U.S. patent application number 12/379941 was filed with the patent office on 2009-08-06 for liquid crystal display.
This patent application is currently assigned to Sharp Kabushiki Kaisha. Invention is credited to Tomokazu Ohtsubo, Masanori Takeuchi, Toshihide Tsubata.
Application Number | 20090195488 12/379941 |
Document ID | / |
Family ID | 36142671 |
Filed Date | 2009-08-06 |
United States Patent
Application |
20090195488 |
Kind Code |
A1 |
Takeuchi; Masanori ; et
al. |
August 6, 2009 |
Liquid crystal display
Abstract
A segmented-pixel liquid crystal display has a plurality of
pixels of which each has three sub-pixels 10a-10c, namely one
middle and two side sub-pixels, arranged next to one another in the
column or row direction. The sub-pixels 10a-10c have different
brightness levels when the pixel as a whole is in a given middle
halftone state, and the middle sub-pixel 10a has the highest
brightness level. This eliminates unnaturalness as is
conventionally produced when an image with a straight border is
displayed, and further improves the gamma characteristic.
Inventors: |
Takeuchi; Masanori;
(Tsu-Shi, JP) ; Ohtsubo; Tomokazu; (Suzuka-Shi,
JP) ; Tsubata; Toshihide; (Tsu-Shi, JP) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
Sharp Kabushiki Kaisha
Osaka
JP
|
Family ID: |
36142671 |
Appl. No.: |
12/379941 |
Filed: |
March 4, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11663889 |
Jan 3, 2008 |
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PCT/JP2005/018313 |
Oct 4, 2005 |
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12379941 |
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Current U.S.
Class: |
345/90 ;
345/87 |
Current CPC
Class: |
G09G 3/3648 20130101;
G02F 1/134336 20130101; G09G 2300/0443 20130101; G09G 2300/0876
20130101; G02F 1/134345 20210101; G02F 1/136213 20130101; G09G
2300/0447 20130101; G09G 2320/0276 20130101; G02F 1/13624
20130101 |
Class at
Publication: |
345/90 ;
345/87 |
International
Class: |
G09G 3/36 20060101
G09G003/36 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 6, 2004 |
JP |
JP 2004-293218 |
Claims
1. A liquid crystal display having a plurality of pixels arrayed in
a matrix, each pixel having a plurality of electrodes for applying
an electric field to a liquid crystal layer, wherein, each pixel
has at least three sub-pixels, said at least three sub-pixels
having at least two different brightness levels when the pixel as a
whole is in a given middle halftone state, and wherein, of said at
least three sub-pixels, one having a highest brightness level is
called a bright sub-pixel and at least two others are called dim
sub-pixels, one of the dim sub-pixels is located at one end of the
pixel, another of the dim sub-pixels is located at an opposite end
of the pixel, and the bright sub-pixel is located between said one
an another of the dim sub-pixels.
2. The liquid crystal display according to claim 1, wherein said
one and another of the dim sub-pixels have an identical brightness
level.
3. The liquid crystal display according to claim 1, wherein a ratio
of an aperture area of the bright sub-pixel to a total aperture
area of the dim sub-pixels is in a range from 1:1 to 1:4.
4. The liquid crystal display according to claim 1, wherein a ratio
between aperture areas of said one and another of sub-pixels is in
a range from 1:1 to 1:4.
5. The liquid crystal display according to claim 1, wherein said at
least three sub-pixels each have a liquid crystal capacitance
between a sub-pixel electrode and an common electrode disposed
opposite each other across the liquid crystal layer and an
auxiliary capacitance between an auxiliary capacitance electrode
electrically connecting to the sub-pixel electrode and an auxiliary
capacitance common electrode disposed opposite the auxiliary
capacitance electrode and connecting to an auxiliary capacitance
conductor, wherein a single electrode is shared as the common
electrodes of said at least three sub-pixels, and wherein at least
two different auxiliary capacitance conductors are provided one for
the bright sub-pixel and another for the dim sub-pixels.
6. The liquid crystal display according to claim 5, wherein an
insulating layer is interposed between the auxiliary capacitance
electrode and the auxiliary capacitance common electrode.
7. The liquid crystal display according to claim 5, the liquid
crystal display further having scanning lines extending in the row
direction, signal lines extending in the column direction, and, for
each pixel, at least two switching devices provided one for the
bright sub-pixel and another for the dim sub-pixels, the switching
devices connecting to a scanning line and a signal line each common
to the three sub-pixels of the pixel, wherein the switching devices
are turned on and off by a scanning signal voltage supplied to the
common scanning line and, when the switching devices are on, a
display signal voltage is supplied from the common signal line to
the sub-pixel electrode and the auxiliary capacitance electrode of
each of the bright and dim sub-pixels, and wherein, after the
switching devices are turned off, auxiliary capacitance common
voltages at the auxiliary capacitance common electrodes of the
bright and dim sub-pixels vary such that variations in the
auxiliary capacitance common voltages as defined by directions and
degrees in which the auxiliary capacitance common voltages vary
differ between the bright sub-pixel and the dim sub-pixels.
8. The liquid crystal display according to claim 7, wherein the
switching devices are TFTs, and these TFTs are formed with a single
semiconductor layer.
9. The liquid crystal display according to claim 5, wherein the
auxiliary capacitance common voltages invert polarities thereof
periodically.
10. The liquid crystal display according to claim 9, wherein the
auxiliary capacitance common voltage applied to the auxiliary
capacitance common electrode of the bright sub-pixel and the
auxiliary capacitance common voltage applied to the auxiliary
capacitance common electrodes of the dim sub-pixels are 180 degrees
out of phase with each other.
11. The liquid crystal display according to claim 10, wherein the
auxiliary capacitance common voltage applied to the auxiliary
capacitance common electrode of the bright sub-pixel and the
auxiliary capacitance common voltage applied to the auxiliary
capacitance common electrodes of the dim sub-pixels have an equal
amplitude.
12. The liquid crystal display according to claim 9, wherein,
between every two mutually adjacent signal lines, display signal
voltages applied thereto have opposite polarities and, between
every two pixels mutually adjacent in the row direction, the
auxiliary capacitance electrodes and the auxiliary capacitance
common electrodes of the bright and dim sub-pixels are disposed in
reversed patterns.
13. The liquid crystal display according to claim 5, wherein the
scanning lines are laid between mutually adjacent pixels, and, in
each pixel, the two auxiliary capacitance conductors are laid
parallel to the scanning lines.
14. The liquid crystal display according to claim 13, wherein a
conductor electrode via which the display signal voltage is
supplied to the sub-pixel electrode of the bright sub-pixel crosses
the two auxiliary capacitance conductors.
15. The liquid crystal display according to claim 1, wherein the
sub-pixel electrodes of said at least three sub-pixels are separate
from one another.
16. The liquid crystal display according to claim 1, wherein the
sub-pixel electrodes of said one and another of side sub-pixels are
continuous with each other.
17. The liquid crystal display according to claim 1, wherein a
metal layer is formed under a contact hole via which a conductor
electrode via which the display signal voltage is supplied connects
to a sub-pixel electrode, with an insulating layer interposed
between the metal layer and the contact hole.
18. The liquid crystal display according to claim 13, wherein the
two auxiliary capacitance conductors are laid between the
sub-pixels.
Description
TECHNICAL FIELD
[0001] The present invention relates to liquid crystal displays,
and more particularly to segmented-pixel liquid crystal
displays.
BACKGROUND ART
[0002] Liquid crystal displays are flat displays that boast of high
resolution, slimness, lightweight, low electric power consumption,
and other advantages. In recent years, liquid crystal displays have
seen improvement of display performance, improvement of fabrication
capacity, and price competitiveness against other types of display,
and accordingly they have been enjoying a rapidly growing
market.
[0003] More recently, as the display quality of liquid crystal
displays has been further improved, a problem in viewing angle
characteristics has become apparent: the gamma characteristic
varies between in orthogonal viewing and in oblique viewing; in
other words, the gamma characteristic depends on the viewing angle.
Here, the gamma characteristic denotes the dependence of display
brightness on halftone levels. Thus, the fact that the gamma
characteristic varies between in orthogonal viewing and in oblique
viewing means that the way different halftone levels are displayed
varies with the direction of viewing. This is annoying especially
during the display of photograph images or of TV broadcasts, among
others.
[0004] This problem of the viewing-angle dependence of the gamma
characteristic is more remarkable in the multi-domain vertical
alignment mode (MVA mode, as disclosed in JP-A-H11-242225) and in
the axisymmetric aligned mode (ASM mode, as disclosed in
JP-A-H10-186330) than in the in-plane switching mode (IPS mode, as
disclosed in JP-B-S63-021907). On the other hand, with the IPS
mode, it is more difficult, than with the MVA or ASM mode, to
fabricate liquid crystal panels that offer high contrast in
orthogonal viewing with satisfactory productivity. Hence,
improvements addressing the viewing-angle dependence of the gamma
characteristic are sought especially eagerly in MVA and ASM mode
liquid crystal displays.
[0005] Under this background, the applicant of the present
invention once proposed a technology whereby each pixel is
segmented into two sub-pixels to which different voltages are
applied to mitigate the viewing-angle dependence of the gamma
characteristic (for example, in Patent Document 1 listed below).
[0006] Patent Document 1: JP-A-2004-078157, Claims [0007] Patent
Document 2: JP-A-H6-332009, Claims [0008] Patent Document 3:
JP-A-2004-062146, Embodiments
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0009] Incidentally, the human eye tends to recognize pixels and
borders by being attracted by light spots and areas. On the other
hand, the recent trend toward increasingly large-screen liquid
crystal displays has resulted in their having larger pixels than
they have conventionally had. Under these circumstances, segmenting
each pixel into two sub-pixels causes an inconvenience: as a result
of the human eye recognizing pixels by tracing the lighter
sub-pixel of each pixel, when an image with a straight border is
displayed, the line of sight moves in a zigzag along the border,
from one pixel with one halftone level to another having a
different halftone level, often causing the viewer to perceive
unsmoothness or unnatural hues. To be sure, in conventional liquid
crystal displays, certain improvements have been made to address
the viewing-angle dependence of the gamma characteristic; these
improvements, however, are not quite satisfactory.
[0010] An object of the present invention is to provide a
segmented-pixel liquid crystal display that does not produce
unnaturalness even when displaying an image with a straight border
and that offers a further improved gamma characteristic.
Means for Solving the Problem
[0011] To achieve the above object, according to one aspect of the
invention, in a liquid crystal display, a plurality of pixels, each
having a plurality of electrodes for applying an electric field to
a liquid crystal layer, are arrayed in a matrix; in each pixel,
three sub-pixels, namely one middle and two side sub-pixels, are
arranged next to one another in the column or row direction; the
three sub-pixels have at least two different brightness levels when
the pixel as a whole is in a given middle halftone state, and the
middle sub-pixel has the highest brightness level. In the present
specification, "a middle halftone state" denotes any intermediate
state between the highest and lowest halftone levels.
[0012] Here, preferably, the two side sub-pixels are given an
identical brightness level.
[0013] From the viewpoint of further improving the gamma
characteristic of the liquid crystal display, preferably, the ratio
of the aperture area of the middle sub-pixel to the total aperture
area of the two side sub-pixels is in the range from 1:1 to 1:4
and, preferably, the ratio between aperture areas of the two side
sub-pixels is in the range from 1:1 to 1:4.
[0014] According to a preferred embodiment, in the liquid crystal
display, preferably, the three sub-pixels each have: a liquid
crystal capacitance between a sub-pixel electrode and an common
electrode disposed opposite each other across the liquid crystal
layer; and an auxiliary capacitance between an auxiliary
capacitance electrode electrically connecting to the sub-pixel
electrode and an auxiliary capacitance common electrode disposed
opposite the auxiliary capacitance electrode and connecting to an
auxiliary capacitance conductor. Moreover, a single electrode may
be shared as the common electrodes of the three sub-pixels;
moreover, at least two different auxiliary capacitance conductors
may provided, one for the middle sub-pixel and another for the side
sub-pixels. Here, preferably, an insulating layer is interposed
between the auxiliary capacitance electrode and the auxiliary
capacitance common electrode.
[0015] Preferably, there are provided: scanning lines extending in
the row direction; signal lines extending in the column direction;
and, for each pixel, at least two switching devices that are
provided one for the middle sub-pixel and another for the side
sub-pixels and that connect to a scanning line and a signal line
each common to the three sub-pixels of the pixel. Moreover,
preferably, the switching devices are turned on and off by a
scanning signal voltage supplied to the common scanning line and,
when the switching devices are on, a display signal voltage is
supplied from the common signal line to the sub-pixel electrode and
the auxiliary capacitance electrode of each of the middle and side
sub-pixels; moreover, preferably, after the switching devices are
turned off, the auxiliary capacitance common voltages at the
auxiliary capacitance common electrodes of the middle and side
sub-pixels vary such that the variations in those voltages as
defined by the directions and degrees in which they vary differ
between the middle sub-pixel and the side sub-pixels.
[0016] Here, for a higher aperture ratio, preferably, the switching
devices are TFTs, and these TFTs are formed with a single
semiconductor layer.
[0017] The auxiliary capacitance common voltages may invert the
polarities thereof periodically. Preferably, the auxiliary
capacitance common voltage applied to the auxiliary capacitance
common electrode of the middle sub-pixel and the auxiliary
capacitance common voltage applied to the auxiliary capacitance
common electrodes of the side sub-pixels are 180 degrees out of
phase with each other. Preferably, the auxiliary capacitance common
voltage applied to the auxiliary capacitance common electrode of
the middle sub-pixel and the auxiliary capacitance common voltage
applied to the auxiliary capacitance common electrodes of the side
sub-pixels have an equal amplitude.
[0018] Preferably, between every two mutually adjacent signal
lines, display signal voltages applied thereto are given opposite
polarities and, between every two pixels mutually adjacent in the
row direction, the auxiliary capacitance electrodes and the
auxiliary capacitance common electrodes of the middle and side
sub-pixels are disposed in reversed patterns.
[0019] From the viewpoint of improving the aperture ration,
preferably, the scanning lines are laid between mutually adjacent
pixels, and, in each pixel, the two auxiliary capacitance
conductors are laid parallel to the scanning lines and between the
sub-pixels. Here, from the viewpoint of improving image quality,
preferably, the conductor electrode via which the display signal
voltage is supplied to the sub-pixel electrode of the middle
sub-pixel is so formed as to cross the two auxiliary capacitance
conductors.
[0020] The sub-pixel electrodes of the three sub-pixels may be
separate from one another, or may be continuous with each
other.
[0021] From the viewpoint of preventing disturbed alignment in the
liquid crystal layer and improving display quality, preferably, a
metal layer is formed under a contact hole via which the conductor
electrode via which the display signal voltage is supplied connects
to the sub-pixel electrode, with an insulating layer interposed
between the metal layer and the contact hole.
Advantages of the Invention
[0022] According to the present invention, in each pixel, three
sub-pixels are formed that are arranged next to one another in the
column or row direction. This helps further mitigate the
viewing-angle dependence of the gamma characteristic as compared
with that conventionally observed. Moreover, the three sub-pixels
have at least two different brightness levels when the pixel as a
whole is in a given middle halftone state, and the middle sub-pixel
has the highest brightness level. Thus, even when an image with a
straight border is displayed, the line of sight, as it moves along
the border, moves across pixels having the same halftone level,
unlike in a case where each pixel has two sub-pixels. This prevents
the viewer from perceiving unsmoothness or unnatural hues at a
border between different halftone levels.
[0023] Here, preferably, the two side sub-pixels are given an
identical brightness level. This helps reduce the numbers of
switching devices, auxiliary capacitance conductors, and other
elements, and thus helps prevent undue lowering of the aperture
ratio.
[0024] Preferably, the ratio of the aperture area of the middle
sub-pixel to the total aperture area of the two side sub-pixels is
in the range from 1:1 to 1:4 and, preferably, the ratio between
aperture areas of the two side sub-pixels is in the range from 1:1
to 1:4. This helps further improve the gamma characteristic of the
liquid crystal display.
[0025] Preferably, the three sub-pixels each have: a liquid crystal
capacitance between a sub-pixel electrode and an common electrode
disposed opposite each other across the liquid crystal layer; and
an auxiliary capacitance between an auxiliary capacitance electrode
electrically connecting to the pixel electrode and an auxiliary
capacitance common electrode disposed opposite the auxiliary
capacitance electrode and connecting to an auxiliary capacitance
conductor; moreover, a single electrode may be shared as the common
electrodes of the three sub-pixels; moreover, at least two
different auxiliary capacitance conductors may provided, one for
the middle sub-pixel and another for the side sub-pixels. This
helps improve the controllability of the voltages applied to the
sub-pixels.
[0026] Here, preferably, an insulating layer is interposed between
the auxiliary capacitance electrode and the auxiliary capacitance
common electrode. This allows those electrodes to overlap to form
an auxiliary electrode, and thus helps increase the aperture ratio.
Preferably, there are provided: scanning lines extending in the row
direction; signal lines extending in the column direction; and, for
each pixel, at least two switching devices that are provided one
for the middle sub-pixel and another for the side sub-pixels and
that connect to a scanning line and a signal line each common to
the three sub-pixels of the pixel. Moreover, preferably, the
switching devices are turned on and off by a scanning signal
voltage supplied to the common scanning line and, when the
switching devices are on, a display signal voltage is supplied from
the common signal line to the sub-pixel electrode and the auxiliary
capacitance electrode of each of the middle and side sub-pixels;
moreover, preferably, after the switching devices are turned off,
the auxiliary capacitance common voltages at the auxiliary
capacitance common electrodes of the middle and side sub-pixels
vary such that the variations in those voltages as defined by the
directions and degrees in which they vary differ between the middle
sub-pixel and the side sub-pixels. This helps further improve the
controllability of the voltages applied to the sub-pixels.
[0027] Here, preferably, the switching devices are TFTs, and these
TFTs are formed with a single semiconductor layer. This helps
increase the aperture ratio of the pixel.
[0028] The auxiliary capacitance common voltages may invert the
polarities thereof periodically; preferably, the auxiliary
capacitance common voltage applied to the auxiliary capacitance
common electrode of the middle sub-pixel and the auxiliary
capacitance common voltage applied to the auxiliary capacitance
common electrodes of the side sub-pixels are 180 degrees out of
phase with each other; and, preferably, the auxiliary capacitance
common voltage applied to the auxiliary capacitance common
electrode of the middle sub-pixel and the auxiliary capacitance
common voltage applied to the auxiliary capacitance common
electrodes of the side sub-pixels have an equal amplitude. This
helps further improve the controllability of the voltages applied
to the sub-pixels.
[0029] Preferably, between every two mutually adjacent signal
lines, display signal voltages applied thereto are given opposite
polarities, in which case, preferably, between every two pixels
mutually adjacent in the row direction, the auxiliary capacitance
electrodes and the auxiliary capacitance common electrodes of the
middle and side sub-pixels are disposed in reversed patterns. This
allows the center sub-pixel to have the highest brightness.
[0030] Preferably, the scanning lines are laid between mutually
adjacent pixels, and, in each pixel, the two auxiliary capacitance
conductors are laid parallel to the scanning lines and between the
sub-pixels. This helps improve the aperture ratio. Moreover,
preferably, the conductor electrode via which the display signal
voltage is supplied to the sub-pixel electrode of the middle
sub-pixel is so formed as to cross the two auxiliary capacitance
conductors. This helps cancel out the two parasitic capacitances
formed where the conductor electrode crosses the auxiliary
capacitance conductors, and thus helps improve image quality.
[0031] Preferably, a metal layer is formed under a contact hole via
which the conductor electrode via which the display signal voltage
is supplied connects to the sub-pixel electrode. This helps shield
disturbed alignment in the liquid crystal layer, and thus helps
improve image quality .
BRIEF DESCRIPTION OF DRAWINGS
[0032] [FIG. 1] A plan view schematically showing the pixel
structure in a liquid crystal display according to the
invention.
[0033] [FIG. 2] A cross-sectional view along line A-A shown in FIG.
1.
[0034] [FIG. 3] A cross-sectional view along line B-B shown in FIG.
1.
[0035] [FIG. 4] A graph showing the viewing-angle dependence of the
gamma characteristic in relation to the ratio of the aperture area
of the middle sub-pixel to the ratio of the total aperture area of
the side sub-pixels.
[0036] [FIG. 5] An enlarged plan view of the TFTs shown in FIG.
1.
[0037] [FIG. 6] A circuit diagram electrically equivalent to the
pixel structure in the liquid crystal display shown in FIG. 1.
[0038] [FIG. 7] A diagram schematically showing an example of the
voltage waveforms with which the liquid crystal display according
to the invention is driven.
[0039] [FIG. 8] A plan view schematically showing the pixel
structure in another liquid crystal display according to the
invention.
[0040] [FIG. 9] A plan view schematically showing another example
of the sub-pixel electrodes usable in the invention.
LIST OF REFERENCE SYMBOLS
[0041] 10a, 10b, 10c sub-pixels [0042] 11a, 11b, 11c, 11d sub-pixel
electrodes [0043] 12 scanning lines [0044] 13 signal lines [0045]
14O, 14E auxiliary capacitance conductors [0046] 15a, 15b, 15c TFTs
(switching devices) [0047] 16a, 16b, 16c, 16d, 16e, 16a', 16b'
extensions from drain electrodes [0048] 17a, 17b auxiliary
capacitance electrodes [0049] 18a, 18b, 18c contact holes [0050] 19
metal layer [0051] 21 common electrode [0052] 141, 142, 141', 142'
auxiliary capacitance common electrode [0053] SC semiconductor
layer [0054] ClcO, ClcE.sub.1, ClcE.sub.2 liquid crystal
capacitances [0055] CcsO, CcsE auxiliary capacitances
BEST MODE FOR CARRYING OUT THE INVENTION
[0056] Hereinafter, liquid crystal displays embodying the present
invention will be described with reference to the accompanying
drawings. It should be understood that these embodiments are not
meant to limit in any way how the invention is implemented.
[0057] FIG. 1 is a plan view schematically showing the pixel
structure on the active matrix substrate of a liquid crystal
display according to the invention, focusing on the pixel at line
n, column m. FIGS. 2 and 3 are cross-sectional view along lines A-A
and B-B, respectively, shown in FIG. 1. Sub-pixel electrodes
11a-11c are arranged next to one another in the column direction. A
scanning line 12(n) is laid between pixels to run laterally as seen
in FIG. 1; a signal line 13(m) is laid between pixels to run
longitudinally as seen in FIG. 1. Two auxiliary capacitance
conductors 14O and 14E are laid parallel to the scanning line
12(n), between the sub-pixel electrodes 11a, 11b, and 11c. As
switching devices, TFTs 15a-15c are provided at the intersection
between the scanning line 12(n) and the signal line 13(m).
[0058] A drain electrode extension 16a from the TFT 15a runs over
the auxiliary capacitance conductor 14E to reach above the
auxiliary capacitance conductor 14O, where a portion of the drain
electrode extension 16a faces, across an insulating layer
(unillustrated), an auxiliary capacitance common electrode 141
formed integrally with the auxiliary capacitance conductor 14O to
function as an auxiliary capacitance electrode 17a. In this
auxiliary capacitance electrode 17a, a contact hole 18a is formed
to connect the drain electrode extension 16a to the sub-pixel
electrode 11a. Likewise, drain electrode extensions 16b and 1 6c
merge together on the way to reach above the auxiliary capacitance
conductor 14E, where a portion of the drain electrodes extension
16b and 16c faces, across an insulating layer (unillustrated), an
auxiliary capacitance common electrode 142 formed integrally with
the auxiliary capacitance conductor 14E to function as an auxiliary
capacitance electrode 17b. In this auxiliary capacitance electrode
17b, a contact hole 18b is formed to connect the drain electrodes
extension 16b and 16c to the sub-pixel electrode 11b (see FIG. 2).
From the auxiliary capacitance electrode 17b, a drain electrode
extension 16d further extends to run over the auxiliary capacitance
conductor 14O to reach above the sub-pixel electrode 11c, where the
drain electrode extension 16d connects via a contact hole 18c to
the sub-pixel electrode 11c (see FIG. 3).
[0059] As shown in FIGS. 2 and 3, under the contact hole 18b, the
auxiliary capacitance conductor 14E is formed with an insulating
layer 21a interposed in between; under the contact hole 18c, a
metal layer 19 forming an island is formed with an insulating layer
21b interposed in between. This helps shield disturbed alignment in
the liquid crystal layer, and thus helps improve image quality. The
insulating layer 21a, which forms an auxiliary capacitance, and the
insulating layer 21b under the contact hole 18c are each, for
example, the gate insulating layer of a TFT.
[0060] With this structure, an equal effective voltage is applied
to the sub-pixel electrodes 11b and 11c. Moreover, as will be
described later, by supplying different auxiliary capacitance
common voltages to the two auxiliary capacitance conductors 14O and
14E, it is possible to make the effective voltage at the sub-pixel
electrode 11a higher than the effective voltage at the sub-pixel
electrodes 11b and 11c. Thus, it is possible to make the brightness
level of a sub-pixel 10a higher than the brightness level of
sub-pixels 10b and 10c. This helps eliminate unnaturalness as is
conventionally produced when an image with a straight border is
displayed, and also helps further mitigate the viewing-angle
dependence of the gamma characteristic.
[0061] Through experiments, the applicant has come to know that an
effective way to mitigate the viewing-angle dependence of the gamma
characteristic is to reduce the proportion of the aperture area of
the sub-pixel 10a that has a higher brightness level. In FIG. 4 is
a graph showing the viewing-angle dependence in relation to the
ratio of the aperture area of the higher-brightness sub-pixel 10a
(indicated as "high" in the graph) to the total aperture area of
the lower-brightness sub-pixels 10b and 10c (indicated as "low" in
the graph). In FIG. 4, the horizontal axis represents the halftone
level observed in orthogonal viewing, and the vertical axis
represents the viewing-angle dependence of the gamma characteristic
observed at different aperture area ratios, namely "with no pixel
segmentation", "at a high-to-low ratio of 1:1", "at a high-to-low
ratio of 1:3", and "at a high-to-low ratio of 1:4", by using the
halftone level observed in oblique viewing from 45 degrees upward,
downward, leftward, and rightward. This graph shows the following.
As the proportion of the "high" brightness aperture area decreases,
the gamma characteristic becomes increasingly close to the ideal
straight line, becoming closest to it when the high-to-low ratio is
1:3; as the "high" brightness aperture area further decreases (to
1:4), the gamma characteristic then becomes increasingly less close
to the ideal straight line. Hence, the ratio of the aperture area
of the higher brightness sub-pixel 10a to the total aperture area
of the lower sub-pixels 10b and 10c is preferably in the range
between 1:1 to 1:4, and further preferably in the range between
1:2.5 to 1:3.5. Incidentally, the relationship between, on one
hand, the just mentioned viewing-angle dependence of the gamma
characteristic in relation to the aperture area ratio and, on the
other hand, transmissivity is explained in JP-A-2004-062146, a
prior application by the same applicant.
[0062] Moreover, the ratio between the aperture areas of the
sub-pixels 10b and 10c is preferably in the range from 1:1 to 1:4,
and further preferably in the range from 1:1 to 1:2. With the
higher-brightness sub-pixel located in a deviated position, an
evaluation of the display quality of an image of a person revealed
an unintended change in color at the border of a skin-color area,
like where a skin-color area, such as representing the chin of a
person, overlaps a single-color background, such as clothes. This
phenomenon was alleviated when the higher-brightness sub-pixel was
located closer to the center.
[0063] In the embodiment under discussion, TFTs (thin-film
transistors) are used as switching devices. FIG. 5 is an enlarged
view of the TFTs in the liquid crystal display shown in FIG. 1. On
top of a gate electrode G formed as part of the scanning line
12(n), a gate insulating film (unillustrated) is formed, and,
further on top, a semiconductor layer SC is formed. On top of this
semiconductor layer SC, a source electrode S and three drain
electrodes D.sub.1, D.sub.2, and D.sub.3 are formed. From the
source electrode S, a plurality of extensions extend substantially
in the shape of a comb. The drain electrodes D.sub.1, D.sub.2, and
D.sub.3 are formed between these extensions, with a predetermined
distance secured from them.
[0064] Forming the three TFTs 15a to 15c on a single semiconductor
layer SC in this way helps give the pixel a larger aperture ratio
than when they are formed separately. Moreover, by varying the
width W and length L of the channel regions formed between the
extensions of the source electrode S and the drain electrodes
D.sub.1, D.sub.2, and D.sub.3, it is possible to supply the desired
current that suits the capacity of the pixel.
[0065] There are no particular restrictions on the shapes of the
source electrode S, the drain electrodes D.sub.1, D.sub.2, and
D.sub.3, and the semiconductor layer SC; these may be given any
shapes so long as no current leakage occurs. As switching devices,
any conventionally known switching devices other than TFTs may
instead be used, such as MIMs (metal insulator metals).
[0066] FIG. 6 is a schematic diagram showing a circuit equivalent
to the liquid crystal display shown in FIG. 1. In this diagram, the
liquid crystal capacitance corresponding to the sub-pixel 10a is
indicated as ClcO, and the liquid crystal capacitances
corresponding to the sub-pixels 10b and 10c is indicated as
ClcE.sub.1 and ClcE.sub.2. The liquid crystal capacitances ClcO,
ClcE.sub.1, and ClcE.sub.2 of the sub-pixels 10a, 10b, and 10c are
formed by the sub-pixel electrodes 11a to 11c, a common electrode
21, and the liquid crystal layer lying in between. The sub-pixel
electrodes 11a to 11c are connected via the TFTs 15a to 15c to the
signal line 13(m), and the gate electrode G (shown in FIG. 5) of
the TFTs is connected to the scanning line 12(n)
[0067] A first auxiliary capacitance provided for the sub-pixel 10a
and a second auxiliary capacitance provided for the sub-pixels 10b
and 10c are indicated as CcsO and CscE in FIG. 6. The auxiliary
capacitance electrode 17a of the first auxiliary capacitance CcsO
is connected via the drain electrode extension 16a to the drain of
the TFT 15a. The auxiliary capacitance electrode 17b of the second
auxiliary capacitance CcsE is connected via the drain electrode
extensions 16b and 16c to the drains of the TFTs 15b and 15c. The
auxiliary capacitance electrodes 17a and 17b may be connected in
any manner other than specifically illustrated, so long as they are
electrically so connected as to receive voltages equal to those
applied to the corresponding sub-pixel electrodes, namely the
sub-pixel electrode 11a and the sub-pixel electrodes 11b and 11c,
respectively; that is, the sub-pixel electrode 11a and the
sub-pixel electrodes 11b and 11c have simply to be electrically
connected, either directly or indirectly, to the corresponding
auxiliary capacitance electrodes 17a and 17b, respectively.
[0068] The auxiliary capacitance common electrode 141 of the first
auxiliary capacitance CcsO is connected to the auxiliary
capacitance conductor 14O, and the auxiliary capacitance common
electrode 142 of the second auxiliary capacitance CcsE is connected
to the auxiliary capacitance conductor 14E. With this structure, it
is possible to apply different auxiliary capacitance common
voltages to the auxiliary capacitance common electrodes 141 and 142
of the first and second auxiliary capacitance CcsO and CcsE,
respectively. As will be described later, how the auxiliary
capacitance common electrodes 141 and 142 are connected to the
first and second auxiliary capacitances CcsO and CcsE is selected
to suit the driving method adopted (for example, dot-inversion
driving).
[0069] Next, a description will be given of the mechanism by which
different voltages are applied, on one hand, to the sub-pixel
electrode 11a and, on the other hand, to the sub-pixel electrodes
11b and 11c.
[0070] FIG. 7 shows the voltage waveforms of the signals fed to the
pixel (n, m) shown in FIG. 6; that is, it shows how those signals
change their voltage levels over time. In FIG. 7, at (a) is shown
the waveform of the display signal voltage (halftone signal
voltage) Vs supplied to a signal line 13; at (b) is shown the
waveform of the scanning signal voltage Vg supplied to a scanning
line 12; at (c) and (d) are shown the waveforms of the auxiliary
capacitance common voltages VcsO and Vcse supplied to the auxiliary
capacitance conductors 14O and 14E, respectively; at (e) and (f)
are shown the waveforms of the voltages VlcO and VlcE applied to
the liquid crystal capacitances ClcE.sub.1 and ClcE.sub.2 of the
sub-pixel 10a and of the sub-pixels 10b and 10c, respectively.
[0071] The driving method shown in FIG. 7 is adopted when the
invention is applied to a liquid crystal display that operates on a
"1H dot inversion plus frame inversion" basis.
[0072] The display signal voltage Vs applied to a signal line 13
inverts its polarity every time a scanning line is selected (every
1H); in addition, between every two mutually adjacent signal lines,
the display signal voltages applied thereto have opposite
polarities (1H inversion). Moreover, the display signal voltages Vs
on all the signal lines 13 invert their polarities every frame
(frame inversion).
[0073] In the example under discussion, the cycle at which the
auxiliary capacitance common voltages VcsO and Vcse invert their
polarities is 2H; moreover, the auxiliary capacitance common
voltages VcsO and Vcse have waveforms such that they have an equal
amplitude and are 180 degrees out of phase with each other. The
cycle at which the auxiliary capacitance common voltages VcsO and
Vcse invert their polarities may be longer than 2H.
[0074] Now, with reference to FIG. 7, a description will be given
of why the voltages VlcO and VlcE applied to the liquid crystal
capacitance ClcO and to the liquid crystal capacitances ClcE.sub.1,
and ClcE.sub.2 change their voltage levels as shown in FIG. 7.
[0075] At time T.sub.1, the scanning signal voltage Vg turns from
low (VgL) to high (VgH), and thereby brings the TFTs 15a to 15c
into a conducting state, allowing the display signal voltage Vs on
the signal line 13 to be applied to the sub-pixel electrodes 10a to
10c. The voltages applied across the liquid crystal capacitance
ClcO and across the liquid crystal capacitances ClcE.sub.1, and
ClcE.sub.2 are the differences between the voltages at the
sub-pixel electrodes 11a to 11c and the voltage (Vcom) at the
common electrode 21. That is,
VlcO=VlcE.sub.1=VlcE.sub.2=Vs-Vcom.
[0076] At time T.sub.2, the scanning signal voltage Vg turns from
high (VgH) to low (VgL, <Vs), and thereby brings the TFTs 15a to
15c into a non-conducting state (off state), electrically
insulating all the sub-pixels and the auxiliary capacitances from
the signal line 13. At this point, under the influence of the
parasitic capacitances etc. of the TFTs 15a to 15c, the voltages at
the sub-pixel electrodes 11a to 11c momentarily fall by .DELTA.Vd,
a phenomenon called "pulling".
[0077] At time T.sub.3, the voltage VlcO at the liquid crystal
capacitance ClcO changes under the influence of the voltage VcsO at
the auxiliary capacitance common electrode 141 of the auxiliary
capacitance CcsO, which electrode is electrically connected to the
sub-pixel electrode 11a of the liquid crystal capacitance ClcO.
Moreover, the voltage VlcE at the liquid crystal capacitances
ClcE.sub.1and ClcE.sub.2 changes under the influence of the voltage
VcsE at the auxiliary capacitance common electrode 142 of the
second auxiliary capacitance CcsE, which electrode is electrically
connected to the sub-pixel electrodes 11b and 11c of the liquid
crystal capacitances ClcE.sub.1 and ClcE.sub.2.
[0078] Here, suppose that, at time T.sub.3, the auxiliary
capacitance common voltage VcsO increases by VcsOp>0 and the
auxiliary capacitance common voltage VcsE decreases by VcsEp>0.
That is, let the whole amplitude Vp-p of the auxiliary capacitance
common voltage VcsO be VcsOp, and let the whole amplitude of the
auxiliary capacitance common voltage VcsE be VceEp.
[0079] Moreover, let the total capacitance of the liquid crystal
capacitance ClcO and the auxiliary capacitance CcsO be C.sub.pixO,
and let the total capacitance of the liquid crystal capacitances
ClcE.sub.1 and ClcE.sub.2 and the auxiliary capacitance CcsE be
C.sub.pixE. Then,
VlcO=Vs-.DELTA.Vd+VcsOp(CcsO/C.sub.pixO)-Vcom, and
VlcE=Vs-.DELTA.Vd+VcsEp(CcsE/C.sub.pixE)-Vcom.
[0080] Next, at time T.sub.4, likewise under the influence of the
voltages VcsO and VceE at the auxiliary capacitance common
electrodes, the voltages VlcO and VlcE restore their voltages at
time T.sub.2.
VlcO=Vs-.DELTA.Vd-Vcom, and
VlcE=Vs-.DELTA.Vd-Vcom.
[0081] These changes in voltage are repeated until the voltage
Vg(n) turns to VgH in the next frame. As a result, the voltages
VlcO and VlcE come to have different effective values.
Specifically, let the effective value of the voltage VlcO be
VlcO.sub.rms, and let the effective value of the voltage VlcE be
VlcE.sub.rms,then
VlcO.sub.rms=Vs-.DELTA.Vd+(1/2)VcsOp(CcsO/C.sub.pixO)-Vcom, and
VlcE.sub.rms=Vs-.DELTA.Vd-(1/2)VcsEp(CcsE/C.sub.pixE)-Vcom
(provided that (Vs-.DELTA.Vd-Vcom)>>VcsOp(CcsO/C.sub.pixO),
and
(Vs-.DELTA.Vd-Vcom)>>VcsEp(CcsE/C.sub.pixE)).
Hence, let the differences between these effective values be
.DELTA.Vlc=VlcO.sub.rms-VlcE.sub.rms, then
.DELTA.Vlc=[VcsOp(CcsO/C.sub.pixO)+VcsEp(CcsE/C.sub.pixE)]/2.
In this way, by controlling the voltages applied to the auxiliary
capacitance common electrodes 141 and 142 of the auxiliary
capacitances CcsO and CcsE connected to the sub-pixel electrodes
11a to 11c, it is possible to apply different voltages to the
sub-pixel electrode 11a and to the sub-pixel electrodes 11b and
11c.
[0082] By interchanging the voltages VcsO and VcsE, it is possible
to give the voltage VlcO a smaller effective value and the voltage
VlcE a greater effective value. Alternatively, also by reversing
the combination of the auxiliary capacitance conductors 14O and 14E
connected to the auxiliary capacitance common electrodes 141 and
142 of the auxiliary capacitances CcsO and CcsE, it is possible to
give the voltage VlcO a smaller effective value and the voltage
VlcE a greater effective value.
[0083] Here, since the driving method adopted involves frame
inversion, in the next frame, the polarity of the voltage Vs is
inverted, so that Vlc<0. Even then, the same results as
described above can be obtained by inverting the polarities of VcsO
and VcsE in synchronism with frame inversion.
[0084] Moreover, here, since the driving method adopted involves
dot inversion, between every two mutually adjacent signal lines 13
(m) and 13(m+1), the display signal voltages supplied thereto have
opposite polarities. Thus, to make the effective voltage applied to
the sub-pixel electrode 11a' always higher than the effective
voltage applied to the sub-pixel electrodes 11b' and 11c' even in
the pixel (n, m+1) in the next frame, as shown in FIG. 8, it is
necessary that the auxiliary capacitance electrode 17a' of the
sub-pixel electrode 11a' face the auxiliary capacitance common
electrode 142' of the auxiliary capacitance conductor 14E and that
the auxiliary capacitance electrode 17b' of the sub-pixel
electrodes 11b' and 11c' face the auxiliary capacitance common
electrode 141' of the auxiliary capacitance conductor 14O.
[0085] Here, in the pixel (n, m), since the drain electrode
extension 16a of the sub-pixel electrode 11a crosses the two
auxiliary capacitance conductors 14O and 14E, and the voltages
applied to the auxiliary capacitance conductors 14O and 14E are 180
degrees out of phase with each other, the parasitic capacitances
attributable to the drain electrode extension 16a and attributable
to the auxiliary capacitance conductors 14O and 14E cancel out. On
the other hand, in the pixel (n, m+1), although the drain electrode
extension 16a' of the sub-pixel electrode 11a' does not need to
cross the auxiliary capacitance conductor 14O, if the drain
electrode extension 16a' of the sub-pixel electrode 11a' crosses
only the auxiliary capacitance conductor 14E, the above-mentioned
parasitic capacitances do not cancel out, and cause uneven display
between the sub-pixel electrodes 11a and 11a'. To overcome this
inconvenience, it is recommended that a drain electrode extension
16e be formed to extend further from the auxiliary capacitance
electrode 17a' of the sub-pixel electrode 11a' to reach above the
auxiliary capacitance conductor 14O so that together the drain
electrode extensions cross the two auxiliary capacitance conductors
14O and 14E.
[0086] In the liquid crystal display described above, the sub-pixel
electrodes 11a to 11c of the sub-pixels 10a to 10c are formed
separately from one another (see FIG. 1); it is however also
possible to form the sub-pixel electrodes 11b and 11c as a single
sub-pixel electrode 11d as shown in FIG. 9. Even in that case, just
as described previously, by controlling the voltages applied to the
auxiliary capacitance common electrodes 141 and 142 connected to
the sub-pixel electrodes 11a and 11d, it is possible to apply
different voltages to the sub-pixel electrodes 11a and 11d. In the
liquid crystal display described above, the sub-pixels are arranged
next to one another in the column direction; needless to say, it is
also possible to arrange them in the row direction instead.
[0087] The embodiment described above demonstrates that the present
invention contributes to improving the gamma characteristic in
normally black mode liquid crystal displays, in particular MVA mode
liquid crystal displays. It should however be understood that the
present invention finds application in any other type of liquid
crystal display, among others, IPS liquid crystal displays.
INDUSTRIAL APPLICABILITY
[0088] Liquid crystal displays according to the invention offer an
improved gamma characteristic with less viewing angle dependence
than ever, and do not produce unsmoothness or unnatural hues along
a border even when an image with a straight border is displayed.
This makes liquid crystal displays according to the invention
suitable for use in, for example, television monitors with large
screens.
* * * * *