U.S. patent number RE43,640 [Application Number 13/070,295] was granted by the patent office on 2012-09-11 for liquid crystal display device and driving method of the same.
This patent grant is currently assigned to Sharp Kabushiki Kaisha. Invention is credited to Yuichi Inoue, Yoshio Koike, Kazuya Ueda, Hidefumi Yoshida.
United States Patent |
RE43,640 |
Ueda , et al. |
September 11, 2012 |
Liquid crystal display device and driving method of the same
Abstract
The invention relates to a liquid crystal display device used
for a display part of an information equipment and a driving method
thereof, and has an object to provide a liquid crystal display
device in which excellent viewing angle characteristics can be
obtained and a driving method thereof. When a relatively high level
data voltage is applied to a gate electrode of a TFT, a driving
voltage is applied to a liquid crystal layer for a long time and a
white display is realized. When a relatively low level data voltage
is applied to the gate electrode of the TFT, the driving voltage is
not applied to the liquid crystal layer, and a black display is
realized. When an intermediate data voltage between the high level
and the low level is applied to the gate electrode of the TFT, the
TFT keeps an on state for a time determined by a time constant
depending on a capacitance and a resistance. The driving voltage is
applied to the liquid crystal layer for the on time. By this, a
half tone display is realized according to the ratio of the on time
of the TFT in one frame period.
Inventors: |
Ueda; Kazuya (Hyogo-ken,
JP), Yoshida; Hidefumi (Nara, JP), Inoue;
Yuichi (Kanagawa, JP), Koike; Yoshio (Atsugi,
JP) |
Assignee: |
Sharp Kabushiki Kaisha (Osaka,
JP)
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Family
ID: |
27750883 |
Appl.
No.: |
13/070,295 |
Filed: |
March 23, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
10375904 |
Feb 26, 2003 |
7508385 |
Mar 24, 2009 |
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Foreign Application Priority Data
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Feb 27, 2002 [JP] |
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2002-052303 |
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Current U.S.
Class: |
345/204; 345/87;
345/50; 345/63; 345/205 |
Current CPC
Class: |
G02F
1/1393 (20130101); G09G 3/3648 (20130101); G02F
1/13775 (20210101); G09G 2300/0809 (20130101); G09G
3/2014 (20130101); G09G 2320/028 (20130101); G09G
2310/06 (20130101); G02F 2201/128 (20130101); G09G
2300/0842 (20130101) |
Current International
Class: |
G09G
5/00 (20060101) |
Field of
Search: |
;345/43,50,63,87-100,204-215,690-697 ;349/43 |
References Cited
[Referenced By]
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Foreign Patent Documents
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07-064092 |
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10-096929 |
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Apr 1998 |
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JP |
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2000-314891 |
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Nov 2000 |
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JP |
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2001-159883 |
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Jun 2001 |
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JP |
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2001-343941 |
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Dec 2001 |
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JP |
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2002-0012737 |
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Feb 2002 |
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KR |
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2002-0012737 |
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Feb 2002 |
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KR |
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Primary Examiner: Shankar; Vijay
Attorney, Agent or Firm: Greer, Burns & Crain, Ltd.
Claims
What is claimed is:
1. A driving method of a liquid crystal display device, comprising
the steps of: applying a data voltage from a data voltage line to a
first thin film transistor in a pixel; applying a driving voltage
from a driving voltage line to a second thin film transistor in the
pixel only in a predetermined time in one frame period; and
displaying a predetermined gradation to the pixel by changing an
application time of the driving voltage in accordance with a
magnitude of the data voltage from the first thin film transistor
applied to a gate electrode of the second thin film transistor.
2. A driving method of a liquid crystal display device according to
claim 1, wherein the driving voltage is set to a voltage or within
a voltage range in which the liquid crystal can perform a high
speed response.
3. A driving method of a liquid crystal display device according to
claim 1, wherein the liquid crystal is aligned almost vertically to
or parallel to a substrate surface at a time when the driving
voltage is applied or at a time when the driving voltage is not
applied.
4. A liquid crystal display device comprising: a pair of substrates
disposed to be opposite to each other; a liquid crystal sealed
between the pair of substrates; a gate bus line formed on one of
the substrates; a data bus line formed to intersect the gate bus
line through an insulating film, to which a predetermined gradation
voltage is applied; a first thin film transistor formed in a
vicinity of an intersection position of the gate bus line and the
data bus line; a second thin film transistor, a gate electrode of
which is connected to a source electrode of the first thin film
transistor; a pixel electrode connected to a source electrode of
the second thin film transistor; a driving voltage bus line which
is connected to a drain electrode of the second thin film
transistor and to which a driving voltage to drive the liquid
crystal is applied; and a first capacitance and a first resistance
connected in parallel with each other at a side of the source
electrode of the first thin film transistor.
5. A liquid crystal display device according to claim 4, further
comprising a second capacitance and a second resistance connected
in parallel with each other at a side of the source electrode of
the second thin film transistor.
6. A liquid crystal display device according to claim 5, wherein
the second capacitance is a liquid crystal capacitance.
7. A liquid crystal display device according to claim 4, wherein
the pair of substrates respectively include vertical alignment
films on opposite surfaces, and the liquid crystal has a negative
dielectric anisotropy.
8. A liquid crystal display device comprising: a pair of substrates
disposed to be opposite to each other; a liquid crystal sealed
between the pair of substrates; and pixels disposed in a matrix
form and including a plurality of aligned regions, each of the
aligned regions further having a plurality of sub-regions where
pre-tilt angles of the liquid crystal are different from each
other, wherein the liquid crystal includes a photo-cured material
which is obtained by curing a photo-curing composition, and the
pre-tilt angle of at least one of the sub-regions is regulated by
the photo-cured material.
9. A liquid crystal display device according to claim 8, wherein
the pre-tilt angle of at least one of the sub-regions is about
90.degree..
10. A liquid crystal display device according to claim 8, wherein
the pre-tilt angle of all sub-regions is 80.degree. or more.
11. A liquid crystal display device according to claim 8, wherein
the plurality of aligned regions are substantially equally
divided.
12. A liquid crystal display device according to claim 8, wherein
the pre-tilt angle of at least a first one of the sub-regions is
regulated by the photo-cured material cured in the first sub-region
with a voltage applied to the liquid crystal, and the pre-tilt
angle of at least a second one of the sub-regions is regulated by
the photo-cured material cured in the second sub-region while no
voltage is applied to the liquid crystal.
13. A liquid crystal display device according to claim 8, wherein
the pair of substrates respectively include vertical alignment
films on opposite surfaces, and the liquid crystal has a negative
dielectric anisotropy.
14. A liquid crystal display device comprising: a pair of
substrates disposed to be opposite to each other; a liquid crystal
sealed between the pair of substrates; a plurality of pixel regions
disposed in a matrix form; and a pixel electrode formed in each of
the pixel regions, including a plurality of stripe electrodes and
spaces between the stripe electrodes, some of the plurality of
stripe electrodes and spaces being formed to have different widths
from each other.
15. A liquid crystal display device according to claim 14, wherein
the liquid crystal includes a photo-cured material which is
obtained by curing a photo-curing composition, and a pre-tilt angle
is regulated by the photo-cured material.
16. A liquid crystal display device according to claim 14, wherein
the pair of substrates respectively include vertical alignment
films on opposite surfaces, and the liquid crystal has a negative
dielectric anisotropy.
.Iadd.17. A liquid crystal display device comprising: a pair of
substrates disposed to be opposite to each other; a liquid crystal
sealed between the pair of substrates; a plurality of pixel regions
disposed in a matrix form; a pixel electrode formed in at least one
pixel region, the pixel electrode including at least a connection
electrode, a plurality of stripe electrodes connected to the
connection electrode, and a plurality of spaces between the stripe
electrodes, at least one of the plurality of spaces being formed to
have a different width than other spaces, the connection electrode
including a first connection electrode formed in the center of the
pixel region..Iaddend.
.Iadd.18. A liquid crystal display device according to claim 17,
the liquid crystal comprising a photo-cured material regulating a
pre-tilt angle of the liquid crystal..Iaddend.
.Iadd.19. A liquid crystal display device according to claim 18,
wherein the photo-cured material is obtained by curing a
photo-curing compound..Iaddend.
.Iadd.20. A liquid crystal display device according to claim 17,
wherein the pair of substrates respectively include vertical
alignment films on opposite surfaces, and the liquid crystal has a
negative dielectric anisotropy..Iaddend.
.Iadd.21. A liquid crystal display device according to claim 17,
wherein the stripe electrodes are the same width..Iaddend.
.Iadd.22. A liquid crystal display device according to claim 17,
wherein the stripe electrodes and spaces are arranged symmetrically
with respect to the first connection electrode..Iaddend.
.Iadd.23. A liquid crystal display device according to claim 17,
wherein adjacent stripe electrodes are parallel to each
other..Iaddend.
.Iadd.24. A liquid crystal display device according to claim 17,
wherein the connection electrode includes a second connection
electrode formed perpendicular to the first connection electrode,
and the first and the second connection electrodes define four
domains within the pixel region..Iaddend.
.Iadd.25. A liquid crystal display device according to claim 17,
comprising: a bus line for providing a signal to at least one of
the stripe electrodes, wherein the stripe electrode is connected to
the first connection electrode, the first connection electrode
being formed parallel to the bus line..Iaddend.
.Iadd.26. A liquid crystal display device comprising: a pair of
substrates disposed to be opposite to each other; a liquid crystal
sealed between the pair of substrates; a plurality of pixel regions
disposed in a matrix form; a pixel electrode formed in at least one
pixel region, the pixel electrode including at least a connection
electrode and a plurality of stripe electrodes connected to the
connection electrode, and a plurality of spaces between the stripe
electrodes, at least one of the plurality of stripe electrodes
being formed to have a different width than other stripe
electrodes, the connection electrode including a first connection
electrode formed in the center of the pixel region..Iaddend.
.Iadd.27. A liquid crystal display device according to claim 26,
comprising a photo-cured material regulating a pre-tilt angle of
the liquid crystal..Iaddend.
.Iadd.28. A liquid crystal display device according to claim 27,
wherein the photo-cured material is obtained by curing a
photo-curing compound..Iaddend.
.Iadd.29. A liquid crystal display device comprising: a pair of
substrates disposed to be opposite to each other; a liquid crystal
sealed between the pair of substrates; a plurality of pixel regions
disposed in a matrix form; a plurality of stripe electrodes formed
in the pixel regions, the stripe electrodes being placed adjacent
to and connected to each other; a plurality of spaces formed
between the stripe electrodes; a photo-cured material regulating an
alignment of the liquid crystal, wherein at least one of the pixel
regions includes a first pixel region and a second pixel region,
and at least one of (a) some of the stripe electrodes and (b) some
of the spaces in the first pixel region are different in width from
at least one of (c) some of the stripe electrodes and (d) some of
the spaces in the second pixel region..Iaddend.
.Iadd.30. A liquid crystal display device according to claim 29,
comprising: a bus line for providing a signal to at least one of
the stripe electrodes, wherein the stripe electrode is connected to
the first connection electrode, the first connection electrode
being formed parallel to the bus line..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device
used for a display part of an information equipment or the like,
and a driving method of the same.
2. Description of the Related Art
In recent years, a liquid crystal display device has been improved
so as to have a large size, high gradation, and high contrast, and
has been used for a monitor of a PC (Personal Computer) or a
television receiver or the like. In these uses, such excellent
viewing angle characteristics that a display screen can be seen in
any directions are required.
Since a color liquid crystal display device is yet inferior to a
CRT (Cathode-Ray Tube) in the viewing angle characteristics, the
realization of a wide viewing angle is desired. As a method of
widening the viewing angle of the liquid crystal display device,
there is an MVA (Multi-domain Vertical Alignment) mode. FIGS. 27A
and 27B show the schematic sectional construction of a liquid
crystal display device of the MVA mode. FIG. 27A shows a state
where a voltage is not applied to a liquid crystal layer, and FIG.
27B shows a state where a predetermined voltage is applied to the
liquid crystal layer. As shown in FIGS. 27A and 27B, the liquid
crystal display device includes substrates 302 and 304 disposed to
be opposite to each other. Transparent electrodes (not shown) are
formed on both the substrates 302 and 304. Besides, plural linear
protrusions 306 parallel to each other are formed on the one
substrate 302, and plural linear protrusions 308 parallel to each
other are formed on the other substrate 304. The protrusions 306
and 308 are alternately arranged when viewed in a direction
vertical to a substrate surface.
A liquid crystal layer 160 having a negative dielectric anisotropy
is sealed between both the substrates 302 and 304. As shown in FIG.
27A, liquid crystal molecules 312 are aligned almost vertically to
the substrate surface by the alignment regulating force of vertical
alignment films (not shown) formed on opposite surfaces of both the
substrates 302 and 304. The liquid crystal molecules 312 in the
vicinity of the protrusions 306 and 308 are aligned almost
vertically to oblique surfaces formed by the protrusions 306 and
308. That is, the liquid crystal molecules 312 in the vicinity of
the protrusions 306 and 308 are aligned obliquely with respect to
the substrate surfaces.
As shown in FIG. 27B, when the predetermined voltage is applied
between the transparent electrodes of both the substrates 302 and
304, the liquid crystal molecules 312 in the vicinity of the
protrusions 306 and 308 are inclined in the directions vertical to
the extending directions of the protrusions 306 and 308. The
inclination is propagated to the respective liquid crystal
molecules 312 between the protrusions 306 and 308, and the liquid
crystal molecules 312 in a region between the protrusions 306 and
308 are inclined in the same direction.
As stated above, by disposing alignment regulating structures such
as the protrusions 306 and 308, inclination directions of the
liquid crystal molecules 312 can be regulated for each region. When
the alignment regulating structures are formed in two directions
almost vertical to each other, the liquid crystal molecules 312 are
inclined in four directions in one pixel. As a result of the
mixture of viewing angle characteristics of the respective regions,
a wide viewing angle in a white display or a black display can be
obtained. In the liquid crystal display device of the MVA mode, ten
or more contrast ratios are obtained even at an angle of 80.degree.
or more in vertical and horizontal directions from a direction
vertical to a display screen.
In the liquid crystal display device of the MVA mode, a vertical
alignment technique to realize high contrast and high speed
response and an alignment dividing technique to realize a wide
viewing angle are combined and used. In the alignment dividing
technique, the alignment regulating structures, such as the linear
protrusions 306 and 308 or electrode removal parts (slits), are
formed on the substrates. Since the alignment directions of the
liquid crystal molecules 312 are regulated by these alignment
regulating structures, and a rubbing treatment which becomes the
great cause of a drop in productivity becomes unnecessary, high
productivity is realized.
Besides, in order to realize the liquid crystal display device of
the MVA mode having higher display quality, there is a technique in
which a photo-cured material is formed in a liquid crystal layer
160 so that the alignment regulating force of the liquid crystal
molecules 312 is increased. A liquid crystal containing a
photo-curing composition (resin) is injected in a liquid crystal
display panel, and the photo-cured material is formed in a state
where a voltage is applied, so that a predetermined pre-tilt angle
can be given to the whole of each of aligned regions divided by the
alignment regulating structures. By this, alignment abnormal
regions of the liquid crystal molecules 312 are decreased and high
transmission factor can be realized, and further, since the
propagation of inclination of the liquid crystal molecules 312
becomes almost unnecessary, a high speed response can be
realized.
As the alignment regulating structures, in addition to the
protrusions 306 and 308 and the slits, there is also a minute
electrode pattern. FIG. 28 shows one pixel in which the minute
electrode pattern is formed. As shown in FIG. 28, plural gate bus
lines 104 (only one line is shown in FIG. 28) extending in the
horizontal direction in the drawing, and plural drain bus lines 106
(two lines are shown in the drawing) intersecting with the gate bus
lines 104 through a not-shown insulating film and extending in the
vertical direction in the drawing are formed on a TFT substrate
102. A TFT 110 is formed in the vicinity of an intersecting
position of the gate bus line 104 and the drain bus line 106.
Besides, a storage capacitor bus line 108 is formed to cross
substantially the center of a rectangular pixel region defined by
the gate bus line 104 and the drain bus line 106.
Cross-shaped connection electrodes 120 and 122 are formed in the
rectangular pixel region to divide it in four rectangles of the
same shape. The connection electrode 122 is formed to cross the
center of the pixel region and to be parallel to the drain bus line
106, and the connection electrode 120 is formed on the storage
capacitor bus line 108. Besides, plural stripe electrodes 124 are
formed which extend from the connection electrodes 120 and 122 at
an angle of 45.degree. and form the minute electrode pattern. A
space 126 in a state where an electrode is removed is formed
between the adjacent stripe electrodes 124. A pixel electrode is
constituted by the connection electrodes 120 and 122, the plural
stripe electrodes 124 and the spaces 126. Besides, alignment
regulating structures are constituted by the stripe electrodes 124
and the spaces 126. Each of the stripe electrodes 124 is formed to
have a width L1, and each of the spaces 126 is formed to have a
width S1.
FIGS. 29 and 30 show a section of the liquid crystal display device
taken along line B-B of FIG. 28. FIG. 29 shows a state where a
voltage is not applied to the liquid crystal layer 160, and FIG. 30
shows a state where a voltage is applied to the liquid crystal
layer 160. As shown in FIGS. 29 and 30, the TFT substrate 102
includes the stripe electrodes 124 on a glass substrate 150. An
opposite substrate 103 disposed to be opposite to the TFT substrate
102 includes a common electrode 154 on a glass substrate 151.
Vertical alignment films 152 and 153 are formed on surfaces of the
TFT substrate 102 and the opposite substrate 103 in contact with
the liquid crystal layer 160, respectively.
In the state where the voltage is not applied to the liquid crystal
layer 160, as shown in FIG. 29, the liquid crystal molecules 312
are aligned almost vertically to the substrate surface. In the
state where the voltage is applied to the liquid crystal layer 160,
as shown in FIG. 30, the liquid crystal molecules 312 are inclined
toward the connection electrodes 122 and 124 in the extending
directions of the stripe electrodes 124, and are aligned almost
parallel to the substrate surface.
Also by the construction shown in FIG. 28, by dividing the
alignment directions of the liquid crystal molecules 312 in
quarters in one pixel, a wide viewing angle is obtained in a white
display or a black display. However, since the alignment regulating
force of the liquid crystal molecules 312 caused by only the minute
electrode pattern is low, similarly to the above, a photo-cured
material is formed in the liquid crystal layer 160 and the
alignment regulating force is increased. The photo-cured material
is formed in such a way that a photo-curing composition (monomer)
capable of being polymerized by light is mixed in the liquid
crystal layer 160, and irradiated by light such as ultraviolet rays
(UV) in a state where a predetermined voltage is applied.
FIG. 31 is a graph showing transmission characteristics (T-V
characteristics) of the MVA mode liquid crystal display device. The
horizontal axis indicates applied voltage (V) to the liquid crystal
layer 160, and the vertical axis indicates transmission factor (%)
of light. A curved line A expressed by a solid line in the graph
indicates a T-V characteristic in a direction vertical to a display
screen (hereinafter referred to as "front direction"), and a curved
line B expressed by a solid line plotted by .DELTA. marks indicates
a T-V characteristic in a direction of an azimuth angle of
90.degree. and a polar angle of 60.degree. (hereinafter referred to
as "oblique direction"). Here, the azimuth angle is an angle
measured in the counterclockwise direction from almost the center
of the display screen on the basis of the horizontal direction. The
polar angle is an angle with respect to the vertical line drawn at
the center of the display screen. The display mode of the liquid
crystal display device is a normally black mode in which an applied
voltage to the liquid crystal layer 160 is lowered to display
black, and an applied voltage is raised to display white. It is
desirable that the T-V characteristics are constant independent of
the viewing angle.
However, as shown in FIG. 31, the curved line A indicating the T-V
characteristic in the front direction intersects the curved line B
indicating the T-V characteristic at a point in the vicinity of an
applied voltage of about 2.7 V. The transmission factor in the
oblique direction is higher than the transmission factor in the
front direction at an applied voltage of 2.7 V or less, and is
lower than the transmission factor in the front direction at an
applied voltage of 2.7 V or higher. Thus, in the range of the
applied voltage of from 1.5 V to 2.7 V, since the transmission
factor in the oblique direction is higher than that in the front
direction, there arises a problem that when viewed in the oblique
direction, the display image is seen to be whitish. Besides, since
the transmission factor in the oblique direction is lower than the
transmission factor in the front direction in the range of a
relatively high applied voltage, when viewed in the oblique
direction, the contrast in the whole display screen is lowered.
The transmission factor is varied in accordance with the
retardation (.DELTA.nd) of the liquid crystal layer 160. When
viewed in the oblique direction, since the substantial retardation
of the liquid crystal layer 160 is lessened by the liquid crystal
molecules 312 inclined in the oblique direction, the above problem
arises. Besides, also with respect to chromaticity, since the
weight of the transmission factor from each pixel is changed
between a case where it is seen in the front direction and a case
where it is seen in the oblique direction, there arises a problem
that the chromaticity is changed.
FIG. 32 is a graph showing the T-V characteristics when the display
screen of the MVA mode liquid crystal display device is observed at
plural viewing angles. The horizontal axis indicates the applied
voltage (V) to the liquid crystal layer 160, and the vertical axis
indicates the transmission factor (%) of light. A curved line A in
the graph indicates the T-V characteristic in the front direction.
Curved lines B, C, D and E indicate the T-V characteristics in the
directions of an azimuth angle of 90.degree. and polar angles of
20.degree., 40.degree., 60.degree. and 80.degree., respectively. As
shown in FIG. 32, an undulation occurs on the curved line E in a
region F, and there is a range in which even if the applied voltage
is raised, the transmission factor is lowered. Thus, there arises a
problem that a display image is reversed between the front
direction and the direction of a polar angle of 80.degree..
SUMMARY OF THE INVENTION
An object of the present invention is to provide a liquid crystal
display device in which excellent viewing angle characteristics can
be obtained, and a driving method thereof.
The above object is achieved by a driving method of a liquid
crystal display device characterized in that a driving voltage is
applied to a liquid crystal of a pixel only in a predetermined time
in one frame period, and an application time of the driving voltage
is changed to cause the pixel to display a predetermined
gradation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing a schematic construction of a liquid
crystal display device according to a first embodiment of the
present invention;
FIGS. 2A to 2C are views for explaining viewing angle
characteristics of the liquid crystal display device according to
the alignment state of liquid crystal molecules;
FIG. 3 is a graph showing the time change of transmission factor of
the liquid crystal display device according to the first embodiment
of the invention;
FIG. 4 is a graph showing the change of transmission factor with
respect to the change of a duty ratio of a liquid crystal display
device according to example 1-1 of the first embodiment of the
invention;
FIG. 5 is a graph showing an effect of the liquid crystal display
device according to the example 1-1 of the first embodiment of the
invention;
FIGS. 6A and 6B are conceptual views showing a driving method of a
liquid crystal display device according to example 1-2 of the first
embodiment of the invention;
FIG. 7 is a view showing an equivalent circuit of the liquid
crystal display device according to the example 1-2 of the first
embodiment of the invention;
FIG. 8 is a view showing the construction of the liquid crystal
display device according to the example 1-2 of the first embodiment
of the invention;
FIG. 9 is a sectional view showing the construction of the liquid
crystal display device according to the example 1-2 of the first
embodiment of the invention;
FIG. 10 is a sectional view showing the construction of the liquid
crystal display device according to the example 1-2 of the first
embodiment of the invention;
FIGS. 11A to 11F are views showing driving waveforms of the liquid
crystal display device according to the example 1-2 of the first
embodiment of the invention;
FIG. 12 is a view showing a modified example of the construction of
the liquid crystal display device according to the example 1-2 of
the first embodiment of the invention;
FIG. 13 is a sectional view showing the modified example of the
construction of the liquid crystal display device according to the
example 1-2 of the first embodiment of the invention;
FIG. 14 is a sectional view showing a modified example of the
construction of the liquid crystal display device according to the
example 1-2 of the first embodiment of the invention;
FIG. 15 is a sectional view showing the modified example of the
construction of the liquid crystal display device according to the
example 1-2 of the first embodiment of the invention;
FIG. 16 is a view showing the construction of a liquid crystal
display device according to a second embodiment of the
invention;
FIG. 17 is a sectional view showing the construction of the liquid
crystal display device according to the second embodiment of the
invention;
FIG. 18 is a graph showing T-V characteristics of the liquid
crystal display device according to the second embodiment of the
invention;
FIG. 19 is a graph showing an effect of the liquid crystal display
device according to the second embodiment of the invention;
FIGS. 20A and 20B are views showing the principle of a liquid
crystal display device according to a third embodiment of the
invention;
FIG. 21 is a view showing the construction of the liquid crystal
display device according to the third embodiment of the
invention;
FIG. 22 is a graph showing an effect of the liquid crystal display
device according to the third embodiment of the invention;
FIG. 23 is a view showing a modified example of the construction of
the liquid crystal display device according to the third embodiment
of the invention;
FIG. 24 is a view showing another modified example of the
construction of the liquid crystal display device according to the
third embodiment of the invention;
FIG. 25 is a view showing still another modified example of the
construction of the liquid crystal display device according to the
third embodiment of the invention;
FIGS. 26A and 26B are graphs showing an effect of the liquid
crystal display device according to the third embodiment of the
invention;
FIGS. 27A and 27B are sectional views showing the construction of a
conventional MVA mode liquid crystal display device;
FIG. 28 is a view showing the construction of a conventional MVA
mode liquid crystal display device;
FIG. 29 is a sectional view showing the construction of the
conventional MVA mode liquid crystal display device;
FIG. 30 is a sectional view showing the construction of the
conventional MVA mode liquid crystal display device;
FIG. 31 is a graph showing T-V characteristics of the conventional
MVA mode liquid crystal display device; and
FIG. 32 is a graph showing T-V characteristics of the conventional
MVA mode liquid crystal display device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
A liquid crystal display device according to a first embodiment of
the present invention and a driving method thereof will be
described with reference to FIGS. 1 to 15. FIG. 1 shows a schematic
construction of the liquid crystal display device according to this
embodiment. The liquid crystal display device has such a
construction that a TFT substrate 2 on which a thin film transistor
(TFT) and the like are formed and an opposite substrate 4 on which
a color filter (CF) and the like are formed are made to face each
other and are bonded, and a liquid crystal is sealed between the
substrates 2 and 4.
Gate bus line driving circuits 5 and a data bus line driving
circuits 6, on each of which a driver IC for driving plural bus
lines are mounted, are provided on the TFT substrate 2. These
driving circuits 5 and 6 output a scanning signal and a data signal
to a predetermined gate bus line or drain bus line on the basis of
a predetermined signal outputted from a control circuit 7. A
polarizing plate 8 is disposed on a substrate surface of the TFT
substrate 2 opposite to its element formation surface, and a back
light unit 3 is attached to a surface opposite to the TFT substrate
2 of the polarizing plate 8. On the other hand, a polarizing plate
9 disposed in crossed Nicols with respect to the polarizing plate 8
is bonded to a surface opposite to the CF formation surface of the
opposite substrate 4.
In this embodiment, a liquid crystal display device of a normally
black mode is used in which a liquid crystal having a negative
dielectric anisotropy and vertically aligned at the time of no
voltage application is sealed. The change of viewing angle
characteristics according to the alignment state of liquid crystal
molecules will be described with reference to FIGS. 2A to 2C. FIG.
2A shows the alignment state of the liquid crystal molecules when
black is displayed, and FIG. 2B shows the alignment state of the
liquid crystal molecules when white is displayed. FIG. 2C shows the
alignment state of the liquid crystal molecules when a half tone is
displayed. As shown in FIG. 2A, in a state where a voltage is not
applied between a pixel electrode 60 formed on a glass substrate 58
and a common electrode 62 formed on a glass substrate 59, liquid
crystal molecules 56 are aligned almost vertically to the substrate
surface. In this state, when viewed in the front direction, since
the retardation occurring in the liquid crystal layer 57 becomes
approximately zero, black is displayed. Besides, also when viewed
in the oblique direction, as shown in FIG. 31, a black display
having substantially the same transmission factor as that in the
front direction is obtained.
As shown in FIG. 2B, in a state where a predetermined voltage is
applied between the pixel electrode 60 and the common electrode 62,
and the liquid crystal molecules 56 are aligned parallel to the
substrate surface, when viewed in the front direction, since the
retardation occurring in the liquid crystal layer 57 becomes
approximately .lamda./2, white is displayed. Besides, when viewed
in the oblique direction, the transmission factor becomes slightly
lower than that in the front direction, and a gray display is
produced (see FIG. 31).
Besides, as shown in FIG. 2C, in the state where the liquid crystal
molecules 56 are inclined at a predetermined angle with respect to
the substrate surface and are aligned, the retardation of the
liquid crystal layer 57 is changed in the range of from 0 to
.lamda./2, and a half tone is displayed. However, although this
half tone display becomes appropriate when viewed in the front
direction, when viewed in the oblique direction, since the
retardation of the liquid crystal layer 57 is changed in accordance
with the viewing angle, a desired gradation can not be obtained.
For example, when viewed in the oblique direction as shown in FIG.
2C, although gray should be originally displayed, a whitish display
is produced.
In the T-V characteristics shown in FIG. 31, as already described,
in the half tone display in the range in which the applied voltage
is from 1.5 V to 2.7 V, since the transmission factor in the
oblique direction is higher than that in the front direction, the
display image is seen to be whitish. For example, in FIG. 31, in
the case where a voltage of 2 V is applied to obtain a transmission
factor of 1% when viewed in the front direction, the transmission
factor in the oblique direction is approximately 5 times as high as
that. As stated above, in the half tone display in the range where
the applied voltage is from 1.5 V to 2.7 V, the retardation of the
liquid crystal layer 57 is greatly changed in dependence on the
viewing angle, and consequently, the displayed half tone is greatly
changed in accordance with the viewing angle.
On the other hand, in the case where a white image is displayed,
that is, as shown in FIG. 31, in the case where a maximum gradation
voltage of 5 V is applied to the liquid crystal layer 57 and a
transmission factor of 30% when viewed in the front direction is
obtained, a transmission factor approximately 2/3 as high as that
can be obtained in the oblique direction. In this case, only the
contrast is somewhat lowered, and a whitish image does not occur.
Further, as shown in FIG. 31, in the case where a black image is
displayed, variation in transmission factor due to the viewing
angle becomes less. In this embodiment, attention is paid to this
point, and a driving method as shown in FIG. 3 is adopted.
FIG. 3 is a graph showing the time changes of an applied voltage to
the liquid crystal layer 57 of the liquid crystal display device of
the normally black mode according to this embodiment and a
transmission factor. The graph (a) in the upper part of FIG. 3
shows the time change of the applied voltage to the liquid crystal
layer 57 in a pixel, and the graph (b) in the lower part shows the
time change of the transmission factor of the pixel. Both the
graphs (a) and (b) are expressed by a common horizontal axis (time
axis: msec). The vertical axis of the upper graph (a) indicates the
applied voltage (V), and the vertical axis of the lower graph (b)
indicates the transmission factor (%) of light. As shown in the
graph (a), the period of a frame f.sub.n is 16.7 msec ( 1/60 sec),
and the liquid crystal layer is driven by a frame reversal driving
system in which the polarity of a gradation voltage is reversed for
each frame f.sub.n. A gradation voltage P applied to the pixel is
continuously kept at an on level of +5.0 V in, for example, first
50% period ( 1/120 sec) of the positive polarity frame f.sub.n.
Next, the gradation voltage P comes to have an off level, and is
kept at a common potential (for example, 0 V) in the remaining 50%
period ( 1/120 sec).
In a next frame f.sub.n+1, the gradation voltage P applied to the
pixel comes to have the on level, and is continuously kept at a
negative polarity voltage level of -5.0 V in the first 50% period (
1/120 sec). Next, the gradation voltage P comes to have the off
level in the remaining 50% period ( 1/120 sec) and is kept at the
common potential. In this example, the gradation voltage P has the
on level only in the period of 50% (duty ratio 50%) of one frame
period. By changing the duty ratio, plural gradations can be
displayed. As stated above, in the driving method of the liquid
crystal display device according to this embodiment, the driving
voltage is applied to the liquid crystal of the pixel only in the
predetermined time in one frame period, and the application time of
the driving voltage is changed to cause the pixel to display a
predetermined gradation. Not only the application time of the
driving voltage, but also the voltage level of the driving voltage
may be changed.
A curved line C in the lower graph (b) of FIG. 3 indicates the
transmission factor in the front direction, and a curved line D
indicates the transmission factor in the oblique direction. As
shown in the graph (b), the transmission factors of light in the
front direction and the oblique direction are changed in accordance
with the gradation voltage P. When the gradation voltage P is at
the on level (.+-.5.0 V), as indicated by the curved line C and the
curved line D, the transmission factor in the front direction is
larger than the transmission factor in the oblique direction. When
the gradation voltage P is at the off level (0 V), although there
is a portion in which the transmission factor in the oblique
direction is larger than the transmission factor in the front
direction, an average transmission factor in the whole period of
one frame f.sub.n in the front direction becomes larger than that
in the oblique direction. By this, it is possible to prevent a
whitish image which occurs when the transmission factor in the
oblique direction becomes larger than the transmission factor in
the front direction. Accordingly, when the magnitude of the
gradation voltage P applied to each pixel is made the maximum
gradation voltage (for example, .+-.5 V) as described above, and
the duty ratio at which the gradation voltage P is applied is
changed, a whitish display is suppressed and it becomes possible to
carry out the gradation display in each pixel.
Besides, when the gradation is set by using the magnitude of the
gradation voltage P at the on level and the duty ratio as
parameters, the ratio of the transmission factor in the front
direction to that in the oblique direction at an arbitrary applied
voltage shown in FIG. 31 can be applied. Accordingly, by optimizing
the magnitude of the gradation voltage P at the on level and the
duty ratio in view of the transmission factor characteristics and
the response characteristics, excellent viewing angle
characteristics can be obtained over all the gradations. For
example, the voltage applied to the liquid crystal layer 57 is made
a voltage by which the vertical alignment state and the parallel
alignment state excellent in the viewing angle characteristics are
obtained, and on the basis of the transmission factor
characteristics corresponding to those alignment states, the
application time of the voltage is changed and the gradation image
is displayed, so that excellent viewing angle characteristics can
be obtained even in the half tone display.
Besides, this embodiment has a function to suppress chromaticity
change due to gradations when viewed in the front direction and in
the oblique direction. In the example shown in the lower graph (b)
of FIG. 3, as stated above, the optical characteristic
(retardation) when a voltage of 5.0 V is applied is reflected in
all gradation displays. Accordingly, since chromaticity
corresponding to the retardation occurring in the liquid crystal
layer 57 when the voltage of 5.0 V is applied becomes dominant in
all the gradations, the chromaticity change due to the gradation is
greatly lessened, and excellent display characteristics can be
obtained. Besides, when each gradation is set by using the
magnitude of the applied voltage and the application time of the
voltage as parameters, chromaticity corresponding to an arbitrary
retardation can be applied. Accordingly, by selecting the magnitude
of the voltage and the application time of the voltage in view of
the transmission factor characteristics and the response
characteristics, excellent viewing angle characteristics can be
obtained in which the chromaticity change due to the gradation is
small even when viewed in the oblique direction.
In order to decrease the contribution of optical characteristics in
the process where the inclination angle of the liquid crystal
molecule 56 is changed, it is desirable to use a liquid crystal in
which the transmission factor change caused by the change of
applied voltage is steep, that is, a liquid crystal panel excellent
in optical responsiveness. Accordingly, when this embodiment is
applied to a liquid crystal display panel excellent in high speed
responsiveness, more excellent gradation viewing angle
characteristics can be obtained. For similar reasons, when a
voltage or a voltage range in which a liquid crystal can respond at
high speed is used for a driving voltage range, more superior
gradation viewing angle characteristics can be obtained.
Hereinafter, the liquid crystal display device according to this
embodiment and the driving method thereof will be described in more
specifically by use of examples 1-1 and 1 -2.
EXAMPLE 1-1
A liquid crystal display device according to example 1-1 of this
embodiment and a driving method thereof will be described with
reference to FIGS. 4 and 5. First, the liquid crystal display
device used for this example will be described with reference to
FIGS. 1, 27A and 27B. Plural slits each having a width of 10 .mu.m
are formed to be parallel to each other at intervals of 70 .mu.m on
the TFT substrate 2. Plural linear protrusions each having a height
of 1.2 .mu.m, a width of 10 .mu.m and insulating properties are
formed to be parallel to each other at intervals of 70 .mu.m on the
opposite substrate 4. Vertical alignment films (for example, made
by JSR Corporation) are coated on both the substrates 2 and 4,
spherical spacers each having a diameter of 3.0 .mu.m are scattered
and both the substrates 2 and 4 are bonded so that alignment
regulating structures are alternately disposed. A liquid crystal
(for example, made by Merck Ltd.) having a negative dielectric
anisotropy is sealed between the substrates 2 and 4.
Next, the driving method of the liquid crystal display device
according to this example will be described. In the above liquid
crystal display device, a gradation display of each pixel is
performed by changing a duty ratio for each pixel at a frame
reversal frequency of 30 Hz (frame period 1/60 sec). The liquid
crystal molecules 56 of the liquid crystal display device according
to this example are aligned almost vertically to the substrate
surface in the state of no voltage application, and is aligned
almost parallel to the substrate surface in the state where a
voltage of .+-.5.0 V is applied.
FIG. 4 is a graph showing the change of a transmission factor with
respect to the change of a duty ratio in the case where the driving
method of the liquid crystal display device according to this
example is used. The horizontal axis indicates the duty ratio of
the gradation voltage P applied to the liquid crystal layer 57, and
the vertical axis indicates the transmission factor (%) of light. A
curved line E expressed by a solid line in the graph indicates the
transmission factor in the front direction, and a curved line F
expressed by a solid line plotted by .diamond. marks indicates the
transmission factor in the oblique direction. As shown in FIG. 4,
according to this example, a difference between the transmission
factor in the front direction and the transmission factor in the
oblique direction becomes very small as compared with the case
where the conventional driving method of the liquid crystal display
device is used as shown in FIG. 31, and excellent gradation viewing
angle characteristics are obtained.
FIG. 5 is a graph showing display characteristics in the case where
the driving method of the liquid crystal display device according
to this example is used and those in the conventional case. The
horizontal axis indicates normalized transmission factor. With
respect to the conventional driving method, the respective
transmission factors are normalized by a transmission factor at the
time when a gradation voltage is 5.0 V. With respect to the driving
method of this example, the respective transmission factors are
normalized by a transmission factor at the time when a gradation
voltage of 5.0 V is applied to the whole of one frame. The vertical
axis indicates the logarithm of the ratio (transmission factor
ratio) of the transmission factor in the oblique direction to the
transmission factor in the front direction. A curved line G
expressed by a solid line plotted by .DELTA. marks indicates the
transmission factor ratio in the case where the driving method of
the conventional liquid crystal display device is used, and a
curved line H expressed by a solid line plotted by .diamond. marks
indicates the transmission factor ratio of the case where the
driving method of the liquid crystal display device according to
this example is used. In the graph shown in FIG. 5, when the
transmission factor ratio is constant independent of the normalized
transmission factor, that is, the profile is flat, gradation
viewing angle characteristics are superior. Besides, when the
transmission factor ratio is close to 1.0, the gradation viewing
angle characteristics are more superior. As shown in FIG. 5,
according to this example, the transmission factor ratio at a low
transmission factor, which is conventionally high, is lowered and
the profile is flatter, and further, the transmission factor ratio
is close to 1.0 on the whole. Accordingly, it is understood that
superior gradation viewing angle characteristics can be obtained as
compared with the conventional example.
According to this example, the chromaticity change due to gradation
is also greatly improved. For example, with respect to white
chromaticity, although a chromaticity shift of about 0.04 occurs
conventionally in an x-y chromaticity diagram, a chromaticity shift
in this example can be suppressed to be less than 0.01. Besides,
also with respect to the chromaticity change of single color of red
(R), green (G) and blue (B), which is difficult to improve, a
similar effect can be obtained.
In this embodiment, although the MVA mode liquid crystal display
device has been taken as an example, the gradation viewing angle
characteristics can be similarly improved in a liquid crystal
display device of another mode. For example, also in the liquid
crystal display device of homogeneous orientation in which a liquid
crystal having a positive dielectric anisotropy is sealed,
excellent viewing angle characteristics can be obtained by applying
this embodiment.
EXAMPLE 1-2
Next, a liquid crystal display device according to example 1-2 of
this embodiment and a driving method thereof will be described with
reference to FIGS. 6A to 15. FIGS. 6A and 6B are conceptual views
showing the driving method of the liquid crystal display device
according to this example. FIG. 6A shows a driving state in a case
where gray close to black is displayed, and FIG. 6B shows a driving
state in a case where gray close to white is displayed. In FIGS. 6A
and 6B, the horizontal direction indicates a time of one frame. As
shown in FIG. 6A, in the case where gray close to black is
displayed, for example, white is displayed in a 1/4 period of one
frame period (duty ratio 25%), and black is displayed in the
remaining period. Besides, as shown in FIG. 6B, in the case where
gray close to white is displayed, for example, white is displayed
in a period of one frame period (duty ratio 83%), and black is
displayed in the remaining period. As stated above, in this
example, only the black display and the white display are used, and
a time when white is displayed is changed in one frame period, so
that a half tone display is realized by time division (duty
ratio).
For example, in a PDP (Plasma Display Panel), 64 gradations are
realized by combining plural subfields (for example, 1, 2, 4, 8,
16, 32) in which relative ratios of luminance are different.
However, in the liquid crystal display device, when the response
characteristics of a liquid crystal and the response
characteristics of a TFT are considered, it is very difficult to
adopt the method used for the PDP. On the other hand, when this
example is adopted, a multi-gradation display by time division can
be easily realized using a normal liquid crystal and TFT.
Next, the driving method of the liquid crystal display device
according to this example will be described. FIG. 7 shows an
equivalent circuit of the liquid crystal display device according
to this example for one pixel. As shown in FIG. 7, plural data bus
lines 74 and plural driving voltage bus lines 76 are formed
substantially parallel to each other in the vertical direction in
the drawing (in FIG. 7, only one line is shown for the respective
lines). Plural gate bus lines 12 are formed to intersect the data
bus lines 74 and the driving voltage bus lines 76 at almost right
angles through a not-shown insulating film (in FIG. 7, only one
line is shown). The respective data bus lines 74 and the respective
driving voltage bus lines 76 are driven by a data bus line driving
circuit 6, and the respective gate bus lines 12 are driven by a
gate bus line driving circuit 5.
Two TFTs 70 and 72 are formed in one pixel. A gate electrode G1 of
the first TFT 70 is connected to the gate bus line 12. A drain
electrode D1 of the TFT 70 is connected to the data bus line 74. A
source electrode S1 of the TFT 70 is connected to one ends of a
capacitance (first capacitance) C1 and a resistance (first
resistance) R1 which are connected in parallel with each other, and
is further connected to a gate electrode G2 of the second TFT 72.
The other ends of the capacitance C1 and the resistance R1 are
connected to a not-shown power source circuit. A drain electrode D2
of the TFT 72 is connected to the driving voltage bus line 76, and
a source electrode S2 is connected to a pixel electrode 60. A
liquid crystal capacitance (second capacitance) Clc is formed by
the pixel electrode 60, a common electrode, and a liquid crystal
sandwiched therebetween. Besides, a resistance (second resistance)
R2 is formed of a liquid crystal layer. By this, the source
electrode S2 of the TFT 72 is connected to one ends of the liquid
crystal capacitance Clc and the resistance R2 which are connected
in parallel with each other. The other ends of the liquid crystal
capacitance Clc and the resistance R2 are kept at a common
potential.
When a predetermined gate voltage is applied to the gate electrode
G1 through the gate bus line 12, the TFT 70 is turned on. When the
TFT 70 is turned on, a data voltage applied to the data bus line 74
is applied to the gate electrode G2 of the TFT 72, and a
predetermined electric charge is charged to the capacitance C1.
When the data voltage exceeding a threshold voltage is applied to
the gate electrode G2 of the TFT 72, the TFT 72 is turned on, and a
driving voltage from the bus line 76 is applied to the pixel
electrode 60.
When the TFT 70 is turned off, a gate voltage Vg2 of the gate
electrode G2 is lowered with the lapse of time in accordance with a
time constant depending on the capacitance C1 and the resistance
R1. When the gate voltage Vg2 becomes the threshold voltage or
less, the TFT 72 is turned off. When the TFT 72 is turned off, the
driving voltage applied to the pixel electrode 60 is lowered with
the lapse of time in accordance with a time constant depending on
the liquid crystal capacitance Clc and the resistance R2.
When the data voltage of a relatively high level is applied from
the data bus line 74 through the TFT 70 to the capacitance C1, the
resistance R1 and the gate electrode G2 of the TFT 72, since the
TFT 72 keeps the on state for a relatively long time, the driving
voltage is applied to the liquid crystal layer 57 for a long time,
and a white display is realized.
When the data voltage of a relatively low level (threshold voltage
of the TFT 72 or less) is applied from the data bus line 74 through
the TFT 70 to the capacitance C1, the resistance R1 and the gate
electrode G2 of the TFT 72, since the TFT 72 keeps the off state,
the driving voltage is not applied to the liquid crystal layer 57,
and a black display is realized.
When the data voltage of an intermediate level between the high
level and the low level is applied from the data bus line 74
through the TFT 70 to the capacitance C1, the resistance R1 and the
gate electrode G2 of the TFT 72, after the TFT 70 is turned off,
the TFT 72 keeps the on state for a time determined by the time
constant depending on the capacitance C1 and the resistance R1. The
driving voltage is applied to the liquid crystal layer 57 for the
on time. By this, a half tone display is realized according to the
ratio of the on time of the TFT 72 in one frame period.
FIG. 8 shows the construction of one pixel of the liquid crystal
display device according to this example. FIG. 9 is a sectional
view taken along line C-C of FIG. 8, and FIG. 10 is a sectional
view taken along line D-D of FIG. 8. As shown in FIGS. 8 to 10, the
gate bus line 12 extending in the horizontal direction and a common
bus line 78 extending substantially parallel to the gate bus line
12 are disposed to have a predetermined interval therebetween and
to be adjacent to each other and is formed of the same formation
material on the TFT substrate 2. The data bus line 74 and the
driving voltage bus line 76 are formed at both sides of a pixel
region to intersect the gate bus line 12 and the common bus line 78
at almost right angles through an insulating film. The data bus
line 74 and the driving voltage bus line 76 are formed of the same
formation material.
The TFT 70 is formed in the vicinity of an intersection position of
the gate bus line 12 and the data bus line 74. The drain electrode
D1 of the TFT 70 is connected to the data bus line 74. The source
electrode S1 is connected to a connection wiring line 84 formed
substantially parallel to the common bus line 78 through a contact
hole 82. A source wiring line 86 substantially parallel to the
common bus line 78 extends from the source electrode S1. The source
wiring line 86 is connected to the common bus line 78 through a
dielectric 90 formed on a contact hole 80 and having a relatively
small resistance value. A resist or ultraviolet curing resin is
used for a formation material of the dielectric 90. The connection
wiring line 84 is connected to the gate electrode G2 of the TFT 72.
The drain electrode D2 of the TFT 72 is connected to the driving
voltage bus line 76, and the source electrode S2 is connected to
the pixel electrode 60 through a contact hole 222. In this example,
the dielectric 90 functions as the resistance R1 and the dielectric
of the capacitance C1. Besides, the liquid crystal layer 57
functions as the resistance R2.
Next, the driving method of the liquid crystal display device
according to this example will be described on the basis of
specific examples. FIGS. 11A to 11F show driving waveforms of two
frames of the liquid crystal display device according to this
example. FIG. 11A shows a waveform of a data voltage Vd applied to
the data bus line 74, and FIG. 11B shows a waveform of a gate
voltage Vg1 applied to the gate electrode G1 of the TFT 70. FIG.
11C shows a waveform of a gate voltage Vg2 applied to the gate
electrode G2 of the TFT 72, and FIG. 11D shows waveforms of a
liquid crystal driving voltage Vdd applied to the driving voltage
bus line 76 and a common voltage Vcom. Besides, FIG. 11E shows a
waveform of a gradation voltage Vp applied to the pixel electrode
60, and FIG. 11F shows, for example, a waveform of display
luminance Tp in the front direction of a pixel. The horizontal
direction in the drawing indicates a time. The vertical direction
of FIGS. 11A to 11E indicates a voltage level, and the vertical
direction of FIG. 11F indicates a luminance level.
In this example, as shown in FIG. 11D, the common voltage is
Vcom=+5 V. Then, a positive polarity driving voltage Vddp=+10 V and
a reverse polarity driving voltage Vddn=0 V are alternately
outputted for each frame f to the driving voltage bus line 76. By
this, the liquid crystal layer 57 is subjected to frame reversal
driving with the driving voltage of .+-.5 V. Hereinafter, a frame
period in which the positive polarity driving voltage Vddp is
applied to the driving voltage bus line 76 is called a positive
polarity frame period, and a frame period in which the reverse
polarity driving voltage Vddn is applied to the driving voltage bus
line 76 is called a reverse polarity frame period.
The positive polarity driving voltage Vddp=+10 V is applied to the
driving voltage bus line 76 in the positive polarity frame period.
In order to certainly turn off the TFT 72 in this positive polarity
frame period, it is necessary that the gate voltage Vg2 of the TFT
72 is made lower than the minimum value of the drain voltage (that
is, the positive polarity driving voltage Vddp=+10 V) by about 5 V.
Besides, in order to certainly turn on the TFT 72 in the positive
polarity frame period, it is necessary that the gate voltage Vg2 of
the TFT 72 is made higher than the maximum value of the drain
voltage (that is, the positive polarity driving voltage Vddp=+10
V). Then, as shown in FIG. 11A, in the positive polarity frame
period, a voltage between +5 V and +15 V (voltage width of 10 V) is
applied as the data voltage Vdp applied to the data bus line
74.
The reverse polarity driving voltage Vddn=0 V is applied to the
driving voltage bus line 76 in the reverse polarity frame period.
In order to certainly turn off the TFT 72 in the reverse polarity
frame period, it is necessary that the gate voltage Vg2 of the TFT
72 is made lower than the minimum value of the drain voltage (that
is, the reverse polarity driving voltage Vddn=0 V) by about 5 V.
Besides, in order to certainly turn on the TFT 72 in the reverse
polarity frame period, it is necessary that the gate voltage Vg2 of
the TFT 72 is made higher than the maximum value of the drain
voltage (that is, the reverse polarity driving voltage Vddn=0 V).
Then, as shown in FIG. 11A, in the reverse polarity frame period, a
voltage between -5 V and +5 V (voltage width of 10 V) is applied as
the data voltage Vdn applied to the data bus line 74.
Accordingly, as shown in FIG. 11A, the data voltage Vd is varied
within a voltage width of 20 V from -5 V to +15 V. Thus, in order
to certainly make the TFT 70 operate as a switching element, as
shown in FIG. 11B, the gate voltage Vg1 applied to the gate bus
line 12 is made the off time voltage Vg1 (off)=-10 V and the on
time voltage Vg1 (on)=+20 V of TFT 70.
Next, a driving operation will be described in sequence.
(1) In the case of positive polarity frame period:
For example, it is assumed that the gradation voltage Vdp=+12 V is
outputted to the data bus line 74 (see FIG. 11A). Next, the gate
pulse Vg1 (on) is outputted to the gate bus line 12 and the TFT 70
is turned on. When the TFT 70 is in the on state, the data voltage
Vdp is applied to the gate electrode G2 of the TFT 72, and the
capacitance C1 is charged. The gate voltage Vg2 (onp) (=+12 V) is
applied to the gate electrode G2 of the TFT 72 as shown in FIG. 1C,
and the TFT 72 is turned on.
Next, when the gate voltage becomes Vg1 (off) and the TFT 70 is
turned off, the electric charge of the capacitance C1 is discharged
at a predetermined time constant, and as shown in FIG. 1C, the
level of the voltage applied to the gate electrode G2 of the TFT 72
is gradually decreased. This voltage level gradually approaches the
threshold voltage Vth of the TFT 72, then it becomes less than
that, and finally becomes Vg2 (offp) (=Vcom=+5 V).
As stated above, the on time of the TFT 72 is determined by the
magnitude of the data voltage Vdp supplied to the gate electrode G2
and the time constant of attenuation depending on the capacitance
C1 and the resistance R1. In the on state of the TFT 72, the
positive polarity driving voltage Vddp=+10 V shown in FIG. 11D is
written in the pixel electrode 60 as the gradation voltage Vp, and
the voltage level is kept during the on time of the TFT 72 (see
FIG. 11E). In this period, as shown in FIG. 11F, the predetermined
transmission factor Tp is obtained in the liquid crystal layer.
When the gate voltage Vg2 of the TFT 72 becomes the predetermined
threshold voltage Vth or less, the TFT 72 is turned off, and the
gradation voltage Vp is decreased to the common voltage Vcom at the
time constant depending on the liquid crystal capacitance Clc and
the liquid crystal resistance R2 (see FIG. 11E). By this, the
transmission factor Tp of the liquid crystal layer is decreased as
shown in FIG. 11F.
(2) In the case of reverse polarity frame period:
A description will be given of, as an example, a case where the
same gradation is displayed subsequently to the foregoing positive
polarity frame. First, the gate voltage Vg2 (offn) of the gate
electrode G2 of the TFT 72 is kept at -5 V by a not-shown circuit
through the capacitance C1 and the resistance R1.
Next, the reverse polarity gradation voltage Vdn=+2 V is outputted
to the data bus line 74 (see FIG. 11A). Next, the gate pulse Vg1
(on) is outputted to the gate bus line 12 and the TFT 70 is turned
on. While the TFT 70 is in the on state, the data voltage Vdn is
applied to the gate electrode G2 of the TFT 72, and the capacitance
C1 is charged. As shown in FIG. 11C, the gate voltage Vg2 (onn)
(=+2 V) is applied to the gate electrode G2 of the TFT 72 and the
TFT 72 is turned on.
Next, when the gate voltage becomes Vg1 (off) and the TFT 70 is
turned off, the electric charge of the capacitance C1 is discharged
at the predetermined time constant, and as shown in FIG. 1C, the
level of the voltage applied to the gate electrode G2 of the TFT 72
is gradually decreased. This voltage level gradually approaches the
threshold voltage Vth of the TFT 72, then it becomes less than
that, and finally becomes Vg2 (offn) (=-5 V).
As stated above, the on time of the TFT 72 is determined by the
magnitude of the data voltage Vdn supplied to the gate electrode G2
and the time constant of attenuation depending on the capacitance
C1 and the resistance R1. In the on state of the TFT, the reverse
polarity driving voltage Vddn=0 V shown in FIG. 11D is written in
the pixel electrode 60 as the gradation voltage Vp, and the voltage
level is kept during the on time of the TFT 72 (see FIG. 11E). In
this period, as shown in FIG. 11F, the predetermined transmission
factor Tp is obtained in the liquid crystal layer.
When the gate voltage Vg2 of the TFT 72 becomes the predetermined
threshold voltage Vth or less, the TFT 72 is turned off, and the
gradation voltage Vp is decreased to the common voltage Vcom at the
time constant depending on the liquid crystal capacitance Clc and
the liquid crystal resistance R2 (see FIG. 11E). By this, as shown
in FIG. 11F, the transmission factor Tp of the liquid crystal layer
is decreased.
As stated above, according to this example, the on time of the TFT
72 can be controlled in accordance with the magnitude of the data
voltage Vd outputted to the data bus line 74. When the TFT 72 is in
the on state, the driving voltage Vdd of +10 V or 0 V is applied to
the liquid crystal layer 57, and in the off state, it becomes equal
to the common voltage Vcom=+5 V. Thus, in accordance with the
magnitude of the data voltage Vd, a time in which white is
displayed in one frame can be controlled.
Accordingly, when the data voltage Vd is made maximum, the TFT 72
is kept in the on state in approximately one frame period and a
white display can be obtained. On the other hand, when the data
voltage Vd is made minimum, the TFT 72 is kept in the off state for
approximately one period and a black display can be obtained.
By setting the data voltage to an arbitrary value between the
maximum and the minimum, the TFT 72 is kept in the on state in an
arbitrary time for one frame period, and then, it can be brought
into the off state. By this, the half tone can be displayed.
According to this example, an effect similar to the example 1-1 can
be obtained by using the general liquid crystal material and the
TFT construction.
Besides, in this example, since it is necessary that the voltage
applied to the liquid crystal layer 57 is discharged and not kept,
it is not necessary to use a liquid crystal having a high
resistance value. Thus, a liquid crystal such as a cyano system one
can be used, and the response speed of the liquid crystal display
device can be enhanced. Further, since it is possible to use a
chlorinated liquid crystal which has a large refractivity
anisotropy An, but a small resistance value, a cell thickness d can
be made thin. Thus, the liquid crystal display device with further
high speed response can be realized.
Next, a modified example of the liquid crystal display device
according to this example will be described. FIG. 12 shows the
construction of a liquid crystal display device according to this
modified example. FIG. 13 shows a section taken along line E-E of
FIG. 12. As shown in FIGS. 12 and 13, in the liquid crystal display
device according to this modified example, the common bus line 78
is not formed differently from the construction shown in FIG. 8. A
contact hole 202 bored in an insulating film 88 is formed over a
connection wiring line 84. A source electrode S1 of a TFT 70 is
connected to the connection wiring line 84 and a common electrode
62 of an opposite substrate 4 through a resistor 91 formed on the
contact hole 202. The resistor 91 is a pillar spacer having a
predetermined cell thickness. As a formation material of the
resistor 91, for example, a resist is preferable. In this modified
example, the resistor 91 functions as the resistance R1 and the
dielectric of the capacitance C1. Also in this example, the same
effect as the above example can be obtained.
Next, another modified example of the liquid crystal display device
according to this example will be described. FIG. 14 shows the
construction of a liquid crystal display device according to this
modified example. FIG. 15 shows a section taken along line F-F of
FIG. 14. In the liquid crystal display device according to this
modified example, similarly to the construction shown in FIG. 8, a
common bus line 78 is formed. A connection wiring line 84 is formed
between a gate bus line 12 and the common bus line 78. A source
electrode S1 of a TFT 70 is connected to one end of the connection
wiring line 84 through a contact hole 206. The sectional
construction of the vicinity of the contact hole 206 is similar to
the vicinity of the contact hole 82 shown in FIG. 10. The
connection wiring line 84 is connected through a resistor 210
formed on a contact hole 208 to one end of a second connection
wiring line 85 formed of the same formation material as a data bus
line 74 and a driving voltage bus line 76. The sectional
construction of the vicinity of the contact hole 208 is similar to
the vicinity of the contact hole 80 shown in FIG. 9. The other end
of the second connection wiring line 85 is connected to the common
bus line 78 through a contact hole 220. A pixel electrode 60 is
connected to the common bus line 78 through a resistor 92 formed on
a contact hole 204. In this modified example, the resistor 210
functions as the resistance R1 and the dielectric of the
capacitance C1, and the resistor 92 functions as the resistance R2.
Also according to this example, the same effect as the above
example can be obtained.
As described above, according to this embodiment, it is possible to
fabricate the liquid crystal display device in which excellent
viewing angle characteristics can be obtained.
Second Embodiment
Next, a liquid crystal display device according to a second
embodiment of the invention will be described with reference to
FIGS. 16 to 19. Incidentally, structural elements having the same
function and operation as the structural elements of the liquid
crystal display device according to the first embodiment are
designated by the same reference symbols and the explanation is
omitted. FIG. 16 shows the basic construction of two pixels of the
liquid crystal display device according to this embodiment when
viewed from a display screen side. FIG. 17 shows a section of the
liquid crystal display device taken along line A-A shown in FIG.
16. As shown in FIG. 16, two pixel regions of illustrated
longitudinal rectangular shapes are defined by a light-shielding
film (BM) 50 formed on an opposite substrate, and the inside of
each pixel is divided in two in the longitudinal direction.
Besides, as shown in FIGS. 16 and 17, linear protrusions 52
extending obliquely with respect to the a pixel region end part are
formed in the respective pixel regions on the opposite substrate
side. Slits 54 extending obliquely with respect to the pixel region
end part are formed in the respective pixels on the TFT substrate
side so that the slits 54 and the protrusions 52 are alternately
disposed. The pixel region is divided by the protrusions 52 and the
slits 54 into four regions (aligned regions) in which the alignment
directions of liquid crystal molecules 56 are different from one
another.
As shown in FIG. 17, each of the pixels includes a region A in the
vicinity of a region where an alignment regulating structure such
as the protrusion 52 or the slit 54 is formed, and a region B
between the two regions A. A pre-tilt angle (angle from a substrate
surface) of a liquid crystal molecule is different between the
region A and the region B. Thus, when viewed locally, the regions A
and B having two different T-V characteristics exist in one pixel.
However, in an actual display observed on the whole, a T-V
characteristic of the average of the respective T-V characteristics
can be obtained.
It is necessary that, one of the plural divided regions A and B has
a pre-tilt angle of approximately 90.degree. (the alignment
direction is almost vertical to the substrate surface) in order to
keep high contrast. In this example, the pre-tilt angle of the
region B is approximately 90.degree.. Further, similarly, it is
desirable that all the pre-tilt angles of the regions A and B are
80.degree. or more in order to keep the high contrast. In this
example, the pre-tilt angle of the liquid crystal molecule 56 in
the region A is 80.degree. or more. Besides, when the T-V
characteristics of the respective regions A and B are equally
averaged, a gradual T-V curved line can be obtained on the whole.
When the gradual T-V curved line is obtained on the whole, a
difference in transmission factor between the front direction and
the oblique direction can be made small. Accordingly, it is
desirable that the regions A and B are almost equally divided.
Next, a manufacturing method of the liquid crystal display device
according to this example will be described. Vertical alignment
films (made by, for example, JSR Corporation) are coated on
opposite surfaces of the TFT substrate 2 on which the plural slits
54 each having a width of 10 .mu.m are formed at intervals of 70
.mu.m to be parallel to each other, and the opposite substrate 4 on
which the plural linear insulating protrusions 52 each having a
height of 1.2 .mu.m and a width of 10 .mu.m are formed at intervals
of 70 .mu.m to be parallel to each other. Next, for example,
spherical spacers (made by, for example, Sekisui Fine Chemical Co.,
Ltd.) each having a diameter of 4.0 .mu.m are scattered. Next, the
TFT substrate 2 and the opposite substrate 4 are bonded so that the
slits 54 and the protrusions 52 are alternately disposed, and an
n-type liquid crystal (made by, for example Merck Ltd.) in which a
photo-curing composition (made by, for example, Merck Ltd.) of 0.3
wt % is added is sealed. As shown in FIG. 16, the slits 54 and the
protrusions 52 are formed to extend to the upper right, at
45.degree. and a lower right, at 45.degree. in one pixel. The
liquid crystal inclination directions of the aligned regions
alignment-divided by the slits 54 and the protrusions 52 in four
directions become directions inclined by 45.degree. from the
vertical direction or the horizontal direction of the display
screen. The areas of the respective aligned regions are almost
equal in one pixel.
After the liquid crystal is sealed between the TFT substrate and
the opposite substrate, in a state where a photomask patterned so
that light is irradiated to only the region A having a width of
17.5 .mu.m with respect to each of the alignment regulating
structures as the center is stacked on a liquid crystal display
panel, UV light with an irradiation energy of 4000 mJ is irradiated
to the liquid crystal layer while a dc voltage of 20 V is applied
between a pixel electrode 60 and a common electrode 62. By this,
the photo-curing composition of the region A is selectively cured.
Subsequently, in a state of no voltage application, UV light with
an irradiation energy of 4000 mJ is irradiated on the whole surface
of the liquid crystal display panel, and the photo-curing
composition of the region B having a width of 17.5 .mu.m is cured.
By this process, the pre-tilt angle of the liquid crystal molecule
in the region A becomes about 80.degree., and the pre-tilt angle of
the liquid crystal molecule 56 in the region B becomes about
90.degree..
FIG. 18 shows T-V characteristics of the liquid crystal display
device according to this example. The horizontal axis indicates
applied voltage (V) to the liquid crystal layer 57, and the
vertical axis indicates transmission factor (%) of light. A curved
line A expressed by a solid line in the graph indicates the T-V
characteristic in the front direction, and a curved line B
expressed by a solid line plotted by .diamond. marks indicates the
T-V characteristic in the oblique direction. When the T-V
characteristics shown in FIG. 18 are compared with the T-V
characteristics of the conventional liquid crystal display device
shown in FIG. 31, since a difference in transmission factor between
the front direction and the oblique direction is small, excellent
gradation viewing angle characteristics can be obtained in which
the chromaticity shift of a display image between the front
direction and the oblique direction is small. When several images
are actually displayed on the liquid crystal display device, and
the display screen is seen in the oblique direction, it is
confirmed that in this embodiment, a difference from a display
image seen in the front direction is small, and excellent gradation
viewing angle characteristics are capable of being obtained.
Incidentally, a conventional liquid crystal display device used for
the comparison is manufactured under the same condition as this
embodiment except that a photo-curing composition is not contained
in a liquid crystal, and a UV light irradiation treatment is not
performed.
In this embodiment, although the two regions A and B in which the
pre-tilt angles of the liquid crystal molecules 56 are different
from each other are formed, even if three or more regions where the
pre-tilt angles of the liquid crystal molecules 56 are different
from one another are formed, the same or more excellent display
characteristics can be obtained. Besides, there is a trade-off
relation that when the pre-tilt angle becomes small, more excellent
gradation viewing angle characteristics can be obtained, however,
the contrast is lowered. Thus, it is necessary to select the
pre-tilt angle on the basis of use environment or the like of the
liquid crystal display device.
FIG. 19 is a graph showing the effect of the liquid crystal display
device according to this example. The horizontal axis indicates
normalized transmission factor, and each transmission factor is
normalized by the transmission factor at the time when the
gradation voltage is 6.0 V. The vertical axis indicates the
logarithm of the ratio (transmission factor ratio) of the
transmission factor in the oblique direction to the transmission
factor in the front direction. A curved line C expressed by a solid
line plotted by .DELTA. marks in the graph indicates the
transmission factor ratio of the conventional liquid crystal
display device, and a curved line D expressed by a solid line
plotted by .diamond. marks indicates the transmission factor ratio
of the liquid crystal display device according to this embodiment.
As shown in FIG. 19, according to this embodiment, the transmission
factor ratio at a low transmission factor, which is conventionally
high, is lowered and the profile becomes flatter, and further, the
transmission factor ratio is close to 1.0 on the whole.
Accordingly, according to this embodiment, as compared with the
conventional example, excellent gradation viewing angle
characteristics can be obtained.
Third Embodiment
Next, a liquid crystal display device according to a third
embodiment of the invention will be described with reference to
FIGS. 20A to 26B. Incidentally, structural elements having the same
function and operation as the structural elements of the liquid
crystal display device according to the first and second
embodiments are designated by the same reference symbols, and the
explanation is omitted. First, the principle of the liquid crystal
display device according to this embodiment will be described.
FIGS. 20A and 20B show the relation between an inclination angle of
a liquid crystal molecule of the liquid crystal display device
according to this embodiment and a viewing angle direction. FIG.
20A shows the relation in a conventional liquid crystal display
device, and FIG. 20B shows the relation in the liquid crystal
display device according to this embodiment. As shown in FIG. 20A,
in the conventional liquid crystal display device, there is a case
where the retardation of a liquid crystal layer 57 when viewed in
the oblique direction is changed by an inclination angle of a
liquid crystal molecule 56. For example, in the liquid crystal
layer in which the major axis of the molecule is aligned in the
viewing angle direction of an arrow in the drawing, the retardation
in the viewing angle direction becomes minimum. On the other hand,
in the liquid crystal display device according to this embodiment,
like liquid crystal molecules 56 and 56' shown in FIG. 20B, plural
regions in which inclination angles gradually vary are provided in
one pixel, so that a substantial variation in retardation when
viewed in the oblique direction is suppressed.
The liquid crystal display device according to this embodiment is
characterized in that the pre-tilt angles of the liquid crystal
molecules 56 and 56' are made to vary in one pixel similarly to the
second embodiment. FIG. 21 shows the construction of the liquid
crystal display device according to this embodiment. As shown in
FIG. 21, plural gate bus lines 12 extending in the horizontal
direction in the drawing and plural drain bus lines 14 intersecting
the gate bus lines 12 through a not-shown insulating film and
extending in the vertical direction in the drawing are formed on a
TFT substrate 2. A TFT 16 is formed in the vicinity of an
intersection position of the gate bus line 12 and the drain bus
line 14. A storage capacitor bus line 20 is formed to cross
substantially the center of a rectangular pixel region defined by
the gate bus line 12 and the drain bus line 14.
Cross-shaped connection electrodes 26 and 28 dividing the pixel
region in four rectangles of the same shape are formed in the
rectangular pixel region. The connection electrode 26 is formed to
be parallel to the drain bus line 14 in the center of the pixel
region, and the connection electrode 28 is formed on the storage
capacitor bus line 20. Plural stripe electrodes 22 and 22' of a
minute electrode pattern extending at an angle of 45.degree. from
the connection electrodes 26 and 28 are formed. Each of the stripe
electrodes 22 is formed to have a width L1, and each of the stripe
electrodes 22' is formed to have a width L2 (>L1). A space 24 in
a state where an electrode is removed is formed between the
adjacent stripe electrodes 22. Besides, a space 24' is formed
between the adjacent stripe electrodes 22'. The space 24 is formed
to have a width S1, and the space 24' is formed to have a width S2
(>S1). A pixel electrode is constituted by the connection
electrodes 26 and 28, the plural stripe electrodes 22 and 22', and
the spaces 24 and 24', and partial stripe electrodes 22 and 22' are
electrically connected to a source electrode of the TFT 16. The
stripe electrodes 22 and 22' and the spaces 24 and 24' constitute
alignment regulating structures.
FIG. 22 is a graph showing the change of transmission factor with
respect to the change of the widths of the stripe electrode 22 and
the space 24 in the liquid crystal display device according to this
embodiment. The horizontal axis indicates the width L (.mu.m) of
the stripe electrode 22, and the vertical axis indicates the width
S (.mu.m) of the space 24. The transmission factor (%) indicates
the transmission factor in the front direction when a voltage of
3.4 V is applied to the liquid crystal layer 57.
As shown in FIG. 22, when the widths L of the stripe electrodes 22
and 22' are formed to be wide, and the widths S of the spaces 24
and 24' are formed to be narrow, the transmission factor at the
time when a predetermined voltage is applied becomes high. As is
apparent from FIG. 22, the transmission factor is changed at a high
sensitivity in the vertical axis direction, that is, with respect
to the change of the widths S of the spaces 24 and 24'.
As stated above, when the stripe electrodes 22 and 22' and the
spaces 24 and 24' are formed to have different widths in one pixel,
plural T-V characteristics locally different from each other can be
obtained in one pixel. Accordingly, a gradation display can be
obtained as one T-V characteristic of the average of the plural T-V
characteristics.
FIG. 23 shows a modified example of the liquid crystal display
device according to this embodiment. As shown in FIG. 23, in this
modified example, stripe electrodes 22, 22' and 22'' are formed to
have three different widths L1, L2 and L3, and spaces 24, 24' and
24'' are formed to have three different widths S1, S2 and S3.
FIG. 24 shows another modified example of the liquid crystal
display device according to this embodiment. As shown in FIG. 24,
in this modified example, stripe electrodes 22 are formed to have
substantially the same width Li, and spaces 24 and 24' are
respectively formed to have two different widths S1 and S2.
FIG. 25 shows still another modified example of the liquid crystal
display device according to this embodiment. As shown in FIG. 25,
in this modified example, plural stripe electrodes 23 and plural
spaces 25 and 25' are formed to be parallel to a drain bus line 14.
The stripe electrodes 23 are formed to have substantially the same
width L1, and the spaces 25 and 25' are formed to have different
widths S1 and S2, respectively. The plural stripe electrodes 23 are
electrically connected to one another by a connection electrode 29
formed in substantially the center of a pixel region to be parallel
to a gate bus line 12. Part of the stripe electrodes 23 is
electrically connected to a source electrode of a TFT 16.
FIGS. 26A and 26B are graphs showing effects of this embodiment.
FIG. 26A shows local T-V characteristics in respective regions in
the case where three regions having different T-V characteristics
are formed in one pixel in the liquid crystal display device
according to this embodiment. Besides, FIG. 26B shows an averaged
T-V characteristic of the case where three regions having different
T-V characteristics are almost uniformly formed in one pixel. In
both the graphs, the horizontal axis indicates applied voltage, and
the vertical axis indicates transmission factor.
As shown in FIG. 26B, by substantially uniformly forming the three
regions having different T-V characteristics in one pixel, the
three T-V characteristics shown in FIG. 26A are uniformly mixed,
and consequently, one T-V curve can be obtained which is smooth on
the whole and in which undulation does not occur. Accordingly, it
becomes possible to eliminate such a viewing angle that gradation
reversal as shown in FIG. 32 occurs, and the display
characteristics of the liquid crystal display device can be greatly
improved.
Next, a manufacturing method of the liquid crystal display device
according to this embodiment will be described. A film of, for
example, ITO (Indium Tin Oxide) is formed on a glass substrate on
which the TFT 16 is formed, and is patterned to form the pixel
electrodes having the minute electrode pattern as shown in FIG. 21.
Next, vertical alignment films made of polyimide or the like are
formed on the TFT substrate 2 and the opposite substrate 4. Next,
the TFT substrate 2 and the opposite substrate 4 are bonded at a
predetermined position. Next, a liquid crystal composition in which
a liquid crystal having a negative dielectric anisotropy and a
monomer capable of being polymerized by UV light are mixed is
sealed between the substrates 2 and 4.
Next, a gate voltage (for example, DC 30 V) and a gradation voltage
(for example, DC 5V) are applied to a liquid crystal display panel
in which the liquid crystal is sealed. At this time, the common
electrode of the opposite substrate 4 is kept at a ground
potential. The voltage is applied to the liquid crystal layer 57,
so that the liquid crystal molecules 56 are gradually aligned into
a stable state. In this state, UV light is irradiated to form a
photo-cured material in the liquid crystal layer 57. A polarizing
plate having a predetermined optic axis is bonded to the liquid
crystal display panel at a predetermined arrangement, so that the
liquid crystal display device is completed.
The invention is not limited to the above embodiment, but can be
variously modified.
For example, in the above embodiment, although the MVA mode liquid
crystal display device has been taken as an example, the invention
is not limited to this, but can be applied to another liquid
crystal display device such as a TN mode one.
Besides, in the above embodiment, although the normally black mode
liquid crystal display device has been taken as an example, the
invention is not limited to this, but can be applied to a normally
white mode liquid crystal display device.
Further, in the above embodiment, although the transmission-type
liquid crystal display device has been taken as an example, the
invention is not limited to this, but can be applied to another
liquid crystal display device such as a reflection type or
transflective type one.
As described above, according to the invention, the liquid crystal
display device can be realized in which excellent viewing angle
characteristics can be obtained.
* * * * *