which the embodiment of the
present invention is applied, and FIG. 20A is a perspective view as
seen from the front, and FIG. 20B is a perspective view as seen
from the rear;
[0040] FIG. 21 is a perspective view illustrating the appearance of
a laptop personal computer to which the embodiment of the present
invention is applied;
[0041] FIG. 22 is a perspective view illustrating the appearance of
a video camcorder to which the embodiment of the present invention
is applied; and
[0042] FIGS. 23A to 23G are external views illustrating a mobile
phone to which the embodiment of the present invention is applied,
and FIG. 23A is a front view of the mobile phone in an open
position, FIG. 23B is a side view thereof, FIG. 23C is a front view
thereof in a closed position, FIG. 23D is a left side view thereof,
FIG. 23E is a right side view thereof, FIG. 23F is a top view
thereof, and FIG. 23G is a bottom view thereof.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] The embodiments of the present invention provide the drive
transistor with the capabilities to control the light emission and
non-light emission of the same element as well as current-drive the
electro-optical element. This makes it possible to make up each
pixel with fewer components, i.e., merely the write and drive
transistors. At the same time, a sufficient video signal write gain
can be secured by providing the auxiliary capacitance in addition
to the holding capacitance.
[0044] Further, the other electrode of the auxiliary capacitance is
connected, for each pixel, to one of the auxiliary electrodes which
are disposed in rows, in columns or in a grid form for the pixels
of the pixel array section arranged in a matrix form. This makes it
possible to apply a fixed potential to the other electrode without
providing any cathode wiring in the TFT layer. As a result, the
auxiliary capacitance can be formed for the fixed potential while
at the same time suppressing the wiring resistance. This suppresses
horizontal crosstalk caused by the wiring resistance, thus
providing improved on-screen image quality.
[0045] A detailed description will be given below of the preferred
embodiment of the present invention with reference to the
accompanying drawings.
[Display Device as a Prerequisite for the Present Invention]
[0046] FIG. 1 is a system configuration diagram illustrating the
schematic configuration of an active matrix display device which is
a prerequisite for the embodiment of the present invention.
[0047] Here, a description will be given taking, as an example, an
active matrix organic EL display device. The organic EL display
device uses, as a light emitting element of each of the pixels
(pixel circuits), an organic EL element (organic electroluminescent
element) which is a current-driven electro-optical element whose
light emission brightness changes according to the current flowing
through the element.
[0048] As illustrated in FIG. 1, an organic EL display device 10
includes a pixel array section 30 and driving sections. The pixel
array section 30 has pixels (PXLCs) 20 arranged two-dimensionally
in a matrix form. The driving sections are disposed around the
pixel array section 30 and adapted to drive the pixels 20. Among
the driving sections adapted to drive the pixels 20 are a write
scan circuit 40, power supply scan circuit 50 and horizontal drive
circuit 60.
[0049] The pixel array section 30 has one of scan lines 31-1 to
31-m and one of power supply lines 32-1 to 32-m disposed for each
pixel row and one of signal lines 33-1 to 33-n disposed for each
pixel column for the pixels arranged in m rows by n columns.
[0050] The pixel array section 30 is typically formed on a
transparent insulating substrate such as glass substrate to provide
a flat panel structure. The pixels 20 of the pixel array section 30
may be formed with amorphous silicon TFTs (Thin Film Transistors)
or low-temperature polysilicon TFTs. When low-temperature
polysilicon TFTs are used, the write scan circuit 40, power supply
scan circuit 50 and horizontal drive circuit 60 can also be
implemented on a display panel (substrate) 70 on which the pixel
array section 30 is formed.
[0051] The write scan circuit 40 includes shift registers or other
components adapted to sequentially shift (transmit) a start pulse
sp in synchronism with a clock pulse ck. During the writing of a
video signal to the pixels 20 of the pixel array section 30, the
same circuit 40 sequentially supplies write pulses WS1 to WSm (scan
signals) respectively to the scan lines 31-1 to 31-m so as to scan
the pixels 20 of the pixel array section 30 in succession on a
row-by-row basis (progressive scan).
[0052] The power supply scan circuit 50 includes shift registers or
other components adapted to sequentially shift (transmit) the start
pulse sp in synchronism with the clock pulse ck. The same circuit
50 sequentially and selectively supplies power supply line
potentials DS1 to DSm respectively to the power supply lines 32-1
to 32-m in synchronism with the progressive scan by the write scan
circuit 40 so as to control the light emission and non-light
emission of the pixels 20. The power supply line potentials DS1 to
DSm are each switched between two different potentials, i.e., a
first potential Vccp and a second potential Vini lower than the
first potential Vccp.
[0053] The horizontal drive circuit 60 selects, as appropriate,
either a video signal voltage Vsig (hereinafter may be simply
written as "signal voltage") appropriate to the brightness
information or an offset voltage Vofs supplied from a signal supply
source (not shown) so as to, for example, write the selected
voltage to the pixels 20 of the pixel array section 30 via the
signal lines 33-1 to 33-n on a row-by-row basis. That is, the
horizontal drive circuit 60 employs progressive writing adapted to
sequentially write the video signal voltage Vsig on a row-by-row
(line-by-line) basis.
[0054] Here, the offset voltage Vofs is a reference voltage (e.g.,
voltage corresponding to the black level) which serves as a
reference for the video signal voltage Vsig. On the other hand, the
second potential Vini is set to a potential lower than the offset
voltage Vofs. For example, letting the threshold voltage of the
drive transistor 22 be denoted by Vth, the second potential Vini is
set to a potential lower than Vofs-Vth, and preferably to a
potential sufficiently lower than Vofs-Vth.
(Pixel Circuit)
[0055] FIG. 2 is a circuit diagram illustrating a specific example
of the configuration of the pixel (pixel circuit) 20.
[0056] As illustrated in FIG. 2, the pixel 20 includes, for
example, as a light emitting element, an organic EL element 21
which is a type of current-driven electro-optical element whose
light emission brightness changes according to the current flowing
through the element. In addition to the same element 21, the pixel
20 includes a drive transistor 22, write transistor 23 and holding
capacitance 24 as its components. That is, the pixel 20 is made up
of two transistors (Tr) and one capacitor (C).
[0057] In the pixel 20 configured as described above, N-channel
TFTs are used as the drive transistor 22 and write transistor 23.
It should be noted, however, that the combination of conductivity
types of the drive transistor 22 and write transistor 23 given here
is merely an example, and the embodiment of the present invention
is not limited to this combination.
[0058] The organic EL element 21 has its cathode electrode
connected to a common power supply line 34 which is disposed
commonly for all the pixels 20. The drive transistor 22 has its
source electrode connected to the anode electrode of the organic EL
element 21 and its drain electrode connected to the power supply
line 32 (one of 32-1 to 32-m).
[0059] The write transistor 23 has its gate electrode connected to
the scan line 31 (one of 31-1 to 31-m). The same transistor 23 has
one of the source and drain electrodes connected to the signal line
33 (one of 33-1 to 33-n) and the other of the source and drain
electrodes connected to the gate electrode of the drive transistor
22.
[0060] The holding capacitance 24 has one of its electrodes
connected to the gate electrode of the drive transistor 22. The
same capacitance 24 has its other electrode connected to the source
electrode of the drive transistor 22 (anode electrode of the
organic EL element 21).
[0061] In the pixel 20 made up of two transistors and one
capacitor, the write transistor 23 conducts in response to the scan
signal applied to its gate electrode by the write scan circuit 40
via the scan line 31. As the same transistor 23 conducts, it
samples either the video signal voltage Vsig appropriate to the
brightness information or offset voltage Vofs supplied from the
horizontal drive circuit 60 via the signal line 33 and writes the
sampled voltage to the pixel 20.
[0062] The written signal voltage Vsig or offset voltage Vofs is
applied to the gate electrode of the drive transistor 22 and at the
same time held by the holding capacitance 24. When the potential DS
of the power supply line 32 (one of 32-1 to 32-m) is at the first
potential Vccp, the drive transistor 22 is supplied with a current
from the power supply line 32. As a result, the drive transistor 22
supplies the organic EL element with a drive current whose level is
appropriate to the voltage level of the signal voltage Vsig held by
the holding capacitance 24, thus current-driving the same element
21 to emit light.
(Circuit Operation of the Organic EL Display Device)
[0063] A description will be given next of the circuit operation of
the organic EL display device 10 configured as described above
based on the timing waveform diagram shown in FIG. 3 and using the
operation explanatory diagrams shown in FIGS. 4 to 6. It should be
noted that the write transistor 23 is represented by a switch
symbol for simplification in the operation explanatory diagrams
shown in FIGS. 4 to 6. It should also be noted that because the
organic EL element 21 has a capacitive component, an EL capacitance
25 thereof is also shown.
[0064] The timing waveform diagram in FIG. 3 illustrates the
variations of the potential (write pulse) WS of the scan line 31
(one of 31-1 to 31-m), potential DS (Vccp/Vini) of the power supply
line 32 (one of 32-1 to 32-m) and gate potential Vg and source
potential Vs of the drive transistor 22.
<Light Emission Period>
[0065] In the timing diagram shown in FIG. 3, the organic EL
element 21 emits light prior to time t1 (light emission period). In
the light emission period, the potential DS of the power supply
line 32 is at the first potential Vccp, and the write transistor 23
is not conducting.
[0066] At this time, because the drive transistor 22 is designed to
operate in the saturation region, a drive current (drain-to-source
current) Ids appropriate to the gate-to-source voltage Vgs of the
drive transistor 22 is supplied to the organic EL element 21 from
the power supply line 32 via the drive transistor 22 as illustrated
in FIG. 4A. As a result, the organic EL element 21 emits light at
the brightness appropriate to the level of the drive current
Ids.
<Preparatory Period for Threshold Correction>
[0067] Then, at time t1, the progressive scan of a new field
begins. The potential DS of the power supply line 32 changes from
the first potential (hereinafter written as "high potential") Vccp
to the second potential (hereinafter written as "low potential")
Vini which is sufficiently lower than Vofs-Vth (Vofs: offset
voltage of the signal line 33).
[0068] Here, letting the threshold voltage of the organic EL
element 21 be denoted by Vel and the potential of the common power
supply line 34 by Vcath and assuming that Vini<Vel+Vcath for the
low potential Vini, the source potential Vs of the drive transistor
22 is almost equal to the low potential Vini. As a result, the
organic EL element 21 is reverse-biased, causing it to stop
emitting light.
[0069] Next, at time t2, the potential WS of the scan line 31
changes from the low to high potential, bringing the write
transistor 23 into conduction as illustrated in FIG. 4C. At this
time, the horizontal drive circuit 60 supplies the offset voltage
Vofs to the signal line 33. Therefore, the gate potential Vg of the
drive transistor 22 becomes equal to the offset voltage Vofs.
Further, the source potential Vs of the drive transistor 22 is at
the low potential Vini which is sufficiently lower than the offset
voltage Vofs.
[0070] At this time, the gate-to-source voltage Vgs of the drive
transistor 22 is Vofs-Vini. Here, the threshold correction
operation may not be performed unless Vofs-Vini is larger than the
threshold voltage Vth of the drive transistor 22. Therefore, the
potential relationship Vofs-Vini>Vth have to be established.
Thus, the preparatory operation for threshold correction includes
of fixing the gate potential Vg and source potential Vs of the
drive transistor 22 respectively to the offset voltage Vofs and low
potential Vini for initialization.
<First Threshold Correction Period>
[0071] Next, at time t3, as the potential DS of the power supply
line 32 changes from the low potential Vini to the high potential
Vccp as illustrated in FIG. 4D, the source potential Vs of the
drive transistor 22 begins to rise, initiating the first threshold
correction period. In the first threshold correction period, as the
source potential Vs of the drive transistor 22 rises, the
gate-to-source voltage Vgs of the drive transistor 22 reaches a
given potential Vx1. The potential Vx1 is held by the holding
capacitance 24.
[0072] Next, at time t4 in the second half of the horizontal
interval (1H), the horizontal drive circuit 60 supplies the video
signal voltage Vsig to the signal line 33 as illustrated in FIG.
5A, changing the potential of the signal line 33 from the offset
voltage Vofs to the signal voltage Vsig. In this period, the signal
voltage Vsig is written to the pixels in other row.
[0073] At this time, in order to prevent the signal voltage Vsig
from being written to the pixels in the own row, the potential WS
of the scan line 31 changes from the high to low potential,
bringing the write transistor 23 out of conduction. This
disconnects the gate electrode of the drive transistor 22 from the
signal line 33, leaving the gate electrode floating.
[0074] Here, if the gate electrode of the drive transistor 22 is
floating and if the source potential Vs of the drive transistor 22
varies due to the connection of the holding capacitance 24 between
the gate and source electrodes of the drive transistor 22, the gate
potential Vg of the same transistor 22 also varies with variation
(varies to follow the variation) in the source potential Vs. This
is the bootstrapping action by the holding capacitance 24.
[0075] At time t4 and beyond, the source potential Vs of the drive
transistor 22 continues to rise by Va1 (Vs=Vofs-Vx1+Va1). At this
time, the gate potential Vg of the drive transistor 22 also rises
by Val (Vg=Vofs+Va1) with the rise of the source potential Vs of
the same transistor 22 because of the bootstrapping action.
<Second Threshold Correction Period>
[0076] At time t5, a next horizontal interval begins. As
illustrated in FIG. 5B, the potential WS of the scan line 31
changes from the low to high potential, bringing the write
transistor 23 into conduction. At the same time, the horizontal
drive circuit 60 supplies the offset voltage Vofs, rather than the
signal voltage Vsig, to the signal line 33, initiating the second
threshold correction period.
[0077] In the second threshold correction period, as the write
transistor 23 conducts, the offset voltage Vofs is written.
Therefore, the gate potential Vg of the drive transistor 22 is
initialized again to the offset voltage Vofs. The source potential
Vs declines with the decline of the gate potential Vg at this time.
Then, the source potential Vs of the drive transistor 22 begins to
rise again.
[0078] Then, as the source potential Vs of the drive transistor 22
rises in the second threshold correction period, the gate-to-source
voltage Vgs of the same transistor 22 reaches a given potential
Vx2. The potential Vx2 is held by the holding capacitance 24.
[0079] Next, at time t6 in the second half of the horizontal
interval, the horizontal drive circuit 60 supplies the signal
voltage Vsig to the signal line 33 as illustrated in FIG. 5C,
changing the potential of the signal line 33 from the offset
voltage Vofs to the signal voltage Vsig. In this period, the signal
voltage Vsig is written to the pixels in other row (row next to the
row in which the pixels were written the last time).
[0080] At this time, in order to prevent the signal voltage Vsig
from being written to the pixels in the own row, the potential WS
of the scan line 31 changes from the high to low potential,
bringing the write transistor 23 out of conduction. This
disconnects the gate electrode of the drive transistor 22 from the
signal line 33, leaving the gate electrode floating.
[0081] At time t6 and beyond, the source potential Vs of the drive
transistor 22 continues to rise by Va2 (Vs=Vofs-Vx1+Va2). At this
time, the gate potential Vg of the drive transistor 22 also rises
by Va2 (Vg=Vofs+Va2) with the rise of the source potential Vs of
the same transistor 22 because of the bootstrapping action.
<Third Threshold Correction Period>
[0082] At time t7, a next horizontal interval begins. As
illustrated in FIG. 5D, the potential WS of the scan line 31
changes from the low to high potential, bringing the write
transistor 23 into conduction. At the same time, the horizontal
drive circuit 60 supplies the offset voltage Vofs, rather than the
signal voltage Vsig, to the signal line 33, initiating the third
threshold correction period.
[0083] In the third threshold correction period, as the write
transistor 23 conducts, the offset voltage Vofs is written.
Therefore, the gate potential Vg of the drive transistor 22 is
initialized again to the offset voltage Vofs. The source potential
Vs declines with the decline of the gate potential Vg at this time.
Then, the source potential Vs of the drive transistor 22 begins to
rise again.
[0084] As the source potential Vs of the drive transistor 22 rises,
the gate-to-source voltage Vgs of the same transistor 22 will
converge to the threshold voltage Vth of the same transistor 22
before long. As a result, the voltage corresponding to the
threshold voltage Vth is held by the holding capacitance 24.
[0085] As a result of the third threshold correction operation
described above, the threshold voltage Vth of the drive transistor
22 in each of the pixels is detected, and the voltage corresponding
to the threshold voltage Vth held by the holding capacitance 24. It
should be noted that, in the third threshold correction period, the
potential Vcath of the common power supply line 34 is set so that
the organic EL element 21 goes into cutoff. This is done to ensure
that a current flows merely to the holding capacitance 24 and not
to the organic EL element 21.
<Signal Write Period and Mobility Correction Period>
[0086] Next, at time t8, the potential WS of the scan line 31
changes to the low potential, bringing the write transistor 23 out
of conduction as illustrated in FIG. 6A. At the same time, the
potential of the signal line 33 changes from the offset voltage
Vofs to the video signal voltage Vsig.
[0087] As the write transistor 23 stops conducting, the gate
electrode of the drive transistor 22 is left floating. However, the
gate-to-source voltage Vgs of the drive transistor 22 is equal to
the threshold voltage Vth of the same transistor 22. Therefore, the
same transistor 22 is in cutoff. As a result, the drain-to-source
current Ids does not flow through the drive transistor 22.
[0088] Next, at time t9, the potential WS of the scan line 31
changes to the high potential, bringing the write transistor 23
into conduction as illustrated in FIG. 6B. As a result, the same
transistor 23 samples the video signal voltage Vsig and writes the
voltage to the pixel 20. This writing of the signal voltage Vsig by
the write transistor 23 brings the gate potential Vg of the drive
transistor 22 equal to the signal voltage Vsig.
[0089] Then, when the drive transistor 22 drives the organic EL
element 21 with the video signal voltage Vsig, the threshold
voltage Vth of the drive transistor 22 is cancelled by the voltage
held by the holding capacitance 24 which corresponds to the
threshold voltage Vth, thus achieving the threshold correction. The
principle of the threshold correction will be described later.
[0090] At this time, the organic EL element 21 is in cutoff (high
impedance state) at first. Therefore, the current flowing from the
power supply line 32 to the drive transistor 22 according to the
video signal voltage Vsig (drain-to-source current Ids) flows into
the EL capacitance 25 of the organic EL element 21, thus initiating
the charging of the same capacitance 25.
[0091] Because of the charging of the EL capacitance 25, the source
potential Vs of the drive transistor 22 rises over time. At this
time, the variation of the threshold voltage Vth of the drive
transistor 22 has already been corrected (by the threshold
correction). As a result, the drain-to-source current Ids of the
drive transistor 22 is dependent merely upon the mobility .mu. of
the same transistor 22.
[0092] When the source potential Vs of the drive transistor 22
rises to the potential equal to Vofs-Vth+.DELTA.V before long, the
gate-to-source voltage Vgs of the same transistor 22 becomes equal
to Vsig-Vofs+Vth-.DELTA.V. That is, the increment .DELTA.V of the
source potential Vs acts so that it is subtracted from the voltage
(Vsig-Vofs+Vth) held by the holding capacitance 24, in other words,
so that the charge stored in the holding capacitance 24 is
discharged. This means that a negative feedback is applied.
Therefore, the increment .DELTA.V of the source potential Vs of the
drive transistor 22 is a feedback amount of the negative
feedback.
[0093] As described above, if the drain-to-source current Ids
flowing through the drive transistor 22 is negatively fed back to
the gate input, i.e., the gate-to-source voltage Vgs, of the same
transistor 22, the dependence of the drain-to-source current Ids of
the same transistor 22 upon the mobility .mu. can be cancelled.
That is, the variation of the mobility .mu. between the pixels can
be corrected.
[0094] More specifically, the higher the video signal voltage Vsig,
the larger the drain-to-source current Ids, and therefore the
larger the absolute value of the negative feedback amount
(correction amount) .DELTA.V. As a result, the mobility is
corrected according to the light emission brightness. If the video
signal voltage Vsig is maintained constant, the larger the mobility
.mu. of the drive transistor 22, the larger the absolute value of
the negative feedback amount .DELTA.V. This makes it possible to
eliminate the variation of the mobility .mu. between the pixels.
The principle of the mobility correction will be described
later.
<Light Emission Period>
[0095] Next, at time t10, the potential WS of the scan line 31
changes to the low potential, bringing the write transistor 23 out
of conduction as illustrated in FIG. 6C. This disconnects the gate
electrode of the drive transistor 22 from the signal line 33,
leaving the gate electrode floating.
[0096] When the gate electrode of the drive transistor 22 is left
floating and at the same time the drain-to-source current Ids of
the same transistor 22 begins to flow into the organic EL element
21, the anode potential of the same element 21 rises according to
the drain-to-source current Ids of the same transistor 22.
[0097] The rise of the anode potential of the organic EL element 21
is nothing other than the rise of the source potential Vs of the
drive transistor 22. As the source potential Vs of the drive
transistor 22 rises, the gate potential Vg of the same transistor
22 will also rise because of the bootstrapping action.
[0098] At this time, assuming that the bootstrap gain is unity
(ideal value), the increment of the gate potential Vg is equal to
the increment of the source potential Vs. In the light emission
period, therefore, the gate-to-source voltage Vgs of the drive
transistor 22 is maintained constant at Vsig-Vofs+Vth-.DELTA.V.
Then, at time t11, the potential of the signal line 33 changes from
the video signal voltage Vsig to the offset voltage Vofs.
[0099] As is clear from the above description of the operation, the
threshold correction period spans three horizontal intervals, i.e.,
one horizontal interval during which the signal writing and
mobility correction are performed and two horizontal intervals
preceding the one horizontal interval. This provides a sufficient
time for the threshold correction period, thus allowing to reliably
detect the threshold voltage Vth of the drive transistor 22 and
hold the voltage in the holding capacitance 24 for the reliable
threshold correction operation.
[0100] Although the threshold correction period spans three
horizontal intervals, this is merely an example. If the one
horizontal interval during which the signal writing and mobility
correction are performed is sufficient for the threshold correction
period, there is no need to provide a threshold correction period
spanning the preceding horizontal intervals. On the other hand, if
one horizontal interval becomes shorter as a result of providing a
higher definition and if three horizontal intervals are not
sufficient for the threshold correction period, this period may
span four horizontal intervals or longer.
(Principle of the Threshold Correction)
[0101] Here, a description will be given of the principle of the
threshold correction of the drive transistor 22. The drive
transistor 22 is designed to operate in the saturation region.
Therefore, the same transistor 22 functions as a constant current
source. As a result, the constant drain-to-source current (drive
current) Ids, given by the following formula (1), is supplied to
the organic EL element 21 from the drive transistor 22:
Ids=(1/2).mu.(W/L)Cox(Vgs-Vth).sup.2 (1)
[0102] where W is the channel width, L the channel length, and Cox
the gate capacitance per unit area.
[0103] FIG. 7 illustrates the characteristic of the drain-to-source
current Ids of the drive transistor 22 vs. gate-to-source voltage
Vgs of the same transistor 22.
[0104] As illustrated in this characteristic diagram, unless the
variation of the threshold voltage Vth of the drive transistor 22
between the pixels is corrected, the drain-to-source current Ids
appropriate to the gate-to-source voltage Vgs is Ids1 when the
threshold voltage Vth is Vth1.
[0105] In contrast, when the threshold voltage Vth is Vth2
(Vth2>Vth1), the drain-to-source current Ids appropriate to the
same gate-to-source voltage Vgs is Ids2 (Ids2<Ids). That is, the
drain-to-source current Ids changes with change in the threshold
voltage Vth of the drive transistor 22 even if the gate-to-source
voltage Vgs remains unchanged.
[0106] In the pixel (pixel circuit) 20 configured as described
above, on the other hand, the gate-to-source voltage Vgs of the
drive transistor 22 during light emission is Vsig-Vofs+Vth-.DELTA.V
as mentioned earlier. Substituting this into the formula (1), the
drain-to-source current Ids is expressed as follows:
Ids=(1/2).mu.(W/L)Cox(Vsig-Vofs-.DELTA.V).sup.2 (2)
[0107] That is, the term of the threshold voltage Vth of the drive
transistor 22 is cancelled. The drain-to-source current Ids
supplied from the drive transistor 22 to the organic EL element 21
is independent of the threshold voltage Vth of the drive transistor
22. As a result, the drain-to-source current Ids remains unchanged
irrespective of the variation of the threshold voltage Vth of the
drive transistor 22 from one pixel to another due to the
manufacturing process variation or change over time. This makes it
possible to maintain the light emission brightness of the organic
EL element 21 constant.
(Principle of the Mobility Correction)
[0108] A description will be given next of the principle of the
mobility correction of the drive transistor 22. FIG. 8 illustrates
a characteristic curve comparing a pixel A with the relatively
large mobility .mu. of the drive transistor 22 and a pixel B with
the relatively small mobility .mu. of the drive transistor 22. If
the drive transistor 22 includes, for example, a polysilicon thin
film transistor, it is inevitable that the mobility .mu. varies
from one pixel to another as with the pixels A and B.
[0109] If the video signal voltage Vsig at the same level is, for
example, applied to the pixels A and B when there is a variation in
the mobility .mu. between the two pixels, there will be a large
difference between a drain-to-source current Ids1' flowing through
the pixel A with the large mobility .mu. and a drain-to-source
current Ids2' flowing through the pixel B with the small mobility
.mu., unless the mobility .mu. is corrected in one way or another.
Thus, the screen uniformity is impaired in the event of a large
difference in the drain-to-source current Ids as a result of the
variation of the mobility .mu. between the pixels.
[0110] As is clear from the transistor characteristic formula (1)
given above, the larger the mobility .mu., the larger the
drain-to-source current Ids. Therefore, the larger the mobility
.mu., the larger the negative feedback amount .DELTA.V. As
illustrated in FIG. 8, a feedback amount .DELTA.V1 of the pixel A
with the large mobility .mu. is larger than a feedback amount
.DELTA.V2 of the pixel B with the small mobility .mu..
[0111] For this reason, if the drain-to-source current Ids of the
drive transistor 22 is negatively fed back to the video signal
voltage Vsig by the mobility correction operation, the larger the
mobility .mu., the greater the extent to which a negative feedback
is applied. This suppresses the variation of the mobility .mu. from
one pixel to another.
[0112] More specifically, if the pixel A with the large mobility
.mu. is corrected with the feedback amount .DELTA.V1, the
drain-to-source current Ids declines significantly from Ids1' to
Ids1. On the other hand, the feedback amount .DELTA.V2 of the pixel
B with the small mobility .mu. is small. Therefore, the
drain-to-source current Ids declines merely from Ids2' to Ids2,
which is not a significant drop. As a result, the drain-to-source
current Ids1 of the pixel A becomes almost equal to the
drain-to-source current Ids2 of the pixel B, thus correcting the
variation of the mobility .mu. from one pixel to another.
[0113] Summing up the above, if the pixels A and B have the
different mobilities .mu., the feedback amount .DELTA.V1 of the
pixel A with the large mobility .mu. is larger than the feedback
amount .DELTA.V2 of the pixel B with the small mobility .mu.. That
is, the larger the mobility .mu., the larger the feedback amount
.DELTA.V, and the more the drain-to-source current Ids
declines.
[0114] Therefore, the level of the drain-to-source current Ids of
the drive transistor 22 can be made uniform between the pixels with
the different mobilities .mu. by negatively feeding back the
drain-to-source current Ids of the drive transistor 22 to the video
signal voltage Vsig. This makes it possible to correct the
variation of the mobility .mu. from one pixel to another.
[0115] Here, a description will be given of the relationship
between the video signal potential (sampling potential) Vsig and
drain-to-source current Ids of the drive transistor 22 in the pixel
(pixel circuit) 20 shown in FIG. 2 with reference to FIGS. 9A to
9C. The above relationship will be described in different cases
with and without the threshold and mobility corrections.
[0116] In FIGS. 9A to 9C, FIG. 9A illustrates the case in which
neither the threshold correction nor the mobility correction is
performed. FIG. 9B illustrates the case in which the threshold
correction is performed, but not the mobility correction. FIG. 9C
illustrates the case in which both the threshold and mobility
corrections are performed. As illustrated in FIG. 9A, if neither
the threshold correction nor the mobility correction is performed,
there is a large difference in the drain-to-source current Ids
between the pixels A and B as a result of the variation of the
threshold voltage Vth and mobility .mu. between the two pixels.
[0117] In contrast, if merely the threshold correction is
performed, the variation of the drain-to-source current Ids can be
reduced to some extent by the threshold correction as illustrated
in FIG. 9B. However, the difference remains in the drain-to-source
current Ids between the pixels A and B caused by the variation of
the mobility .mu. between the two pixels.
[0118] If both the threshold and mobility corrections are
performed, the difference in the drain-to-source current Ids
between the pixels A and B caused by the variation of the threshold
voltage Vth and mobility .mu. between the two pixels can be almost
completely eliminated as illustrated in FIG. 9C. This ensures
constant brightness of the organic EL element 21 free from
variation, thus providing a high-quality on-screen image.
[0119] Further, the following advantageous effects can be achieved
by providing the pixel 20 shown in FIG. 2 with the bootstrapping
function mentioned earlier in addition to the threshold and
mobility correction functions.
[0120] That is, even if the source potential Vs of the drive
transistor 22 changes with change in the I-V characteristic of the
organic EL element 21 over time, the gate-to-source voltage Vgs of
the same transistor 22 is maintained constant thanks to the
bootstrapping action of the holding capacitance 24. As a result,
the current flowing through the organic EL element 21 remains
unchanged. Therefore, the light emission brightness of the organic
EL element 21 is maintained constant. This provides an on-screen
image free from brightness deterioration even in the event of a
change of the I-V characteristic of the organic EL element 21 over
time.
[Problems Attributable to Reduced Capacitance Value of the
Capacitive Component of the Organic EL Element]
[0121] As described above, in the organic EL display device 10
having the threshold and mobility correction functions, as the
pixel size becomes finer as a result of providing a higher
definition, the electrodes forming the organic EL element 21 grow
smaller in size. As a result, the capacitance value of the
capacitive component of the same element 21 becomes smaller. This
leads to a decline in the write gain of the video signal voltage
Vsig by as much as the decline in the capacitance value of the
capacitive component of the organic EL element 21.
[0122] Here, letting the capacitance value of the EL capacitance 25
be denoted by Cel and the capacitance value of the holding
capacitance 24 by Cs, the voltage Vgs held by the holding
capacitance 24 when the video signal voltage Vsig is written is
expressed as follows:
Vgs=Vsig.times.{1-Cs/(Cs+Cel)} (3)
[0123] Therefore, the ratio between the voltage Vgs held by the
holding capacitance 24 and the signal voltage Vsig, i.e., a write
gain G (=Vgs/Vsig), can be expressed as follows:
G=1-Cs/(Cs+Cel) (4)
As is clear from this formula (4), if the capacitance value Cel of
the capacitive component of the organic EL element 21 declines, the
write gain G will decline by as much as the decline therein.
[0124] In order to compensate for the decline in the write gain G,
an auxiliary capacitance need merely be attached to the source
electrode of the drive transistor 22. Letting the capacitance value
of the auxiliary capacitance be denoted by Csub, the write gain G
can be expressed as follows:
G=1-Cs/(Cs+Cel+Csub) (5)
[0125] As is clear from the formula (5), the larger the capacitance
value Csub of the auxiliary capacitance to be attached, the closer
the write gain G is to unity. The voltage Vgs close to the video
signal voltage written to the pixel 20 can be held by the holding
capacitance 24. This makes it possible to provide a light emission
brightness appropriate to the video signal voltage written to the
pixel 20.
[0126] As is clear from the above description, the write gain G of
the video signal voltage Vsig can be adjusted by adjusting the
capacitance value Csub of the auxiliary capacitance. On the other
hand, the drive transistor 22 differs in size depending upon the
light emission color of the organic EL element 21. Therefore, white
balance can be achieved by adjusting the capacitance value Csub of
the auxiliary capacitance according to the emission color of the
organic EL element 21, i.e., the size of the drive transistor
22.
[0127] On the other hand, letting the drain-to-source current of
the drive transistor 22 be denoted by Ids and the voltage increment
corrected by the mobility correction by .DELTA.V, a mobility
correction period t during which the aforementioned mobility
correction is to be performed is determined as follows:
T=(Cel+Csub).times..DELTA.V/Ids (6)
As is clear from the formula (6), the mobility correction period t
can be adjusted by the capacitance value Csub of the auxiliary
capacitance.
[Pixel Configuration Having an Auxiliary Capacitance]
[0128] FIG. 10 is a circuit diagram illustrating the pixel
configuration having an auxiliary capacitance. In FIG. 10, like
components are designated by the same reference numerals as in FIG.
2.
[0129] As illustrated in FIG. 10, the pixel 20 includes the organic
EL element 21 as a light-emitting element. The pixel 20 includes,
in addition to the organic EL element 21, the drive transistor 22,
write transistor 23 and holding capacitance 24. The pixel
configured as described above further includes an auxiliary
capacitance 26. The same capacitance 26 has one of its electrodes
connected to the source electrode of the drive transistor 22 and
the other electrode connected to the common power supply line 34
serving as a fixed potential.
[0130] Here, if the cathode wiring is routed in the TFT layer
(corresponding to a TFT layer 207 in FIGS. 16 to 18) in order to
form the auxiliary capacitance 26, problems occurs such as
horizontal crosstalk which is caused by the limited layout area of
the pixel 20 or wiring resistance in the pixel 20. Horizontal
crosstalk occurs due to the wiring resistance for the following
reason.
[0131] If the cathode wiring is routed in the TFT layer, a wiring
resistance R mediates between the cathode electrode of the organic
EL element 21 and common power supply line 34 as illustrated in
FIG. 11. As a result, the cathode potential of the organic EL
element 21 fluctuates synchronously with the variation of the
potential of the signal line 33 as illustrated in FIG. 12. When a
black window is displayed, for example, as illustrated in FIG. 13,
this fluctuation of the cathode potential is visually identified as
a crosstalk brighter than the regions above and below the black
window on the display screen (horizontal crosstalk).
[Features of the Present Embodiment]
[0132] The present embodiment is, therefore, defined in that the
auxiliary capacitance 26 is formed by positively using auxiliary
electrodes 35. The auxiliary electrodes 35 are each electrically
connected to the common power supply line 34 serving as the cathode
electrode of the organic EL element 21. In the same layer (anode
layer) as the anode electrode of the organic EL element 21, the
auxiliary electrodes 35 are at a fixed potential (cathode
potential) and disposed, for example, in rows (one for each pixel
row) for the pixels of the pixel array section 30 arranged in a
matrix form as illustrated in FIG. 14. The other electrode of the
auxiliary capacitance 26 is electrically connected to the auxiliary
electrode 35 (contact is established therebetween) for each of the
pixels 20.
[0133] In FIG. 14, the auxiliary electrodes 35 are disposed in rows
for the pixels 20 of the pixel array section 30. However, this is
merely an example. The auxiliary electrodes 35 may be disposed in
columns (one for each pixel column) or in a grid form (one for each
pixel row and for each pixel column) for the pixels 20 of the pixel
array section 30. Also in these cases, contact can be established
between the auxiliary electrode 35 and other electrode of the
auxiliary capacitance 26 for each of the pixels 20 as when the
auxiliary electrodes 35 are disposed in rows.
(Pixel Layout Structure)
[0134] FIG. 15 is a plan view schematically illustrating a pixel
layout structure of the pixel 20 having the auxiliary capacitance
26.
[0135] As illustrated in FIG. 15, the scan line 31 (one of 31-1 to
31-m) is disposed along the row (in the row direction of pixels)
close to the upper pixel row. The power supply line 32 (one of 32-1
to 32-m) is disposed downward from the middle portion. The
auxiliary electrode 35 is disposed along the row above the lower
pixel row. Further, the signal line 33 (one of 33-1 to 33-n) is
disposed along the column (in the column direction of pixels) close
to the pixel column on the left.
[0136] The drive transistor 22, write transistor 23 and holding
capacitance 24 are formed in the region between the scan line 31
and power supply line 32 of the pixel 20. The auxiliary capacitance
26 is formed in the region between the power supply line 32 and
auxiliary electrode 35 of the pixel 20. Contact (electrical
connection) is established between the other electrode of the
auxiliary capacitance 26 and the auxiliary electrode 35 by a
contact portion 36 for each of the pixels. The auxiliary electrode
35 is applied with a fixed potential (cathode potential) from the
common power supply line 34.
[0137] As described above, the auxiliary electrodes 35 are applied
with a fixed potential from the common power supply line 34 serving
as the cathode electrode of the organic EL element 21. The same
electrodes 35 are disposed in rows, in columns or in a grid form
for the pixels arranged in a matrix form. For the organic EL
display device configured as described above, specific examples
will be described below as to how to establish contact between the
other electrode of the auxiliary capacitance 26 and the auxiliary
electrode 35 for each of the pixels 20 so as to apply a fixed
potential to the other electrode of the auxiliary capacitance 26
and form the auxiliary capacitance 26 for the fixed potential.
EXAMPLE 1
[0138] FIG. 16 is a sectional view illustrating the sectional
structure of a pixel 20A according to example 1. The sectional view
of FIG. 16 is a sectional view taken along line A-A of FIG. 15.
[0139] As illustrated in FIG. 16, the pixel 20A has the gate
electrode of the drive transistor 22 formed on a glass substrate
201 as a first wiring 202. A gate insulating film 203 is formed on
the first wiring 202. A semiconductor layer 204 is formed, for
example, with polysilicon on the gate insulating film 203. The same
layer 204 forms the source and drain regions of the drive
transistor 22. The power supply line 32 is formed as a second
wiring 206 above the semiconductor layer 204 via an interlayer
insulating film 205.
[0140] Here, the layer which includes the first wiring 202, gate
insulating film 203, semiconductor layer 204 and interlayer
insulating film 205 serves as the TFT layer 207. Further, an
insulating planarizing film 208 and window insulating film 209 are
formed successively on the interlayer insulating film 205 and
second wiring 206. The organic EL element 21 is formed in a concave
portion 209A provided in the window insulating film 209.
[0141] The organic EL element 21 includes an anode electrode 211
made of a metal or other material formed on the bottom of the
concave portion 209A of the window insulating film 209. The same
element 21 further includes an organic layer (electron transporting
layer, light-emitting layer and hole transporting/injection layer)
212 formed on the anode electrode 211. The same element 21 still
further includes a cathode electrode 213 (common power supply line
34) made, for example, of a transparent conductive film formed on
the organic layer 212 commonly for all the pixels. Here, the layer
which includes the second wiring 206 and insulating planarizing
film 208 serves as an anode layer 210.
[0142] In the organic EL element 21, the organic layer 212 is
formed by depositing the electron transporting layer,
light-emitting layer and hole transporting/injection layer (none of
these layers are shown) successively on the anode electrode 211. As
the organic EL element 21 is current-driven by the drive transistor
22 shown in FIG. 2, a current flows from the drive transistor 22 to
the organic layer 212 via the anode electrode 211. This causes
electrons and holes to recombine in the light-emitting layer of the
organic layer 212, thus causing light to be emitted.
[0143] The pixel 20, which includes the organic EL element 21,
drive transistor 22, write transistor 23 and holding capacitance
24, is basically structured as described above.
[0144] In this basic pixel structure, the auxiliary capacitance 26
of the pixel 20A according to example 1 has the following
structure. That is, one of electrodes 261 is formed with the
semiconductor layer 204 made of polysilicon which forms the source
and drain regions of the drive transistor 22. Other electrode 262
is formed with the same metallic material and by the same process
as for the second wiring 206 so that the other electrode 262 is
opposed to the one of the electrodes 261 via the interlayer
insulating film 205. The auxiliary capacitance 26 is formed between
the opposed regions of the parallel plates of the electrodes 261
and 262.
[0145] Contact is established between the other electrode 262 of
the auxiliary capacitance 26 and the auxiliary electrode 35 by the
contact portion 36. This ensures electrical connection, for each
pixel, between the other electrode 262 of the auxiliary capacitance
26 and the auxiliary electrodes 35 which are disposed, for example,
in rows for the pixels arranged in a matrix form. As a result, a
fixed potential is applied from the common power supply line 34 via
the auxiliary electrodes 35.
[0146] As described above, the auxiliary capacitance 26 is formed
with the electrodes 261 and 262. The one of the electrodes 261 is
made of polysilicon as for the semiconductor layer 204 of the drive
transistor 22. The other electrode 262 is made of the same metallic
material as for the second wiring 206. The other electrode 262 is
electrically connected, for each pixel, to the auxiliary electrodes
35 which are disposed, for example, in rows for the pixels arranged
in a matrix form. This makes it possible to apply a fixed potential
to the other electrode 262 of the auxiliary capacitance 26 without
providing any cathode wiring in the TFT layer 207, thus allowing to
form the auxiliary capacitance 26 for the fixed potential. As a
result, problems such as horizontal crosstalk caused by the limited
layout area of the pixel 20 or wiring resistance in the pixel 20
can be resolved.
[0147] In the case of example 1, the capacitance value of the
auxiliary capacitance 26 is determined by the following, i.e., the
area of the opposed regions of the parallel plates of the
electrodes 261 and 262, the gap between the electrodes 261 and 262
(film thickness of the interlayer insulating film 205), and the
specific inductive capacity of the insulator (interlayer insulating
film 205 in this example) mediating between the electrodes 261 and
262.
EXAMPLE 2
[0148] FIG. 17 is a sectional view illustrating the sectional
structure of a pixel 20B according to example 2. In FIG. 17, like
components are designated by the same reference numerals as in FIG.
16. The sectional view of FIG. 17 is a sectional view taken along
line A-A of FIG. 15.
[0149] The pixel 20B according to example 2 has the basic pixel
structure as described in example 1. The auxiliary capacitance 26
of the pixel 20B has the following structure. That is, the other
electrode 262 is formed first on the glass substrate 201 with the
same metallic material and by the same process as for the first
wiring 202. The one of the electrodes 261 is formed via the gate
insulating film 203 with polysilicon which forms the semiconductor
layer 204 of the drive transistor 22. The one of the electrodes 261
is formed where it is opposed to the electrode 262. The auxiliary
capacitance 26 is formed between the opposed regions of the
parallel plates of the electrodes 261 and 262.
[0150] Contact is established between the other electrode 262 of
the auxiliary capacitance 26 and the second wiring 206 by a contact
portion 37. Contact is also established between the other electrode
262 of the auxiliary capacitance 26 and the auxiliary electrode 35
by the contact portion 36. This ensures electrical connection, for
each pixel, between the other electrode 262 of the auxiliary
capacitance 26 and the auxiliary electrodes 35 which are disposed,
for example, in rows for the pixels arranged in a matrix form. As a
result, a fixed potential is applied from the common power supply
line 34 via the auxiliary electrodes 35.
[0151] As described above, the auxiliary capacitance 26 is formed
with the electrodes 261 and 262. The other electrode 262 is made of
the same metallic material as for the first wiring 202. The one of
the electrodes 261 is made of polysilicon as for the semiconductor
layer 204 of the drive transistor 22. The other electrode 262 is
electrically connected, for each pixel, to the auxiliary electrodes
35 which are disposed, for example, in rows for the pixels arranged
in a matrix form. This makes it possible to apply a fixed potential
to the other electrode 262 of the auxiliary capacitance 26 without
providing any cathode wiring in the TFT layer 207, thus allowing to
form the auxiliary capacitance 26 for the fixed potential. As a
result, problems such as horizontal crosstalk caused by the limited
layout area of the pixel 20 or wiring resistance in the pixel 20
can be resolved.
[0152] In the case of example 2, the capacitance value of the
auxiliary capacitance 26 is determined by the following, i.e., the
area of the opposed regions of the parallel plates of the
electrodes 261 and 262, the gap between the electrodes 261 and 262
(film thickness of the gate insulating film 203), and the specific
inductive capacity of the insulator (gate insulating film 203 in
this example) mediating between the electrodes 261 and 262.
[0153] Here, examples 1 and 2 are compared. Assuming that both the
specific inductive capacity and area of the opposed regions of the
parallel plates are the same, the following can be said. That is,
the gate insulating film 203 is typically thinner than the
interlayer insulating film 205. Therefore, the gap between the
parallel plates can be made smaller in example 2 than in example 1.
As a result, the capacitance value of the auxiliary capacitance 26
can be set larger in example 2 than in example 1.
[0154] Conversely, example 1 has an advantage over example 2 in
that leak caused by interlayer shorting is less likely to occur
because the interlayer insulating film 205 is thicker than the gate
insulating film 203.
EXAMPLE 3
[0155] FIG. 18 is a sectional view illustrating the sectional
structure of a pixel 20C according to example 3. In FIG. 18, like
components are designated by the same reference numerals as in
FIGS. 16 and 17. The sectional view of FIG. 18 is a sectional view
taken along line A-A of FIG. 15.
[0156] The pixel 20C according to example 3 has the basic pixel
structure as described in example 1. The auxiliary capacitance 26
of the pixel 20C has the following structure. That is, an other
first electrode 262A is formed first on the glass substrate 201
with the same metallic material and by the same process as for the
first wiring 202. The one of the electrodes 261 is formed via the
gate insulating film 203 with polysilicon which forms the
semiconductor layer 204 of the drive transistor 22. The one of the
electrodes 261 is formed where it is opposed to the electrode 262.
Further, an other second electrode 262B is formed with the same
metallic material and by the same process as for the second wiring
206 so that it is opposed to the electrode 261 via the interlayer
insulating film 205. The auxiliary capacitance 26 is formed
electrically in parallel between the opposed regions of the
parallel plates of the electrodes 262A, 261 and 262B.
[0157] Contact is established between the other first electrode
262A of the auxiliary capacitance 26 and the other second electrode
262B by the contact portion 37. Contact is also established between
the other first electrode 262A of the auxiliary capacitance 26 and
the auxiliary electrode 35 by the contact portion 36. This ensures
electrical connection, for each pixel, between the other first and
second electrodes 262A and 262B of the auxiliary capacitance 26 and
the auxiliary electrodes 35 which are disposed, for example, in
rows for the pixels arranged in a matrix form. As a result, a fixed
potential is applied from the common power supply line 34 via the
auxiliary electrodes 35. Further, the capacitance formed between
the electrodes 262A and 261 and that formed between the electrodes
262B and 261 are connected electrically in parallel so that the
auxiliary capacitance 26 is formed as the combined capacitance of
the two capacitances.
[0158] As described above, the auxiliary capacitance 26 is formed
with the other electrodes 262A and 262B and one of electrodes 261.
The other electrodes 262A and 262B are respectively made of the
same metallic materials as for the first and second wirings 202 and
206. The one of electrodes 261 is made of polysilicon as for the
semiconductor layer 204 of the drive transistor 22. The other
electrodes 262A and 262B are electrically connected, for each
pixel, to the auxiliary electrodes 35 which are disposed, for
example, in rows for the pixels arranged in a matrix form. This
makes it possible to apply a fixed potential to the other
electrodes 262A and 262B of the auxiliary capacitance 26 without
providing any cathode wiring in the TFT layer 207, thus allowing to
form the auxiliary capacitance 26 for the fixed potential. As a
result, problems such as horizontal crosstalk caused by the limited
layout area of the pixel 20 or wiring resistance in the pixel 20
can be resolved.
[0159] In particular, a capacitance is formed between the other
first electrode 262A and one of the electrodes 261 and another
between the one of the electrodes 261 and other second electrode
262B. Therefore, assuming that the capacitance values in examples 1
and 2 are the same, the auxiliary capacitance 26 having a
capacitance value roughly twice as large as that in examples 1 and
2 can be formed. In other words, if the auxiliary capacitance 26
need merely have more or less the same capacitance value as in
examples 1 and 2, the electrodes 261, 262A and 262B forming the
auxiliary capacitance 26 can be reduced in size. As a result, the
auxiliary capacitance 26 can be formed in the pixel 20 without
increasing the size of the pixel 20C as compared to examples 1 and
2.
[0160] In the case of example 3, the capacitance value of the
auxiliary capacitance 26 is determined by the combined capacitance
value of the two capacitances. One of the capacitances is
determined by the area of the opposed regions of the parallel
plates of the one of the electrodes 261 and other first electrode
262A, the distance between the electrodes 261 and 262A, and the
specific inductive capacity of the insulator (gate insulating film
203 in this example) mediating between the electrodes 261 and 262A.
The other capacitance is determined by the area of the opposed
regions of the parallel plates of the one of the electrodes 261 and
other second electrode 262B, the distance between the electrodes
261 and 262B, and the specific inductive capacity of the insulator
(interlayer insulating film 205 in this example) mediating between
the electrodes 261 and 262B.
(Advantageous Effects of the Present Embodiment)
[0161] As described above, the pixels 20 of the organic EL display
device each have the auxiliary capacitance 26 to secure a
sufficient write gain of the video signal. In this organic EL
display device, the other electrode or electrodes 262 (262A and
262B) of the auxiliary capacitance 26 are connected, for each of
the pixels 20, to the auxiliary electrodes 35 which are disposed in
rows, in columns or in a grid form for the pixels arranged in a
matrix form and which are applied with a fixed potential. This
makes it possible to apply a fixed potential to the other
electrodes 262 without providing any cathode wiring in the TFT
layer 207, thus allowing to form the auxiliary capacitance 26 for
the fixed potential while at the same time suppressing the wiring
resistance. As a result, horizontal crosstalk caused by the wiring
resistance can be suppressed, thus providing improved on-screen
image quality.
[0162] In the above embodiment, a description was given taking, as
an example, the case in which the present invention was applied to
an organic EL display device using organic EL elements as
electro-optical elements of the pixel circuits. However, the
embodiment of the present invention is not limited to this
application example, but applicable to display devices in general
using current-driven electro-optical elements (light-emitting
elements) whose light emission brightness changes with change in
current flowing through the elements.
[Application Examples]
[0163] The display device according to the embodiment of the
present invention described above is applicable as a display device
of electronic equipment across all fields including those shown in
FIGS. 19 to 23, namely, a digital camera, laptop personal computer,
mobile terminal device such as mobile phone and video camcorder.
These pieces of equipment are designed to display an image or video
of a video signal fed to or generated inside the electronic
equipment.
[0164] As described above, if used as a display device of
electronic equipment across all fields, the display device
according to the embodiment of the present invention can, as is
clear from the aforementioned embodiment, prevent horizontal
crosstalk caused by the wiring resistance because contact is
established, for each of the pixels 20, between the other electrode
of the auxiliary capacitance 26 and the auxiliary electrodes 35
which are disposed in rows, in columns or in a grid form for the
pixels arranged in a matrix form. As a result, the display device
according to the embodiment of the present invention provides
excellent on-screen image quality in all kinds of electronic
equipment.
[0165] It should be noted that the display device according to the
embodiment of the present invention includes that in a modular form
having a sealed configuration. Such a display device corresponds to
a display module formed by attaching an opposed section made, for
example, of transparent glass to the pixel array section 30. The
aforementioned light-shielding film may be provided on the
transparent opposed section, in addition to films such as color
filter and protective film. It should also be noted that a circuit
section, FPC (flexible printed circuit) or other circuitry, adapted
to allow exchange of signals or other information between external
equipment and the pixel array section, may be provided on the
display module.
[0166] Specific examples of electronic equipment to which the
embodiment of the present invention is applied will be described
below.
[0167] FIG. 19 is a perspective view illustrating a television set
to which the embodiment of the present invention is applied. The
television set according to the present application example
includes a video display screen section 101 made up, for example,
of a front panel 102, filter glass 103 and other parts. The
television set is manufactured by using the display device
according to the embodiment of the present invention as the video
display screen section 101.
[0168] FIGS. 20A and 20B are perspective views illustrating a
digital camera to which the embodiment of the present invention is
applied. FIG. 20A is a perspective view of the digital camera as
seen from the front, and FIG. 20B is a perspective view thereof as
seen from the rear. The digital camera according to the present
application example includes a flash-emitting section 111, display
section 112, menu switch 113, shutter button 114 and other parts.
The digital camera is manufactured by using the display device
according to the embodiment of the present invention as the display
section 112.
[0169] FIG. 21 is a perspective view illustrating a laptop personal
computer to which the embodiment of the present invention is
applied. The laptop personal computer according to the present
application example includes, in a main body 121, a keyboard 122
adapted to be manipulated for entry of text or other information, a
display section 123 adapted to display an image, and other parts.
The laptop personal computer is manufactured by using the display
device according to the embodiment of the present invention as the
display section 123.
[0170] FIG. 22 is a perspective view illustrating a video camcorder
to which the embodiment of the present invention is applied. The
video camcorder according to the present application example
includes a main body section 131, lens 132 provided on the
front-facing side surface to image the subject, imaging start/stop
switch 133, display section 134 and other parts. The video
camcorder is manufactured by using the display device according to
the embodiment of the present invention as the display section
134.
[0171] FIGS. 23A to 23G are perspective views illustrating a mobile
terminal device such as mobile phone to which the embodiment of the
present invention is applied. FIG. 23A is a front view of the
mobile phone in an open position. FIG. 23B is a side view thereof.
FIG. 23C is a front view of the mobile phone in a closed position.
FIG. 23D is a left side view. FIG. 23E is a right side view. FIG.
23F is a top view. FIG. 23G is a bottom view. The mobile phone
according to the present application example includes an upper
enclosure 141, lower enclosure 142, connecting section (hinge
section in this example) 143, display 144, subdisplay 145, picture
light 146, camera 147 and other parts. The mobile phone is
manufactured by using the display device according to the
embodiment of the present invention as the display 144 and
subdisplay 145.
[0172] It should be understood by those skilled in the art that
various modifications, combinations, sub-combinations and
alterations may occur depending on design requirements and other
factor in so far as they are within the scope of the appended
claims or the equivalents thereof.
* * * * *