U.S. patent application number 12/363986 was filed with the patent office on 2009-07-09 for display device and driving method thereof.
This patent application is currently assigned to PANASONIC CORPORATION. Invention is credited to Masanori Kimura, Katsuhiko KUMAGAWA, Akio Takimoto, Hiroyuki Yamakita.
Application Number | 20090174828 12/363986 |
Document ID | / |
Family ID | 26601507 |
Filed Date | 2009-07-09 |
United States Patent
Application |
20090174828 |
Kind Code |
A1 |
KUMAGAWA; Katsuhiko ; et
al. |
July 9, 2009 |
DISPLAY DEVICE AND DRIVING METHOD THEREOF
Abstract
An array substrate (10) is provided with a pixel electrode (3)
disposed in a region defined by two adjacent gate wirings (1) and
two adjacent source wirings (2), a switching element (5) for
switching a voltage applied to the pixel electrode (3) from the
source wiring (2) based on a signal voltage supplied from the gate
wiring (1), a common wiring (8) arranged between the two adjacent
gate wirings (1) and a common electrode (4) being electrically
connected to the common wiring (8) and generating an electric field
between the pixel electrode (3) whereto a voltage is applied,
wherein the pixel electrode (1) comprises a first pixel electrode
(1a) and a second pixel electrode (2a), and the opposing electrode
(2) comprises a first opposing electrode (1b) and a second opposing
electrode (2b), wherein a first region generates an electric field
between the first pixel electrode (1a) and the first opposing
electrode (2a) whose light transmittance is lower than that of the
first pixel electrode (1a) and a second region generates an
electric field between the second pixel electrode (1b) and the
second opposing electrode (2b) whose light transmittance is higher
than that of the second pixel electrode (1b) are formed.
Inventors: |
KUMAGAWA; Katsuhiko; (Osaka,
JP) ; Yamakita; Hiroyuki; (Osaka, JP) ;
Kimura; Masanori; (Osaka, JP) ; Takimoto; Akio;
(Osaka, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, NW
WASHINGTON
DC
20005-3096
US
|
Assignee: |
PANASONIC CORPORATION
Osaka
JP
|
Family ID: |
26601507 |
Appl. No.: |
12/363986 |
Filed: |
February 2, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10398385 |
Oct 7, 2003 |
7499115 |
|
|
PCT/JP01/08749 |
Oct 4, 2001 |
|
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12363986 |
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Current U.S.
Class: |
349/37 ;
349/139 |
Current CPC
Class: |
G09G 3/3614 20130101;
G02F 1/134345 20210101; G02F 1/134363 20130101; G09G 2320/0247
20130101; G09G 2300/0434 20130101 |
Class at
Publication: |
349/37 ;
349/139 |
International
Class: |
G09G 3/36 20060101
G09G003/36; G02F 1/1343 20060101 G02F001/1343 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 4, 2000 |
JP |
2000-304557 |
Oct 13, 2000 |
JP |
2000-313155 |
Claims
1. A display device comprising: an array substrate; an opposing
substrate facing the array substrate; and an electro-optic
substance held between the array substrate and the opposing
substrate, wherein the array substrate is provided with: a
plurality of gate wirings and a plurality of source wirings
intersecting each other; a pixel electrode disposed in each region
defined by two adjacent gate wirings and two adjacent source
wirings; a switching element for switching a voltage applied to the
pixel electrode from the source wiring based on a signal voltage
supplied from the gate wiring; a common wiring formed between the
two adjacent gate wirings; and an opposing electrode being
electrically connected to the common wiring and generating an
electric field for driving the electro-optic substance between the
opposing electrode and the pixel electrode whereto a voltage is
applied, wherein the pixel electrode comprises a first pixel
electrode and a second pixel electrode, and the opposing electrode
comprises a first opposing electrode and a second opposing
electrode, wherein a first region is formed in which an electric
field is generated between the first pixel electrode and the first
opposing electrode whose light transmittance is lower than that of
the first pixel electrode, wherein a second region is formed in
which an electric field is generated between the second pixel
electrode and the second opposing electrode whose light
transmittance is higher than that of the second pixel
electrode.
2. The display device according to claim 1, wherein the first
region and the second region are adjacent to each other.
3. The display device according to claim 1, wherein a voltage is
supplied to the first pixel electrode and the second pixel
electrode from the same source wiring based on a signal voltage fed
from the same gate wiring.
4. The display device according to claim 3, wherein the first
region and the second region are formed in a single dot.
5. The display device according to claim 4, wherein the interface
between the first region and the second region is located on the
common wiring, and the first pixel electrode is connected to the
second pixel electrode and the first opposing electrode is
connected to the second opposing electrode through contact holes
formed in insulating layers held in between.
6. The display device according to claim 4, wherein the source
wiring is disposed between the first region and the second region,
and the switching elements are arranged so as to correspond to the
first pixel electrode and the second pixel electrode,
respectively.
7. The display device according to claim 4, wherein a plurality of
the first regions and a plurality of the second regions are formed
and alternately arranged along the gate wiring in such a manner
that groups of two consecutively identical regions are alternately
disposed, and the interface between that groups of two adjacent
first regions and two adjacent second regions exists on the pixel
electrode or the opposing electrode.
8. The display device according to claim 1, wherein a plurality of
the first regions and a plurality of the second regions are formed
and arranged in a manner such that the flicker polarity cyclically
changes along both the gate wiring and the source wiring based on
the prescribed voltage polarity applied to the first pixel
electrode and the second pixel electrode.
9. The display device according to claim 8, wherein the flicker
polarities are inverted at every dot along both the gate wiring and
the source wiring.
10. The display device according to claim 8, wherein the flicker
polarities are inverted at every plurality of dots along both the
gate wiring and the source wiring.
11. The display device according to claim 1, wherein the first
region and the second region each corresponds to a dot.
12. The display device according to claim 1, wherein the first
region and the second region each corresponds to a pixel composed
of three dots of red, green and blue.
13. The display device according to claim 1, further comprising
storage capacitor electrodes electrically connected to the first
pixel electrode and the second pixel electrode, each of the storage
capacitor electrodes is arranged in the first region and the second
region, wherein the two storage capacitor electrodes are located on
the common electrode or the gate wiring with an insulating layer or
insulating layers in between to form storage capacitor regions and
the two storage capacitor regions have substantially the same
capacity.
14. The display device according to claim 13, wherein the two
storage capacitor electrodes are made of the same material and have
substantially the same surface area.
15. The display device according to claim 1, wherein the first
pixel electrode and the second opposing electrode are made of a
transparent material and the first opposing electrode and the
second pixel electrode are made of an opaque material.
16. The display device according to claim 1, wherein the area of
the first pixel electrode in the aperture of the first region and
the area of the second opposing electrode in the aperture of the
second region are substantially the same.
17. The display device according to claim 16, wherein the first
pixel electrode and the second opposing electrode have
substantially the same transmittance.
18. The display device according to claim 16, wherein an opaque
layer is formed on the opposing substrate for blocking light over
some portion of the array substrate and some portion of the first
pixel electrode or the second opposing electrode is covered with
the opaque layer.
19. The display device according to claim 1, wherein drive voltages
applied to the first region and the second region have the same
polarity.
20. The display device according to claim 1, wherein the first
region and the second region have substantially the same absolute
value of brightness difference between the case where the pixel
electrode has a positive electric potential relative to the
opposing electrode and the case where the pixel electrode has a
negative electric potential relative to the opposing electrode.
21. A display device comprising: an array substrate; an opposing
substrate facing the array substrate; and an electro-optic
substance held between the array substrate and the opposing
substrate, wherein the array substrate is provided with: a
plurality of gate wirings and a plurality of source wirings
intersecting each other; a pixel electrode disposed in each region
defined by two adjacent gate wirings and two adjacent source
wirings; a switching element for switching a voltage applied to the
pixel electrode from the source wiring based on a signal voltage
supplied from the gate wiring; a common wiring formed between the
two adjacent gate wirings; an opposing electrode being electrically
connected to the common wiring and generating an electric field for
driving the electro-optic substance between the opposing electrode
and the pixel electrode whereto a voltage is applied; and an
intermediate electrode disposed between the pixel electrode and the
opposing electrode, wherein the intermediate electrode has a
transmittance either higher or lower than both the pixel electrode
and the opposing electrode.
22. The display device according to claim 21, wherein the pixel
electrode and the opposing electrode are formed out of the same
material, and the intervals between the pixel electrode and the
intermediate electrode and between the intermediate electrode and
the opposing electrode are substantially the same.
23. The display device according to claim 21, wherein the
intermediate electrode is resistively connected to the pixel
electrode and the opposing electrode.
24. The display device according to claim 21, wherein the
intermediate electrode is subjected to capacity coupling with the
pixel electrode and the opposing electrode.
25. The display device according to claim 21, wherein the electric
potential of the intermediate electrode becomes the average value
of the electric potential of the pixel electrode whereto a voltage
applied and the electric potential of the opposing electrode which
functions as a standard electric potential.
26. The display device according to claim 1 wherein the
electro-optic substance is liquid crystal.
27. The display device according to claim 26, wherein an
alternating voltage is applied to the pixel electrode.
28. A method of driving a display device having: an array
substrate; an opposing substrate facing the array substrate; and an
electro-optic substance held between the array substrate and the
opposing substrate, the array substrate being provided with: a
plurality of gate wirings and a plurality of source wirings
intersecting each other; a pixel electrode disposed in each region
defined by two adjacent gate wirings and two adjacent source
wirings; a switching element for switching a voltage applied to the
pixel electrode from the source wiring based on a signal voltage
supplied from the gate wiring; a common wiring formed between the
two adjacent gate wirings; and an opposing electrode being
electrically connected to the common wiring and generating an
electric field for driving the electro-optic substance between the
opposing electrode and the pixel electrode whereto a voltage is
applied, in the two adjacent regions each defined by two adjacent
gate wirings and two adjacent source wirings the transmittance of
the pixel electrode disposed in each region being higher than that
of the opposing electrode disposed in the same region and the
transmittance of the pixel electrode disposed in other region being
lower than that of the opposing electrode disposed in that region,
said method comprising the step of inverting the voltage applied to
the pixel electrode for every predetermined adjacent region.
29. The method of driving a display device according to claim 28,
wherein the predetermined regions are adjacent to each other in two
directions, along the gate wiring and the source wiring.
30. The method of driving a display device according to claim 28,
wherein each predetermined region corresponds to a dot.
31. The method of driving a display device according to claim 28,
wherein the predetermined region corresponds to two dots adjacent
in a direction either along the gate wiring or the source
wiring.
32. The method of driving a display device according to claim 28,
wherein the predetermined region corresponds to a pixel composed of
three dots of red, green and blue.
33. The method of driving a display device according to claim 28,
wherein the predetermined region corresponds to two pixels each
composed of three dots of red, green and blue, adjacent in a
direction either along the gate wiring or the source wiring.
34-35. (canceled)
36. A method of driving a display device having: an array
substrate; an opposing substrate facing the array substrate; and an
electro-optic substance held between the array substrate and the
opposing substrate, the array substrate being provided with: a
plurality of gate wirings and a plurality of source wirings
intersecting each other; a pixel electrode disposed in each region
defined by two adjacent gate wirings and two adjacent source
wirings; a switching element for switching a voltage applied to the
pixel electrode from the source wiring based on a signal voltage
supplied from the gate wiring; a common wiring formed between the
two adjacent gate wirings; and an opposing electrode being
electrically connected to the common wiring and generating an
electric field for driving the electro-optic substance between the
opposing electrode and the pixel electrode whereto a voltage is
applied, the pixel electrode comprising a first pixel electrode and
a second pixel electrode, and the opposing electrode comprising a
first opposing electrode and a second opposing electrode, a
plurality of first regions generating an electric field between the
first pixel electrode and the first opposing electrode having a
lower light transmittance than the first pixel electrode being
formed, a plurality of second regions generating an electric field
between the second pixel electrode and the second opposing
electrode having a higher light transmittance than the second pixel
electrode being formed, said method comprising a step of inverting
a voltage applied to the first pixel electrode and the second pixel
electrode based on the arrangement cycles of the first region and
the second region so as to flicker polarities cyclically change
along both the gate wiring and the source wiring.
37. The method of driving a display device according to claim 36,
wherein said step of inverting the voltage comprises the step of
inverting the flicker polarities at every dot along both the gate
wiring and the source wiring.
38. The method of driving a display device according to claim 36,
wherein said step of inverting the voltage comprises the step of
inverting the flicker polarities at every plurality of dots along
one or both of the gate wiring and the source wiring.
39. The method of driving a display device according to claim 28,
wherein the driving frequency of the voltage applied to the pixel
electrode is 60 Hz or higher.
40. The display device according to claim 21, wherein the
electro-optic substance is liquid crystal.
41. The method of driving a display device according to claim 36,
wherein the driving frequency of the voltage applied to the pixel
electrode is 60 Hz or higher.
Description
TECHNICAL FIELD
[0001] The present invention relates to display devices such as
liquid crystal display devices, etc., and driving methods
thereof.
BACKGROUND ART
[0002] Liquid crystal display devices are in wide use as thin and
light flat displays for use in various electronic machines. There
are several display schemes used in liquid crystal display devices.
Among those, a scheme known as IPS (In-Plane Switching), in which
an electric field is applied to liquid crystal in parallel to a
substrate for obtaining a wide viewing angle, is suitably used for
monitor displays for use in personal computers, liquid crystal TV
sets or the like because of its excellent image properties.
[0003] Liquid crystal display devices using IPS are disclosed in
Japanese Unexamined Patent Publication No. 10-10556, for example. A
plan view of a pixel portion thereof is shown in FIG. 47. Such a
liquid crystal display device comprises an array substrate and an
opposing substrate parallel to each other, and liquid crystal held
between the array substrate and the opposing substrate. As shown in
FIG. 47, in the array substrate, gate wirings 101 feeding scanning
signals and source wirings 102 feeding image signals are arranged
so as to intersect at approximately right angles. Nearby each
intersection of the gate wiring 101 and the source wiring 102, a
thin-film transistor (TFT) 104 having a semiconductor layer is
formed as a switching element. To the source wiring 102, a
comb-like pixel electrode 115 is connected via the TFT 104.
Opposing electrodes 116 functioning as a standard potential are
arranged so as to mesh with the pixel electrode 115. The opposing
electrodes 116 are electrically connected to a common wiring 103
parallel to the gate wiring 101 through a contact hole 108. At the
intersection of the common wiring 103 and the pixel electrode 115,
with an insulating layer (not shown) in between, a storage
capacitor region 107 is formed.
[0004] According to such a liquid crystal display device, an
electric field substantially parallel to the substrates is
generated by the difference between the voltage applied to the
pixel electrode 115 and that of the opposing electrode 116, to
which a standard potential is applied, and thereby the liquid
crystal (not shown) held between the electrodes is driven. By
storing electric charge in the storage capacitor region 107 while
the TFT 104 is in an on-status, the liquid crystal remains actuated
while the TFT 104 is in an off-status.
[0005] In prior art IPS style liquid crystal display devices, pixel
electrodes and opposing electrodes are generally made of aluminum
or the like metals. Therefore, the pixel electrodes and opposing
electrodes do not transmit light, leading to the drawback of an
unsatisfactory pixel aperture ratio. Japanese Unexamined Patent
Publication No. 10-10556 proposes a way to enhance the aperture
ratio by forming either or both of the pixel electrode 115 and the
opposing electrode 116 out of a transparent conductive film.
[0006] In the case where both the pixel electrode 115 and the
opposing electrode 116 are made of transparent electrodes, it is
preferable that both the electrodes be formed as a same layer in
order to avoid a more complicated production process and increased
manufacturing costs. However, this arrangement may lower the
manufacturing yield by causing short-circuits between the pixel
electrode 115 and the opposing electrode 116. Therefore, it is more
practical that either the pixel electrode or the opposing electrode
be made of a transparent electrode.
[0007] However, forming only one of the pixel electrode and the
opposing electrode out of a transparent electrode and forming the
other out of metal or a like material may cause flicker due to the
difference in the optical properties of the two materials.
[0008] In order to apply a sufficient voltage to liquid crystal
molecules while preventing decomposition or deterioration thereof,
liquid crystal display devices are driven by the alternating
current drive method, where an electric potential alternately
positive and negative relative to that of the opposing electrode is
applied to the pixel electrode at a regular interval (for example,
once every sixtieth seconds). When the alternating current drive
method is employed in a liquid crystal display device in which only
one of the pixel electrode and the opposing electrode is a
transparent electrode, its transmittance changes cyclically between
the period when an electric potential positive relative to that of
the opposing electrode (positive frame) is applied to the pixel
electrode and the period when an electric potential negative
relative to that of the opposing electrode (negative frame) is
applied to the pixel electrode, causing observable differences in
brightness.
DISCLOSURE OF THE INVENTION
[0009] The present invention aims to overcome the drawbacks
described above. An object of the invention is to prevent flicker
of a display device in which an electro-optic material is driven by
applying a voltage between two electrodes having different
transmittances.
[0010] The inventors conducted research into the causes of the
flicker described above and found that the following two factors
greatly affect the occurrence of flicker. A first factor is the
flexoelectric effect. The flexoelectric effect is a polarization
phenomenon brought about by splay deformation (orientation
deformation) of liquid crystal. Regarding the relationship between
the flexoelectric effect and IPS, "Manuscripts of Lectures at the
1999 Japanese Liquid Crystal Conference" (page 514, lecture number
3D06) explains the occurrence of domain reversal in connection with
the positive and negative electrodes and rubbing direction.
[0011] How the flexoelectric effect influences flicker will be
explained below with reference to FIGS. 44(a), 44(b), 44(c) and
44(d). In FIG. 44(a), when a positive voltage is applied to an
electrode 21 and a negative voltage is applied to an electrode 22
in a liquid crystal display device using IPS or the like where a
lateral electric field is applied, a solid line 26 represents a
line of electric force, when the shape effect of the liquid crystal
molecules is left out of consideration. On the electrodes 21 and
22, the lines of electric force splay out. In this figure, 23
represents a liquid crystal layer, 24 represents an opposing
substrate, and 25 represents an array substrate. Liquid crystal
display devices are driven by the alternating current drive method.
Therefore, the direction of the electric field reverses, for
example, once every sixtieth of seconds.
[0012] FIG. 44(b) shows an array of liquid crystal molecules 27
formed out of this splay electric field. To the end of each of the
liquid crystal molecules, a cyano group, a fluorine atom or the
like is introduced to give dielectric anisotropy. These parts
function as negative electrodes of a dipole moment and compose the
larger part of the molecular skeleton. As shown in an enlarged view
of FIG. 44(b) (in the circle), the molecule has a wedge-like shape
opening to the negative electrode side. Because of the shape effect
(excluded volume effect), when a splay shape alternating electric
field is applied to the liquid crystal molecules, they will tend to
be arranged so as to direct the narrower end of the wedge to the
electrode side and the wider end to the center of the liquid
crystal layer. The liquid crystal molecules 27 are uniformly
aligned as described above and this generates an electric field 28
attributable to the liquid crystal molecules. This phenomenon is
known as the flexoelectric effect.
[0013] FIG. 44(c) illustrates a composite electric field 29 shown
by broken lines which is generated by the original electric field
26 and the electric field 28 attributable to the flexoelectric
effect in the liquid crystal molecules. The composite electric
field 29 exhibits a stronger vertical electric field on the
positive electrode 21 side and a weaker vertical electric field on
the negative electrode 22 side.
[0014] As a result, its distribution of transmittance varies
depending on the polarity (i.e., positive or negative) of the
applied voltage. FIG. 44(d) shows the transmittance distribution
when both electrodes 21 and 22 are transparent. Here, the solid
line shows the transmittance distribution when the electrode 21 has
a positive electric potential (positive frame), and the
dash-and-dot line shows the transmittance distribution when the
electrode 21 has a negative electric potential (negative frame).
Both electrodes are symmetric with respect to a longitudinal axis
passing through the midpoint thereof. Therefore, when both
electrodes 21 and 22 are transparent or both electrodes 21 and 22
have opaque properties, very little variance in the transmittance
between the positive and negative frames is observed. When one of
the electrodes transmits light and the other blocks light or the
transmittances of the two electrodes 21 and 22 are significantly
different, the transmittance of the pixel differs between the
positive and negative frames due to the difference of their optical
contribution ratios, causing flicker.
[0015] A second main factor causing flicker is influence by a
peripheral electric potential. FIG. 45(a) shows equipotential lines
when, out of the three electrodes 32, 33 and 34 disposed on an
array substrate 36, a voltage of -5 volts (V) is applied to the end
electrodes 32 and 34 and a voltage of +5 V is applied to the middle
electrode 33. When the electric potential of the interface of
opposing substrate 35 is assumed to be the average of the two
voltages (i.e. 0 V), equipotential lines of 0 V exist on the lines
normal to the substrate passing through points equidistant to any
two adjacent electrodes among 32, 33 and 34. Therefore, when the
flexoelectric effect is left out of consideration, the three
electrodes 32, 33 and 34 are equivalent. Therefore, when the
electrode 33 has a positive electric potential and when it has a
negative electric potential, its transmittance distribution is
shown by the solid line in FIG. 45(b), and this enables the
transmittance of the pixel to remain stable, even when some of the
plurality of electrodes 32, 33 and 34 is/are made transparent,
resulting in no occurrence of flicker.
[0016] However, in an IPS style liquid crystal display device,
there is no electrode on the surface of the opposing substrate and
this makes it difficult to form a desirable electric potential on
the interface 35. Therefore, if the electric potential of the
interface 35 of the opposing substrate is assumed to be -5 V, in
cases where the electrode 33 has a positive electric potential, as
shown in FIG. 46(a), equipotential lines of -5 V form above
electrodes 32 and 34 along the direction normal to the substrate.
In this case, the transmittance distribution is as shown by the
solid line in FIG. 46(b), i.e., the transmittances on the end of
electrodes (negative electrodes) 32 and 34 are higher than that on
the middle electrode (positive electrode) 33. On the other hand,
when the electrode 33 has a negative electric potential, as shown
by the broken line in FIG. 46(b), the transmittances on the end
electrodes (positive electrodes) 32 and 34 become lower than that
on the middle electrode (negative electrode) 33. Therefore, when
some of the plurality of electrodes 32, 33 and 34 is/are made
transparent, frames where the transparent electrode(s) have a
negative electric potential become brighter than frames where the
transparent electrode(s) have a positive electric potential,
causing flicker.
[0017] Taking these phenomena, which are key causes of flicker,
into consideration, transmittances of individual pixels in prior
art display devices are not even but exhibit a certain
distribution, i.e., the transmittance distribution varies between
when a pixel electrode has a positive electric potential relative
to the opposing electrode (positive frame) and when the pixel
electrode has a negative electric potential relative to the
opposing electrode (negative frame). Therefore, for example, when
the pixel electrode is made of a transparent material and the
opposing electrode is made of an opaque material, the transmittance
of the pixel electrode in either the positive or negative frame
becomes higher than that of the other frame. On the other hand, the
opposing electrode does not transmit light and therefore the
transmittance of the opposing electrode does not change between a
positive frame and a negative frame. As a result, the transmittance
variance between frames of the pixel electrode is observed as a
variance in the brightness of the whole pixel.
[0018] Such a flicker phenomenon is not limited to IPS style liquid
crystal display devices but occurs when display devices comprising
two electrodes having different light transmittances are driven by
the alternating current drive method.
[0019] To achieve the above object, the display device of the
invention comprises an array substrate, an opposing substrate
facing the array substrate and an electro-optic substance held
between the array substrate and the opposing substrate. The array
substrate is provided with a plurality of gate wirings and a
plurality of source wirings intersecting each other, a pixel
electrode disposed in each region defined by two adjacent gate
wirings and two adjacent source wirings, a switching element for
switching a voltage applied to the pixel electrode from the source
wiring based on a signal voltage supplied from the gate wiring, a
common wiring formed between the two adjacent gate wirings and an
opposing electrode being electrically connected to the common
wiring and generating an electric field for driving the
electro-optic substance between the opposing electrode and the
pixel electrode whereto a voltage is applied. The pixel electrode
comprises a first pixel electrode and a second pixel electrode, and
the opposing electrode comprises a first opposing electrode and a
second opposing electrode. A first region is formed in which an
electric field is generated between the first pixel electrode and
the first opposing electrode whose light transmittance is lower
than that of the first pixel electrode. A second region is also
formed in which an electric field is generated between the second
pixel electrode and the second opposing electrode whose light
transmittance is higher than that of the second pixel electrode.
According to this display device, flicker can be reduced because
the flicker polarities caused by the variance in transmittance
between the pixel electrode and the opposing electrode can be
cancelled between the first region and the second.
[0020] In the display device, it is preferable that the first
region and the second region be adjacent to each other.
[0021] It is preferable that a voltage is applied to the first
pixel electrode and the second pixel electrode from the same source
wiring based on the signal voltage supplied from the same gate
wiring. This makes the polarities of a voltage applied to the first
pixel electrode and second pixel electrode the same and reliably
cancel flicker polarity.
[0022] Preferably, the first region and the second region be
disposed in the same dot. This makes it possible to locate the
interface of the first region and the second region on the common
electrode. It is also possible to connect the first pixel electrode
to the second pixel electrode and the first opposing electrode to
the second opposing electrode respectively through contact holes
formed in the insulating layers held in between. Thereby, formation
of contact hole in aperture of the display region for connecting
different electrode materials (material transformation) becomes
unnecessary, enhancing a high aperture ratio. It is also possible
to arrange the source wiring between the first region and the
second region. A preferable arrangement is such that the switching
elements each correspond to the first pixel electrode and second
pixel electrode, respectively. This arrangement reduces a defective
ratio of the dot. Furthermore, when a plurality of the first
regions and a plurality of the second regions are formed, it is
preferable that groups of two consecutively identical regions be
alternately arranged along the gate wiring and the interface of the
that groups of two adjacent first regions and the second regions be
located on the pixel electrode or the opposing electrode. This
allows any two adjacent regions to share the pixel electrode or the
opposing electrode, enhancing the aperture ratio.
[0023] When a plurality of the first regions and a plurality of the
second regions are formed, it is preferable that the first regions
and the second regions are arranged in a manner such that the
flicker polarity cyclically changes along both the gate wiring and
the source wiring based on the prescribed voltage polarity applied
to the first pixel electrode and the second pixel electrode. This
reduces flicker and achieves a uniform display without suffering
from vertical or horizontal strips while in operation. In this
case, it is preferable that the flicker polarities be inverted at
every dot along both the gate wiring and the source wiring. When a
checkerboard pattern or the like is displayed, it is preferable the
flicker polarities be inverted at every plurality of dots along
both the gate wiring and the source wiring.
[0024] It is also possible to arrange the first region and the
second region in such a manner that each region corresponds to a
dot or a pixel comprising three dots of red, green and blue. In
both arrangements, flicker reduction can be achieved in a smaller
region.
[0025] When storage capacitor electrodes electrically connected to
the first pixel electrode and the second pixel electrode are formed
and each of them is arranged in the first region and the second
region, the two storage capacitor electrodes are disposed on the
common electrode or the gate wiring with insulating layers in
between to form storage capacitor regions. In this case, it is
preferable that the capacities of the two storage capacitor regions
be made substantially the same. This can be achieved by forming the
two storage capacitor electrodes out of the same material and
making their surface areas substantially the same.
[0026] The first pixel electrode and the second opposing electrode
can be made of transparent materials and the first opposing
electrode and the second pixel electrode can be made of an opaque
material.
[0027] It is preferable that the area of the pixel electrode in the
aperture of the first region and the area of the opposing electrode
in the aperture of the second region be made substantially the
same, reliably canceling flicker polarities and enhancing the
flicker reduction effect. In this case, it is desirable that the
transmittances of the first pixel electrode and the second opposing
electrode be approximately the same. Such an arrangement readily be
achieved by covering some portion of the first opposing electrode
or the second pixel electrode with an opaque layer formed on the
opposing substrate for blocking some portion of the array substrate
from light.
[0028] It is preferable that a driving voltage having the same
polarity is applied to the first region and the second region.
[0029] It is also preferable that first region and the second
region have substantially the same absolute value of bright
difference between the case where the pixel electrode has a
positive electric potential relative to the opposing electrode and
the case where the pixel electrode has a negative electric
potential relative to the opposing electrode.
[0030] An object of the invention is also achieved by a display
device comprises an array substrate, an opposing substrate facing
the array substrate and an electro-optic substance held between the
array substrate and the opposing substrate. The array substrate is
provided with a plurality of gate wirings and a plurality of source
wirings intersecting each other, a pixel electrode disposed in each
region defined by two adjacent gate wirings and two adjacent source
wirings, a switching element for switching a voltage applied to the
pixel electrode from the source wiring based on a signal voltage
supplied from the gate wiring, a common wiring formed between the
two adjacent gate wirings, an opposing electrode being electrically
connected to the common wiring and generating an electric field for
driving the electro-optic substance between the opposing electrode
and the pixel electrode whereto a voltage is applied and an
intermediate electrode disposed between the pixel electrode and the
opposing electrode. The intermediate electrode has a transmittance
either higher or lower than both the pixel electrode and the
opposing electrode.
[0031] In this display device, it is preferable that the pixel
electrode and the opposing electrode be formed out of the same
material, and the intervals between the pixel electrode and the
intermediate electrode and between the intermediate electrode and
the opposing electrode be substantially the same.
[0032] It is preferable that the intermediate electrode be
resistively connected to the pixel electrode and the opposing
electrode or conduct capacity coupling can be performed.
[0033] It is also preferable that the electric potential of the
intermediate electrode becomes the average value of the electric
potential of the pixel electrode whereto a voltage is applied and
the electric potential of the opposing electrode which functions as
a standard electric potential.
[0034] In the display device described above, it is preferable that
the electro-optic substance be liquid crystal and the voltage
applied to the pixel electrode be an alternating voltage.
[0035] An object of the invention can be achieved by applying a
drive method for use in a display device comprising an array
substrate, an opposing substrate facing the array substrate and an
electro-optic substance held between the array substrate and the
opposing substrate. The array substrate is provided with a
plurality of gate wirings and a plurality of source wirings
intersecting each other, a pixel electrode disposed in each region
defined by two adjacent gate wirings and two adjacent source
wirings, a switching element for switching a voltage applied to the
pixel electrode from the source wiring based on a signal voltage
supplied from the gate wiring, a common wiring formed between the
two adjacent gate wirings and an opposing electrode being
electrically connected to the common wiring and generating an
electric field for driving the electro-optic substance between the
opposing electrode and the pixel electrode whereto a voltage is
applied. The pixel electrode and the opposing electrode are made of
the materials having different transmittances. In this drive
method, the voltage applied to the pixel electrode is inverted
every predetermined adjacent regions.
[0036] In this drive method, the flicker polarities can be canceled
between the two adjacent regions, reducing flicker.
[0037] It is preferable that the predetermined regions be adjacent
to each other in two directions along the gate wiring and the
source wiring.
[0038] It is also preferable that each predetermined region
correspond to a dot or two dots adjacent in a direction either
along the gate wiring or the source wiring.
[0039] The predetermined region can correspond to a pixel composed
of three dots of red, green and blue or two adjacent pixels each
composed of three dots of red, green and blue, wherein the any two
adjacent pixels are adjacent to each other in a direction either
along the gate wiring or the source wiring.
[0040] An object of the invention can be achieved by applying a
drive method for use in a display device comprising an array
substrate, an opposing substrate facing the array substrate and an
electro-optic substance held between the array substrate and the
opposing substrate. The array substrate is provided with a
plurality of gate wirings and a plurality of source wirings
intersecting each other, a pixel electrode disposed in each region
defined by two adjacent gate wirings and two adjacent source
wirings, a switching element for switching a voltage applied to the
pixel electrode from the source wiring based on a signal voltage
supplied from the gate wiring, a common wiring formed between the
two adjacent gate wirings and an opposing electrode being
electrically connected to the common wiring and generating an
electric field for driving the electro-optic substance between the
opposing electrode and the pixel electrode whereto a voltage is
applied. The pixel electrode and the opposing electrode are made of
the materials having different transmittances. The voltage applied
to the pixel electrode is inverted by increasing or decreasing the
volume of prescribed brightness compensation voltage.
[0041] This method for driving a display device makes the
brightness differences approximately the same when the polarity of
a voltage applied to the pixel electrode is inverted, reducing
flicker.
[0042] When both the pixel electrode and the opposing electrode are
formed out of transparent electric conductors, this method for
driving a display device can be applied to the case where the total
area of the pixel electrode and the total area of the opposing
electrode occupying the transparent portions in the regions are
different from each other.
[0043] An object of the invention can be achieved by applying a
drive method for use in a display device comprising an array
substrate, an opposing substrate facing the array substrate and an
electro-optic substance held between the array substrate and the
opposing substrate. The array substrate is provided with a
plurality of gate wirings and a plurality of source wirings
intersecting each other, a pixel electrode disposed in each region
defined by two adjacent gate wirings and two adjacent source
wirings, a switching element for switching a voltage applied to the
pixel electrode from the source wiring based on a signal voltage
supplied from the gate wiring, a common wiring formed between the
two adjacent gate wirings and an opposing electrode being
electrically connected to the common wiring and generating an
electric field for driving the electro-optic substance between the
opposing electrode and the pixel electrode whereto a voltage is
applied. The pixel electrode comprises a first pixel electrode and
a second pixel electrode, and the opposing electrode comprises a
first opposing electrode and a second opposing electrode. A
plurality of first regions generating an electric field between the
first pixel electrode and the first opposing electrode whose light
transmittance is lower than that of the first pixel electrode are
formed; and a plurality of second regions generating an electric
field between the second pixel electrode and the second opposing
electrode whose light transmittance is lower than that of the
second pixel electrode are formed. A voltage applied to the first
pixel electrode and the second pixel electrode is inverted based on
the arrangement cycles of the first region and the second region so
as to flicker polarities periodically change along both the gate
wiring and the source wiring.
[0044] This drive method can cancel flicker and prevent vertical or
horizontal strips from appearing on a display during operation.
[0045] It is preferable that the flicker polarities are inverted at
every dot or every plurality of dots along both or either of the
gate wiring and the source wiring. [0046] In the drive method, it
is preferable that the driving frequency of the voltage applied to
the pixel electrode be 60 Hz or higher for cancel apparent
flicker.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 is a plan view showing a structure of one dot serving
as a minimal display unit of an array substrate in a display device
according to Embodiment 1 of the present invention.
[0048] FIGS. 2(a), 2(b) and 2(c) are sectional views of FIG. 1.
[0049] FIGS. 3 and 4 illustrate operations of the structure shown
in FIG. 1.
[0050] FIG. 5 is a plan view showing a structure of one dot serving
as a minimal display unit of an array substrate in a display device
according to Embodiment 2 of the invention.
[0051] FIGS. 6(a), 6(b), 6(c) and 6(d) are sectional views of FIG.
5.
[0052] FIG. 7 is a plan view showing a structure of one dot serving
as a minimal display unit of an array substrate in a display device
according to Embodiment 3 of the invention.
[0053] FIG. 8 is a plan view showing a structure of one dot serving
as a minimal display unit of an array substrate in a display device
according to Embodiment 4 of the invention.
[0054] FIGS. 9(a) and 9(b) are sectional views of FIG. 8.
[0055] FIG. 10 is a plan view showing a structure of one dot
serving as a minimal display unit of an array substrate in a
display device according to Embodiment 5 of the invention.
[0056] FIGS. 11(a) and 11(b) are sectional views of FIG. 10.
[0057] FIGS. 12(a) and 12(b) schematically illustrate arrays of
dots.
[0058] FIGS. 13(a), 13(b), 13(c), 13(d), 13(e) and 13(f)
schematically illustrate several methods for inverting a driving
voltage.
[0059] FIG. 14 is a plan view showing a structure of one dot
serving as a minimal display unit of an array substrate in a
display device according to Embodiment 6 of the invention.
[0060] FIGS. 15(a), 15(b) and 15(c) are sectional views of FIG.
14.
[0061] FIG. 16 is a plan view showing a structure of one dot
serving as a minimal display unit of an array substrate in a
display device according to Embodiment 7 of the invention.
[0062] FIGS. 17(a), 17(b), 17(c) and 17(d) are sectional views of
FIG. 16.
[0063] FIG. 18 shows the equivalent circuit of the structure shown
in FIG. 16.
[0064] FIG. 19 is a plan view showing a structure of one dot
serving as a minimal display unit of an array substrate in a
display device according to Embodiment 8 of the invention.
[0065] FIGS. 20(a), 20(b), 20(c), and 20(d) are plan views of FIG.
19.
[0066] FIG. 21 shows the equivalent circuit of the structure shown
in FIG. 19.
[0067] FIG. 22 is a plan view showing a modification of the
structure shown in FIG. 19.
[0068] FIG. 23 is a plan view showing the structures of two
adjacent dots on an array substrate in a display device according
to Embodiment 9 of the invention.
[0069] FIGS. 24(a), 24(b) and 24(c) are sectional views of FIG.
23.
[0070] FIG. 25 is a plan view showing the structures of two
adjacent dots on an array substrate in a display device according
to Embodiment 10 of the invention.
[0071] FIGS. 26(a), 26(b) and 26(c) are sectional views of FIG.
25.
[0072] FIG. 27 schematically shows an array of dots in a pixel of a
color display device.
[0073] FIG. 28 shows schematic structure of a display device
according to Embodiment 11 of the invention.
[0074] FIG. 29 schematically shows arrases of dots in two adjacent
pixels of a display device according to Embodiment 12 of the
invention.
[0075] FIG. 30 schematically shows arrays of dots in two adjacent
pixels of a display device according to Embodiment 13 of the
invention.
[0076] FIGS. 31(a), 31(b), 31(c), 31(d), 31(e) and 31(f)
schematically show the polarities of drive waveforms on odd frames,
dot structure and flicker polarities of a display device according
to Embodiment 14 of the invention.
[0077] FIGS. 32(a), 32(b), 32(c) and 32(d) schematically show the
polarities of drive waveforms on odd frames, dot structure and
flicker polarities of a display device according to Embodiment 15
of the invention.
[0078] FIGS. 33(a) and 33(b) are sectional view and plan view of a
display device according to Embodiment 16 of the invention.
[0079] FIG. 34 is an expanded sectional view showing the structure
around a switching element of a display device according to
Embodiment 16 of the invention.
[0080] FIG. 35(a) is a plan view showing a 4.times.4 dot section of
pixels and FIGS. 35(b) and 35(c) are schematic diagrams showing
writing polarities to the pixels of a display device according to
Embodiment 16 of the invention.
[0081] FIGS. 36(a) and 36(b) show light transmittance properties of
a pixel portion in a display device according to Embodiment 16 of
the invention.
[0082] FIGS. 37(a) and 37(b) are sectional view and plan view of a
display device according to Embodiment 17 of the invention.
[0083] FIG. 38 is an expanded sectional view showing the structure
around a switching element of a display device according to
Embodiment 17 of the invention.
[0084] FIG. 39(a) is a plan view showing a 4.times.4 dot section of
pixels of a display device according to Embodiment 17 of the
invention, and FIG. 39(b) is a schematic diagram showing the
waveform applied to each of the pixels.
[0085] FIGS. 40(a) and 40(b) show light transmittance properties of
a pixel portion in a display device according to Embodiment 17 of
the invention.
[0086] FIG. 41 is a plan view showing a display device according to
Embodiment 18 of the invention.
[0087] FIG. 42 is a plan view showing another display device
according to Embodiment 18 of the invention.
[0088] FIGS. 43(a) and 43(b) show operation of a display device
according to another embodiment of the invention.
[0089] FIGS. 44(a), 44(b), 44(c) and 44(d) illustrate a first
factor causing flicker.
[0090] FIGS. 45(a) and 45(b), and FIGS. 46(a) and (b) illustrate a
second factor causing flicker.
[0091] FIG. 47 is a plan view showing a prior art display
device.
BEST MODE FOR CARRYING OUT THE INVENTION
[0092] Embodiments of the present invention will be described below
with reference to the drawings.
Embodiment 1
[0093] FIG. 1 is a plan view showing a structure of one dot serving
as a minimal display unit of an array substrate in a display device
according to Embodiment 1 of the invention. FIGS. 2(a), 2(b) and
2(c) are sectional views of FIG. 1 taken along the lines A-A', B-B'
and C-C', respectively. In FIG. 1, gate wirings 4 feeding scanning
signals and source wirings 7 feeding image signals are arranged so
as to intersect at approximately right angles. Nearby each
intersection of the gate wiring 4 and the source wiring 7, a
thin-film transistor (TFT) 5 is formed as a switching element. The
TFT 5 formed on the gate wiring 4 with an insulating layer in
between comprises a semiconductor layer 8 made of amorphous
silicon. On the two sides of the semiconductor layer 8, a
projecting part of the source wiring 7 and a drain electrode 6 are
arranged facing each other.
[0094] To the source wiring 7, a pixel electrode 1 is connected via
the drain electrode 6 of the TFT 5. An opposing electrode 2
functioning as a standard potential is arranged so as to face the
pixel electrode 1. The opposing electrode 2 is disposed between the
two gate wirings 4, 4 in a parallel manner, and electrically
connected to a common wiring 3 which supplies a prescribed electric
potential (opposing voltage) to the opposing electrode 2.
[0095] The pixel electrode 1 comprises a first pixel electrode 1a
made of a transparent electric conductor disposed in the upper half
of the dot and a second pixel electrode 1b made of a metal material
disposed in the lower half of the dot. The opposing electrode 2
comprises a first opposing electrode 2a made of a metal material
which is disposed in the upper half of the dot so as to face the
first pixel electrode 1a and a second opposing electrode 2b made of
a transparent electric conductor disposed in the lower half of the
dot so as to face the second pixel electrode 1b.
[0096] On the gate wiring 4, a storage capacitor region 10
connected to the first pixel electrode 1a is formed with an
insulating layer in between.
[0097] As shown in FIGS. 1, 2(a), 2(b) and 2(c), on an array
substrate 9, the gate wiring 4 the first opposing electrode 2a and
a common wiring 3a are formed out of a first metal layer (ex. a
three-layered structure comprising titanium, aluminum and
titanium). There upon, with an insulating layer 11a in between, the
source wiring 7, the drain electrode 6 and the second pixel
electrode 1b are formed out of a second metal layer (ex. a
three-layered structure comprising titanium, aluminum and
titanium). There upon, with an insulating layer 11b in between, the
first pixel electrode 1a and the second opposing electrode 2b are
formed out of a transparent electric conductor layer (ex.
Indium-Tin-Oxide (ITO)). The semiconductor layer 8 is formed
between the first metal layer and the second metal layer and
subjected to patterning. Both of the metal layers can be a uniform
layer instead of a multilayer. For example, they can be formed of
chromium, aluminum, tantalum or the like. It is also possible to
use an alloy of molybdenum and tungsten, an alloy of molybdenum and
tantalum or like alloys.
[0098] Particularly, using silver alloys (ex. an alloy of silver,
palladium and copper) is advantageous in that it lowers the wiring
resistance and simplifies the manufacturing process. Tin oxide and
like oxides, organic conductive films as well as ITO can be used
for forming the transparent electric conductor layer.
[0099] The first pixel electrode 1a and the second pixel electrode
1b are connected to each other through a contact hole 13 formed in
the insulating layer 11b. The first opposing electrode 2a is
connected to the common wiring 3 formed on the same layer, and the
second opposing electrode 2b is connected to a common wiring
through a contact hole 14 formed in the insulating layers 11a, 11b.
The number of contact holes and layer transformations (connections
between different layers) are adjusted based on the shape and the
number of electrodes.
[0100] Between the array substrate 9 and the opposing substrate
(not shown) structured as described above, liquid crystal (not
shown) is sealed in. Thus, a display device can be obtained.
[0101] The operation of the display device is described below. When
an on-status voltage is applied to the gate wiring 4, a channel is
formed on the semiconductor layer 8 and the gap between the source
wiring 7 and the drain electrode 6 becomes conductive. Then, the
drain electrode 6 and the pixel electrode 1 are charged to have the
same electric potential as that of the source wiring 7. Thereby, a
difference appears between the voltage fed to the pixel electrode 1
and that of the opposing electrode 2, to which a standard electric
potential is applied. This generates electric fields substantially
parallel to the substrates between the first pixel electrode 1a and
the first opposing electrode 2a and between the second pixel
electrode 1b and the second opposing electrode 2b and applied to
the liquid crystal held between each of the electrodes.
[0102] When an off-status voltage is applied to the gate wiring 4,
channel formation is not achieved in the semiconductor layer 8, and
therefore there is no electrical continuity between the source
wiring 7 and the drain electrode 6 and the electric charges charged
in the drain electrode 6 and the pixel electrode 1 are retained. A
storage capacitor electrode 10 forms a storage capacitor region
between the gate wiring 4 and stabilizes the operation of the
display device by compensating for or alleviating the potential
difference due to leakage of electric charge from the pixel
electrode 1. Operation observed in one dot is explained above. In a
display device as a whole, a prescribed electric potential is sent
to each of the dots arranged in a matrix while scanning the gate
wirings one by one and applying to the source wiring a signal
voltage appropriate to the dot scanned.
[0103] Operation of a display device according to the present
embodiment will be described below in more detail with reference to
FIGS. 3 and 4. The structure shown in FIGS. 3 and 4 is the same as
that shown in FIG. 2, and therefore the reference symbols used in
FIG. 2 are omitted in FIGS. 3 and 4 unless needed for
explanation.
[0104] The signal voltage of each dot alternates in every frame in
a manner such that the electric potential of the pixel electrode 1
assumes a positive or negative value relative to the opposing
electrode 2. FIG. 3 shows the condition where the gate voltage is
at the off-level (Vg(OFF)) after creating a positive electric
potential in the pixel electrode 1 in the first frame. FIG. 4 shows
the condition where the gate voltage is at the off-level after
recording a negative electric potential into the pixel electrode in
the second frame. The display device, while the first and the
second frames are being alternately repeated, is driven by the
alternating current drive method. To simplify the explanation, the
electric potential of the opposing electrode 2 is made a constant
ground potential; however, if modulation in accordance with the
polarity of the pixel electric potential is added to the opposing
voltage and the gate voltage, the amplitude of the signal voltage
can be reduced.
[0105] As shown in FIG. 3, in the first frame, the first pixel
electrode 1a and the second pixel electrode 1b have a positive
electric potential and the first opposing electrode 2a and the
second opposing electrode 2b have a ground potential, generating an
electric field as shown by the arrows in the figure. Therefore, in
the upper half of the dot, an electric field is generated from the
transparent first pixel electrode 1a to the opaque first opposing
electrode 2a and the transparent electrode (the shadowed portion of
the figure) has a relatively positive electric potential; however,
in the lower half of the dot, an electric field is generated from
the opaque second pixel electrode 1b to the transparent second
opposing electrode 2b and the transparent electrode (shadowed
portion of the figure) has a relatively negative electric
potential.
[0106] On the other hand, as shown in FIG. 4, in the second frame,
the first pixel electrode 1a and the second pixel electrode 1b have
a negative electric potential and the first opposing electrode 2a
and the second opposing electrode 2b have a ground potential,
generating an electric field as shown by the arrows in the figure.
Therefore, in the upper half of the dot, an electric field is
generated from the opaque first opposing electrode 2a to the
transparent first pixel electrode 1a and the transparent electrode
(shadowed portion of the figure) has a relatively negative electric
potential; however, in the lower half of the dot, an electric field
is generated from the transparent second opposing electrode 2b to
the opaque second pixel electrode 1b and the transparent electrode
(shadowed portion of the figure) has a relatively positive electric
potential.
[0107] The light passing through spaces or transparent electrodes
(shadowed portion of the figure) in a dot, becomes brighter in
portions where, among the pixel electrode 1 and the opposing
electrode 2, the transparent electrode has a negative electric
potential relative to the opaque electrode compared to portions
where the transparent electrode has a positive electric potential
relative to the opaque electrode. Therefore, in the first frame
shown in FIG. 3, the lower half of the dot is brighter and, in the
second frame shown in FIG. 4, the upper half of the dot becomes
brighter. As described above, either the upper half or the lower
half of the dot alternately becomes brighter, and therefore the
contrast within a dot is canceled from frame to frame and the
flicker phenomenon does not occur.
[0108] In the present embodiment, since the partition line which
divides the dot into the upper and lower portions exists on the
common wiring 3, two regions having opposite flicker polarities
(light or dark polarity) can be formed in a single display unit
without an additional electrode layer or switching element.
Therefore, the embodiment has the advantage that flicker can be
reduced or eliminated without increased production costs caused by
a more complicated manufacturing process or lowered aperture ratio
due to formation of a switching element.
[0109] In addition, the layer transformation and the material
transformation (connections between different layers and materials)
of the first and second pixel electrodes 1a, 1b and the first and
second opposing electrodes 2a, 2b occur above the common wiring 3,
and therefore there is no need to form contact holes 13, 14 in the
aperture portion of the display region to make the connections,
improving the aperture ratio. Furthermore, in the structure where
the common wiring 3 is disposed near the center of the display unit
as in the present embodiment, if the connections between the
electrode materials are made above the common wiring 3, the areas
of the two regions having different flicker polarities can be made
almost equal, and a great reduction of flicker can be achieved by a
simple structure. Generally speaking, the above-mentioned
improvement in the aperture ratio can be achieved if the
connections between the electrode materials are made above the
common wiring or the gate wiring.
Embodiment 2
[0110] FIG. 5 is a plan view showing the structure of one dot
serving as a minimal display unit of an array substrate in a
display device according to Embodiment 2 of the invention and FIGS.
6(a) 6(b); 6(c) and 6(d) are sectional views of FIG. 5 taken along
the lines D-D', E-E', F-F' and G-G'. In FIG. 5 and FIGS. 6(a),
6(b), 6(c) and 6(d), those elements which are identical to the
elements of Embodiment 1 shown in FIGS. 1, 2(a), 2(b), and 2(c) are
identified with the same numerical symbols, and repetitious
explanation will be omitted.
[0111] A display device of the present embodiment is different from
that of Embodiment 1 in that a storage capacitor electrode 10 is
formed on the common wiring 3 instead of on the gate wiring 4 and a
storage capacitor region is formed between the common wiring 3 and
the storage capacitor electrode 10. This arrangement makes it
possible to eliminate an additional capacitor above the gate wiring
4 and achieve an uniform display with a reduced distortion of the
scanning voltage even on a wide screen. The principle used to
eliminate flicker is the same as that used in Embodiment 1.
[0112] In Embodiment 1, the storage capacitor electrode 10 is
formed out of the second metal layer in the same layer as the
source wiring 7, the drain electrode 6 and the second pixel
electrode 1b and is connected to the second pixel electrode 1b. Via
the contact hole 13, the storage capacitor electrode 10 is also
connected to the first pixel electrode 1a which is formed out of a
transparent electric conductor layer with an insulating layer 11b
in between.
[0113] Therefore, as Embodiment 1, the structure of the present
embodiment has the following advantages. By dividing the region
(dot) constituting display unit into upper and lower portions and
connections between the different materials (material
transformation) of the pixel electrode 1 and the opposing electrode
2 on the common wiring 3 corresponding to the partition line, it is
possible to reduce or eliminate flicker without suffering from
increased production costs caused by a more complicated
manufacturing process or a lowered aperture ratio attributable to
the formation of a switching element. Furthermore, since
connections between the electrode materials are made above the
wiring, formation of a contact hole in the aperture of the display
region for making connections between different materials becomes
unnecessary, and this enhances the aperture ratio.
[0114] In the following embodiments, the storage capacitor
electrode is formed on the common wiring 3 as in the present
embodiment; however, it can be formed on the gate wiring 4 as in
Embodiment 1.
Embodiment 3
[0115] FIG. 7 is a plan view showing the structure of one dot
serving as a minimal display unit of an array substrate in a
display device according to Embodiment 3 of the invention. In FIG.
7, those elements which are identical to the elements of Embodiment
1 shown in FIG. 1 are identified with the same numerical symbols,
and repetitious explanation will be omitted.
[0116] In the display device of the present embodiment, the region
81 corresponding to the black matrix formed as an opaque layer on
the opposing substrate (not shown) facing the array substrate shown
in FIG. 1 is indicated by the region outlined with broken lines and
filled by oblique lines. In other words, the region 81 is an area
where passing light is blocked, and an aperture is formed in the
center of the dot.
[0117] The outline of the region 81 runs along the middle of the
first opposing electrode 2a and the second opposing electrode 2b in
the longitudinal direction and makes the areas of the transparent
electrode disposed in the apertures in the upper and lower halves
of a dot (i.e. the first pixel electrode 1a and the second opposing
electrode 2b) equal. As a result, it is possible to reliably cancel
the flicker polarities in a dot. This structure is particularly
useful when the actual areas of the transparent electrode differ in
the upper and lower halves of a dot. In the present embodiment, the
areas of the transparent electrode in the apertures in the upper
and lower halves of a dot are made equal using a black matrix;
however, it is also possible to make the areas of the transparent
electrode equal by varying the width of the electrode and adjusting
the length of the electrode. The black matrix can be formed on the
array substrate side. Furthermore, it is also possible to use a
metal layer instead of the black matrix and make it function as an
opaque layer by overlaying it on a part of the transparent
electrode layer. By forming an opaque layer such as a black matrix
or the like on the array substrate side, the effect of any
misalignment of the two substrates is eliminated and the accuracy
of the position of the opaque layer with respect to the electrode
is enhanced. This enhances the ability to eliminate flicker. More
preferably, flicker can be reliably prevented by utilizing the
results of experiments or simulations and adjusting the width of
the opaque layer and electrode and the length of the electrode in a
manner such that the effective areas of the transparent electrode
affecting the transmittance in the upper and lower halves of a dot
becomes equal. This arrangement can be employed not only in a
display device of Embodiment 1 but also in display devices of other
embodiments.
Embodiment 4
[0118] FIG. 8 is a plan view showing the structure of one dot
serving as a minimal display unit of an array substrate in a
display device according to Embodiment 4 of the invention. FIGS.
9(a) and 9(b) are sectional views of FIG. 8 taken along the lines
H-H' and I-I'. In FIGS. 8, 9(a) and 9(b), those elements which are
identical to the elements of Embodiment 1 shown in FIGS. 1, 2(a),
2(b), and 2(c) are identified with the same numerical symbols, and
repetitious explanation will be omitted.
[0119] A display device according to the present embodiment is
designed so that the inside of a dot is divided into right and left
halves and flicker is canceled between the two (left and right)
regions.
[0120] The right region comprises a first pixel electrode 1a made
of a transparent electric conductor and a first opposing electrode
2a made of a metal material. The left region comprises a second
pixel electrode 1b made of a metal material and a second opposing
electrode 2b made of a transparent electric conductor. In the
center of the dot, with the boundary line of the left and right
regions in between, a first central opposing electrode 2c made of a
metal material is formed on the right side and a second central
opposing electrode 2d made of a transparent electric conductor is
formed on the left side. A common wiring 3 is disposed above the
center of the dot.
[0121] As shown in FIGS. 8, 9(a) and 9(b), on an array substrate 9,
a gate wiring 4 the first opposing electrode 2a, the common wiring
3 and the first central opposing electrode 2c are formed out of a
first metal layer. There upon, with an insulating layer 11a in
between, a source wiring 7, a drain electrode 6, the second pixel
electrode 1b and a storage capacitor electrode 10 are formed out of
a second metal layer. Above the second metal layer, with an
insulating layer 11b in between, the first pixel electrode 1a, the
second opposing electrode 2b and the second central opposing
electrode 2d are formed out of a transparent electric conductor
layer. The first pixel electrode 1a is connected to the storage
capacitor electrode 10 through a contact hole 13, and the second
opposing electrode 2b and the second central opposing electrode 2d
are connected to the common wiring 3 through a contact hole 14.
[0122] The display device having the above structure is
advantageous in that it prevents flicker and readily obtains high
definition images owing to a reduced number of contact holes.
Furthermore, it can enhance the manufacturing yield since the ratio
of defects caused by poor contact between the constituent elements
is lowered. The common wiring 3 can also be disposed below the
center of the dot.
Embodiment 5
[0123] FIG. 10 is a plan view showing the structure of one dot
serving as a minimal display unit of an array substrate in a
display device according to Embodiment 5 of the invention. FIGS.
11(a) and 11(b) are sectional views of FIG. 10 taken along the
lines J-J' and K-K'. In FIGS. 10, 11(a) and 11(b), those elements
which are identical to the elements of Embodiment 1 shown in FIGS.
1, 2(a), 2(b), and 2(c) are identified with the same numerical
symbols, and repetitious explanation will be omitted.
[0124] In FIG. 10, the region 41, whose outline is shown by the
broken line, indicates a region of one dot.
[0125] In the present embodiment, the dot is divided into two
subdots SD1 and SD2 having opposite flicker polarities (light or
dark polarity) and the flicker polarities are canceled within the
dot.
[0126] The two subdots SD1 and SD2 are formed by dividing the dot
into left and right portions with the center at a source wiring 7.
The portions receive signals from the same gate wiring 4 and source
wiring 7. The right subdot SD1 comprises a first pixel electrode 1a
made of a transparent electric conductor and a first opposing
electrode 2a made of a metal material. In contrast to SD1, the left
subdot SD2 comprises a second pixel electrode 1b made of a metal
material and a second opposing electrode 2b made of a transparent
electric conductor. The first pixel electrode 1a and the second
pixel electrode 1b are connected to the same source wiring 7
through TFTs 42, 43, respectively. Storage capacitor electrodes
10b, 10c are formed on a common wiring 3 and connected to the first
pixel electrode 1a and the second pixel electrode 1b,
respectively.
[0127] As shown in FIGS. 10, 11(a) and 11(b), on an array substrate
9, the gate wiring 4, the first opposing electrode 2a and the
common wiring 3 are formed out of a first metal layer. There upon,
with an insulating layer 11a in between, the source wiring 7, drain
electrodes 6a, 6b, the second pixel electrode 1b and the storage
capacitor electrodes 10b, 10c are formed out of a second metal
layer. There upon, with an insulating layer 11b in between, the
first pixel electrode 1a and the second opposing electrode 2b are
formed out of a transparent electric conductor layer. The first
pixel electrode 1a is connected to the storage capacitor electrode
10b through the contact hole 13, and the second opposing electrode
2b is connected to the common wiring 3 through the contact hole
14.
[0128] This structure achieves a display which is free from
flicker, since the difference in brightness attributable to the
flexoelectric effect or a peripheral electric potential is offset
between the left and right subdots.
[0129] In the display device of the present embodiment, each dot is
divided into two subdots SD1 and SD2, and the TFTs 42, 43 are
provided in the subdots SD1 and SD2, respectively. Therefore, even
when a defect arises in one of the TFTs 42, 43, the subdot having
the other TFT operates normally. Therefore, the display device has
the advantage that there is a low possibility of having a
non-lighting dot caused by an entire dot being defective.
[0130] Furthermore, the two subdots SD1 and SD2 are arranged so as
to hold the source wiring 7 in between. Since both subdots SD1 and
SD2 use the same source wiring 7, there is no need to increase the
number of source wirings 7.
[0131] As shown in FIG. 10, the electrodes 1a and 2b extend upward
and downward from the contact holes 13, 14. Therefore, the number
of contact holes can be reduced in the present embodiment compared
to the structure in Embodiments 1 and 2 in which the electrodes
extend in only one direction from the contact holes. This makes it
possible to readily provide high definition images and enhance the
manufacturing yield since the probability of a defect caused by
poor contact between the constituent elements is lowered.
[0132] In order to further enhance the flicker reducing effect,
bringing the flicker polarities of the two dots into balance is
desirable. For that purpose, it is desirable that the capacities of
the two storage capacitor electrodes 10b, 10c be made equal and it
is advantageous that the two storage capacitor electrodes 10b, 10c
be designed to be formed of the same material, thereby making the
areas of the two storage capacitor electrodes equal. For that
purpose, in the right subdot SD1 of the present embodiment, the
transparent first pixel electrode 1a makes a connection between
layers and the storage capacitor electrode 10b is made out of a
metal layer. As a result, the design period of the TFT array is
shortened without adversely affecting the design, enhancing the
manufacturing yield by using a design having a high tolerance for
the errors introduced by the manufacturing process.
[0133] Next, examples of the repeated patterns of dots in the
entire array substrate and desirable combinations with driving
methods will be explained. FIGS. 12(a) and 12(b) show repeated
patterns of the subdots SD1 and SD2 shown in FIG. 10, wherein the
left subdot SD2 (the pixel electrode is made out of a metal layer)
is defined as P and the right subdot SD1 (the pixel electrode is
made out of a transparent electrode layer) is defined as Q. As
shown in FIG. 10, between each pair of dots, there is either a
first opposing electrode 2a or a second opposing electrode 2b. With
respect to the pattern design, the structure in which the left and
right dots share the first opposing electrode 2a or the second
opposing electrode 2b is preferable to enhance the aperture ratio.
Therefore, in the dot array arranged along the gate wiring 4, on
the right side of a dot having the two subdots arranged in the
order of PQ, it is preferable to place a dot having the two subdots
arranged in the order of QP. In other words, for any two
horizontally adjacent dots, it is preferable that the arrangement
of the subdots thereof be reversed.
[0134] On the other hand, regarding vertically adjacent dots, it is
preferable that the arrangement cycle of the subdots differ from
the inversion cycle of the driving voltages. If the two cycles are
coincident with each other, the effect of inversion is offset and
vertical lines may arise because dots having the same flicker
polarity are arranged along the source wiring 7.
[0135] Desirable subdot patterns are explained below with reference
to concrete examples. Regarding the subdot arrangement of
vertically adjacent dots, the following two patterns are better
suited for practical use in view of the layout design. In the first
pattern, as shown in FIG. 12(a), the arrangement of the right and
left subdots is reversed between any two vertically adjacent dots,
and in the other, as shown in FIG. 12(b), the arrangement of the
right and left subdots is kept the same without reversing.
[0136] FIGS. 13(a) to 13(f) illustrate conditions in which the
polarities of the voltage applied to each dot are inverted between
the two frames, showing several polarity driving voltage inversion
methods. Among the figures, FIG. 13(a) shows the frame-inversion
drive method and FIG. 13(b) shows the column-inversion drive
method. In both methods, a voltage is applied in such a manner that
dots aligned in the vertical direction have the same polarity. It
is desirable that these driving methods be used with the subdot
arrangement pattern shown in FIG. 12(a). This is because, in
vertically adjacent rows, subdot arrays are inverted and the
polarity of the voltage applied to the pixel electrodes is the
same, making flicker more indistinctive as subdots having the same
flicker polarities are not continuously aligned in the vertical
direction.
[0137] Among the polarity inversion methods shown in FIGS. 13(a) to
13(f), it is preferable that the line-inversion drive method
(row-inversion drive method) shown in FIG. 13(c) and the
dot-inversion drive method shown in FIG. 13(d) be used with the
subdot arrangement pattern shown in FIG. 12(b). This is because, in
vertically adjacent rows, subdot arrays have the same polarity
pattern and the polarity of the voltage applied to the pixel
electrodes is inverted, making flicker more indistinctive as
subdots having the same flicker polarities are not continuously
aligned in the vertical direction.
[0138] In the polarity inversion methods used to drive the liquid
crystal, there are several ways in which inversion is performed
every n lines instead of every line as shown in FIGS. 13(c) and
13(d). The two-line inversion drive method shown in FIG. 13(e)
(inversion is performed every two lines) and the two-line-dot
inversion drive method shown in FIG. 13(f) are the examples of the
case when n is 2. When inversion drive is performed every n lines,
a display free from severe problems in visibility can be achieved
if the subdot array is inverted every n lines and the array cycle
differs from the inversion cycle of the driving voltage.
[0139] When combined with the subdot arrangement shown in FIG.
12(a), the flicker polarities of vertically adjacent subdots are
repeatedly inverted for n lines and a portion appears every n lines
where vertically adjacent subdots have the same flicker polarities.
On the other hand, when combined with the subdot arrangement shown
in FIG. 12(b), vertically adjacent subdots have the same flicker
polarities for n lines and a portion appears every n lines where
the flicker polarities of vertically adjacent subdots are inverted.
Therefore, when n is 2, and either the subdot arrangement shown in
FIG. 12(a) or that of 12(b) is adopted, two lines of subdots having
the same flicker polarities are continuously arranged in the
vertical direction and then inverted, obtaining a desirable
display. When n is 3 or greater, combination with the subdot
arrangement shown in FIG. 12(a) results in an increased number of
inversions of flicker polarity, and is thus desirable.
Embodiment 6
[0140] FIG. 14 is a plan view showing the structure of one dot
serving as a minimal display unit of an array substrate in a
display device according to Embodiment 6 of the invention and FIGS.
15(a), 15(b), 15(c) and 15(d) are sectional views of FIG. 14 taken
along the lines L-L', M-M', N-N' and O-O'. The present embodiment
is a combination of Embodiments 2 and 5, and therefore those
elements which are identical to the elements of Embodiments 2 and 5
are identified with the same numerical symbols, and repetitious
explanation will be omitted.
[0141] In a display device of the present embodiment, the dot 51,
whose outline is shown by the broken line in FIG. 14, is divided
into two subdots SD3 and SD4. The flicker polarities are canceled
between the upper and lower halves of the subdots SD3 and SD4.
[0142] The two subdots SD3 and SD4 are formed by dividing the dot
into left and right portions with the center at a source wiring 7.
The portions receive signals from the same gate wiring 4 and source
wiring 7. The upper portions of the right subdot SD3 and the left
subdot SD4 comprise a first pixel electrode 1a made of a
transparent electric conductor and a first opposing electrode 2a
made of a metal material. The lower portions of the right subdot
SD3 and the left subdot SD4 subdot comprise a second pixel
electrode 1b made of a metal material and a second opposing
electrode 2b made of a transparent electric conductor. The second
pixel electrodes 1b in the subdots SD3 and SD4 are connected to the
same source wiring 7 through TFTs 42, 43, respectively. Storage
capacitor electrodes 10 are formed on the common wiring 3 in the
subdots SD3 and SD4 and connected to the first pixel electrode 1a
and the second pixel electrode 1b, respectively.
[0143] As shown in FIGS. 14, 15(a), 15(b), 15(c) and 15(d) on an
array substrate 9, the gate wiring 4, the first opposing electrode
2a and the common wiring 3 are formed out of a first metal layer.
There upon, with an insulating layer 11a in between, the source
wiring 7, drain electrodes 6a, 6b, the second pixel electrode 1b
and the storage capacitor electrode 10 are formed out of a second
metal layer. There upon, with an insulating layer 11b in between,
the first pixel electrode 1a and the second opposing electrode 2b
are formed out of a transparent electric conductor layer. The first
pixel electrode 1a is connected to the storage capacitor electrode
10 through the contact hole 13, and the second opposing electrode
2b is connected to the common wiring 3 through the contact hole
14.
[0144] In the display device of the present embodiment, as in
Embodiment 5, each dot is divided into two subdots SD3 and SD4, and
the TFTs 42, 43 are provided in the subdots SD3 and SD4,
respectively. Therefore, even when a defect arises in one of the
TFTs 42, 43, the subdot having the other TFT operates normally.
Therefore, the display device has the advantage that there is a low
possibility of having a non-lighting dot caused by an entire dot
being defective.
[0145] Furthermore, as in Embodiment 5, the two subdots SD3 and SD4
are arranged so as to hold the source wiring 7 in between. Since
both subdots SD3 and SD4 use the same source wiring 7, there is no
need to increase the number of source wirings 7.
[0146] A distinctive advantage of the present embodiment is that it
can obtain excellent flicker polarities despite its driving method,
since the two subdots SD3 and SD4 are symmetrically shaped.
Embodiment 7
[0147] FIG. 16 is a plan view showing the structure of one dot
serving as a minimal display unit of an array substrate in a
display device according to Embodiment 7 of the invention and FIGS.
17(a), 17(b) and 17(c) are sectional views of FIG. 14 taken along
the lines P-P', Q-Q' and R-R'. In FIGS. 16, 17(a), 17(b), 17(c) and
17(d), those elements which are identical to the elements of
Embodiment 1 shown in FIGS. 1, 2(a), 2(b), and 2(c) are identified
with the same numerical symbols, and repetitious explanation will
be omitted.
[0148] According to a display device of the present embodiment, a
pixel electrode 1 and an opposing electrode 2 are formed out of a
metal layer and an intermediate electrode 61 made of a transparent
conductive layer is formed between the two electrodes. The widths
of the spaces between the opposing electrode 2 and the intermediate
electrode 61 and between the intermediate electrode 61 and the
pixel electrode 1 are made approximately equal. The pixel electrode
1, the intermediate electrode 61 and the opposing electrode 2 are
electrically connected to each other by a resistor 62 having
belt-shaped ends.
[0149] As shown in FIGS. 16, 17(a), 17(b) and 17(c), on an array
substrate 9, a gate wiring 4, the opposing electrode 2 and a common
wiring 3 are formed out of a first metal layer. There upon, with an
insulating layer 11a in between, a source wiring 7, a drain
electrode 6, the pixel electrode 1 and a storage capacitor
electrode 10 are formed out of a second metal layer. There upon,
with an insulating layer 11b in between, the intermediate electrode
61 is formed out of a transparent electric conductor layer. On the
intermediate electrode 61, with an insulating layer 11c in between,
a resistor 62 is formed out of a metal-oxide layer or a
semiconductor layer. High-resistance ITO, tin oxide or the like can
be used to obtain a metal-oxide layer. An example of a
semiconductor layer include an amorphous silicon layer.
[0150] The pixel electrode 1 is connected to the resistor 62
through a contact hole 67 formed in the insulating layers 11b, 11c.
The opposing electrode 2 is connected to the resistor 62 through a
contact hole 68 formed in the insulating layers 11a, 11b, 11c. The
intermediate electrode 61 is connected to the resistor 62 through a
contact hole 69 formed in the insulating layer 11c.
[0151] FIG. 18 is an equivalent circuit diagram of the array
substrate described above. In this figure, the opposing electrode 2
is assumed to have a ground potential through the common wiring 3
and a signal electric potential (Va) applied to the pixel electrode
1 is assumed to be positive. In this case, by making the
resistances of each resistor 62 approximately the same, the
electric potential of the intermediate electrode 61 becomes the
average value (Va/2) of the electric potentials of the pixel
electrode 1 and the opposing electrode 2.
[0152] In FIG. 16, the distances between the opposing electrode 2
and the intermediate electrode 61 and between the intermediate
electrode 61 and the pixel electrode 1 are assumed to be
substantially the same, and therefore the strengths of the electric
fields generated in spaces S1, S2, S3 and S4 which are formed
between the electrodes become the same and their directions are as
shown by the arrows in the figure. In this case, in the left
intermediate electrode 61, the left half serves as a positive
electrode relative to space S1 and the right half serves as a
negative electrode relative to space S2. On the other hand, in the
right intermediate electrode 61, the left half serves as a negative
electrode relative to space S3 and the right half serves as a
positive electrode relative to space S4. Therefore, between the
left side and the right side of the intermediate electrode 61 made
of a transparent electric conductor, differences in brightness
caused by the flexoelectric effect or a peripheral electric
potential can be cancelled.
[0153] When a negative signal voltage is applied to the next frame,
the directions of the electric fields and operations of each space
serving as a positive or a negative electrode are reversed;
however, as explained above, differences in brightness can be
cancelled between the right and the left sides of the intermediate
electrode 61. Therefore, the brightness of the positive and the
negative frames becomes the same in the dot as a whole, eliminating
flicker.
[0154] The intermediate electrode 61 is resistively connected to
the electrodes to which an electric potential is applied (the pixel
electrode 1 and the opposing electrode 2), and therefore its
electric potential is stable without floating. This makes it
possible to display stable images.
[0155] In the present embodiment, the intermediate electrode 61 is
resistively connected to the pixel electrode 1 and the opposing
electrode 2; however, it can also be arranged so that an external
electric potential is applied to the intermediate electrode 61 to
stabilize the electric potential of the intermediate electrode
61.
[0156] In the present embodiment, the intermediate electrode 61 is
made of a transparent electric conductor and the pixel electrode 1
and the opposing electrode 2 are formed out of a metal layer;
however, by making a connection between layers by forming a contact
hole, etc., it is also possible to form the pixel electrode 1 and
the opposing electrode 2 of a transparent electric conductor and
the intermediate electrode 61 out of a metal layer. This
arrangement increases the number of electrodes made of a
transparent electric conductor, obtaining brighter images.
[0157] It is preferable that the electric potential of the
intermediate electrode 61 be the average of the electric potentials
of the pixel electrode 1 and the opposing electrode 2 as in the
present embodiment; however, setting the electric potential of the
intermediate electrode 61 anywhere between those of the pixel
electrode 1 and the electric potential also achieves a flicker
reduction.
[0158] According to the present embodiment, the resistor 62 is
formed on the intermediate electrode 61 with the insulating layer
11c in between; however, it is not necessary to protect the
intermediate electrode 61 by the insulating layer 11c, if it is
free from damage while the resistor 62 is being subjected to
patterning. Therefore, as shown in FIG. 17(d), it is also possible
to form the portion taken along the line P-P' in FIG. 16 without
having the insulating layer 11c. This allows a reduction of the
manufacturing processes and production costs.
[0159] In this case, a concrete example of a way to obtain the
resistor 62 is as follows. A resin-based resistance-material layer
is formed on the intermediate electrode 61 made of ITO and etched
using a photoresist having a predetermined pattern. It is also
possible to use a photosensitive material as a resin material and
directly conduct patterning by exposure to light. As another
example, it is also possible to apply a resistor to only a
prescribed area by mask deposition.
Embodiment 8
[0160] FIG. 19 is a plan view showing the structure of one dot
serving as a minimal display unit of an array substrate in a
display device according to Embodiment 8 of the invention and FIGS.
20(a), 20(b) and 20(c) are sectional views of FIG. 19 taken along
the lines S-S', T-T' and U-U'. In the present embodiment, instead
of connecting the pixel electrode 1, the intermediate electrode 61
and the opposing electrode 2 by a resistive element, capacitive
coupling is used. In other respects, the construction thereof is
the same as that of Embodiment 5. Therefore, in the present
embodiment, those elements which are identical to the elements of
Embodiment 5 are identified with the same numerical symbols, and
repetitious explanation will be omitted.
[0161] As shown in FIG. 19, according to the present embodiment,
extensions 71 projecting into the left and the right sides from the
top end of the intermediate electrode 61 are formed instead of
forming the resistor 62 shown in FIG. 16. By placing the extension
71 on top of the pixel electrode 1 and the opposing electrode 2,
coupling capacity regions 72a, 72b are formed.
[0162] As shown in FIGS. 19 and 20(a), the extension 71 extending
from the intermediate electrode 61 is formed on the insulating
layer 11b. A coupling capacity region 72a is formed between the
extension 71 and the pixel electrode 1 with an insulating layer 11a
in between. A coupling capacity region 72b is formed between the
extension 71 and the opposing electrode 2 with the insulating
layers 11a, 11b in between.
[0163] FIG. 21 shows an equivalent circuit of the array substrate
described above. Like in Embodiment 7, it is assumed that the
opposing electrode 2 has a ground potential through the common
wiring 3 and that a positive signal electric potential (Va) is
applied to the pixel electrode 1. In this case, by making the
capacitances of the coupling capacity regions 72a, 72b
approximately equal, the electric potential of the intermediate
electrode 61 becomes the average value (Va/2) of the electric
potentials of the pixel electrode 1 and the opposing electrode
2.
[0164] Like in Embodiment 7, In FIG. 19, the distances between the
opposing electrode 2 and the intermediate electrode 61 and between
the intermediate electrode 61 and the pixel electrode 1 are assumed
to be substantially the same, and therefore the strengths of the
electric fields generated in spaces S1, S2, S3 and S4 formed
between the electrodes become the same and their directions are as
shown by the arrows in the figure. Therefore, as in Embodiment 7,
between the left side and the right side of the intermediate
electrode 61 made of a transparent electric conductor, differences
in brightness caused by the flexoelectric effect or a peripheral
electric potential can be cancelled.
[0165] When a negative signal voltage is applied to the next frame,
the directions of the electric fields and the operation of each
space serving as a positive or a negative electrode are reversed;
however, as explained above, differences in brightness can be
cancelled between the right and the left sides of the intermediate
electrode 61. Therefore, the brightness of the positive and the
negative frames become the same in the dot as a whole, eliminating
flicker.
[0166] A display device according to the present embodiment can
reduce fraction defectives and production costs compared to that of
Embodiment 7, since formation of a resistor and a coupling part
(contact hole) connecting the resistor to each electrode becomes
unnecessary.
[0167] In the present embodiment, as in Embodiment 7, by making
connections with different layers by forming a contact hole, etc.,
it is also possible to form the pixel electrode 1 and the opposing
electrode 2 of a transparent electric conductor and the
intermediate electrode 61 out of a metal layer. This arrangement
increases the number of electrodes made of a transparent electric
conductor, obtaining brighter images.
[0168] It is preferable that the electric potential of the
intermediate electrode 61 be the average of the electric potentials
of the pixel electrode 1 and the opposing electrode 2; however,
setting the electric potential of the intermediate electrode 61
anywhere between those of the pixel electrode 1 and the electric
potential also achieves a flicker reduction.
[0169] Taking the difference in the thickness of the insulating
layers in the coupling capacity regions 72a, 72b into
consideration, in order to make the capacities of the coupling
capacity regions 72a, 72b equal, it is preferable to adjust the
opposing area of the electrodes in the coupling capacity regions
72a, 72b. For example, this can be done by varying the widths of
the pixel electrode 1 and the opposing electrode 2 in the portions
where the coupling capacity regions 72a, 72b are formed.
[0170] In the present embodiment, two intermediate electrodes 61
are separately arranged; however, as shown in FIG. 22, it is also
possible to connect the two intermediate electrodes 61 by the
extension 71. This arrangement reliably makes the electric
potentials of the two intermediate electrodes 61 equal. FIG. 20(d)
shows the sectional view taken along the line S-S' in FIG. 22. In
this figure, the left and right coupling capacity regions 72b, 72b
are in parallel, and therefore it is desirable that the total
capacity of the two regions 72b, 72b be equal to that of the
coupling capacity region 72a. Specifically, this is achieved by
varying the widths of the pixel electrode 1 and the opposing
electrode 2 as described above.
Embodiment 9
[0171] FIG. 23 is a plan view showing the structures of two
adjacent dots on an array substrate in a display device according
to Embodiment 9 of the invention. FIGS. 24(a), 24(b) and 24(c) are
sectional views taken along the lines V-V', W-W' and X-X' of FIG.
23.
[0172] In the display device according to Embodiment 5 shown in
FIGS. 10, 11(a) and 11(b), flicker polarities are cancelled in a
dot by dividing the dot into two subdots and making the flicker
polarities different in each subdot. On the other hand, in the
display device according to the present embodiment, two adjacent
dots D1 and D2 are structured so that the flicker polarities are
cancelled between the dots when a signal voltage having the same
polarity is applied to the two dots D1 and D2. In FIGS. 23, 24(a),
24(b) and 24(c), those elements which are identical to the elements
of Embodiment 5 are identified with the same numerical symbols, and
repetitious explanation will be omitted.
[0173] As shown in FIG. 23, the left dot D1 comprises a first pixel
electrode 1a and a first opposing electrode 2a. The first pixel
electrode 1a is made of a transparent electric conductor and the
first opposing electrode 2a is formed out of a metal layer. The
right dot D2 comprises a second pixel electrode 1b and a second
opposing electrode 2b. The second pixel electrode 1b is formed out
of a metal layer and the second opposing electrode 2b is made of a
transparent electric conductor. The first pixel electrode 1a and
the second pixel electrode 1b are connected to separate source
wirings 7 via separate TFTs 5. The structure of the TFT 5 of the
present embodiment is the same as that of Embodiment 1. On a gate
wiring 4, storage capacitor electrodes 10d, 10e are formed and
connected to the first pixel electrode 1a and the second pixel
electrode 1b, respectively.
[0174] As shown in FIGS. 23, 24(a), 24(b) and 24(c), on an array
substrate 9, the gate wiring 4, the first opposing electrode 2a and
a common wiring 3 are formed out of a first metal layer. There
upon, with an insulating layer 11a in between, the source wiring 7,
a drain electrode 6 and the second pixel electrode 1b are formed
out of a second metal layer. There upon, with an insulating layer
11b in between, the first pixel electrode 1a, the second opposing
electrode 2b and the storage capacitor electrodes 10d, 10e are
formed of a transparent electric conductor layer. The first pixel
electrode 1a is connected to the drain electrode 6 through a
contact hole 13. The second opposing electrode 2b is connected to
the common wiring 3 through a contact hole 14.
[0175] In this structure, when a positive voltage is applied to
dots D1 and D2, in the left dot D1, the transparent first pixel
electrode 1a has a relatively positive electric potential, and, in
the right dot D2, the transparent second opposing electrode 2b has
a relatively negative electric potential. When a negative voltage
is applied to dots D1 and D2, in the left dot D1, the transparent
first pixel electrode 1a has a relatively negative electric
potential, and, in the right dot D2, the transparent second
opposing electrode 2b has a relatively positive electric potential.
Thereby, the flicker polarities are cancelled between the two dots
D1 and D2.
[0176] In the display device of the present embodiment, in order to
further enhance the flicker reduction effect, bringing the flicker
polarities of the two dots D1 and D2 into balance is desirable. For
that purpose, it is desirable that the capacities of the two
storage capacitor electrodes 10d, 10e be made equal and it is
advantageous that the two storage capacitor electrodes 10d, 10e be
designed to be formed out of the same material, thereby making the
areas of the two storage capacitor electrodes equal. In the right
subdot SD2 of the present embodiment, as shown in FIG. 24(c), the
transparent second pixel electrode 1b makes a connection between
layers and the storage capacitor electrode 10e is made of a
transparent electric conductor. As a result, the design period of
the TFT array is shortened without adversely affecting the design,
enhancing the manufacturing yield by using a design having a high
tolerance for the errors introduced by the manufacturing process.
As in Embodiment 5, storage capacitor electrodes 10d, 10e can also
be formed on the gate wiring 4 out of a metal layer.
Embodiment 10
[0177] FIG. 25 is a plan view showing the structures of two
adjacent dots on an array substrate in a display device according
to Embodiment 10 of the invention. FIGS. 26(a), 26(b) and 26(c) are
sectional views of FIG. 25 taken along the lines AA-AA', BB-BB' and
CC-CC'.
[0178] In the display device of Embodiment 9, the storage capacitor
electrodes 10d, 10e are formed on the gate wiring 4. On the other
hand, in the display device of the present embodiment, storage
capacitor electrodes 10b, 10c are formed on a common wiring 3. In
FIGS. 25, 26(a), 26(b) and 26(c), those elements which are
identical to the elements of Embodiment 9 are identified with the
same numerical symbols, and repetitious explanation will be
omitted.
[0179] As shown in FIGS. 25, 26(a), 26(b) and 26(c), on an array
substrate 9, a gate wiring 4, a first opposing electrode 2a and a
common wiring 3 are formed out of a first metal layer. There upon,
with an insulating layer 11a in between, a source wiring 7, a drain
electrode 6, a second pixel electrode 1b and storage capacitor
electrodes 10b, 10c are formed out of a second metal layer. There
upon, with an insulating layer 11b in between, a first pixel
electrode 1a and a second opposing electrode 2b are formed of a
transparent electric conductor layer. The first pixel electrode 1a
is connected to the storage capacitor electrode 10b through a
contact hole 13. The second opposing electrode 2b is connected to
the common wiring 3 through a contact hole 14.
[0180] Like that in Embodiment 9, in the display device of the
present embodiment, flicker polarities are cancelled between the
two adjacent dots D1 and D2. The present embodiment has a
distinctive feature in that an uniform display with a reduced
distortion of scanning voltage can be obtained even on a wide
screen because its additional capacitance of the gate wiring 4 is
reduced.
[0181] Like in Embodiment 9, in the display device of the present
embodiment, in order to further enhance the flicker reduction
effect, bringing the flicker polarities of the two dots D1, D2 into
balance is desirable. For that purpose, it is desirable that the
capacities of the two storage capacitor electrodes 10b, 10c be made
equal and it is advantageous that the two storage capacitor
electrodes 10b, 10c be designed to be formed of the same material,
thereby making the areas of the two storage capacitor electrodes
equal. In the left D1 of the present embodiment, the transparent
first pixel electrode 1a makes a connection between layers and the
storage capacitor electrode 10b is formed out of a metal layer. As
a result, the design period of the TFT array is shortened without
adversely affecting the design, enhancing the manufacturing yield
by using a design having a high tolerance for the errors introduced
by the manufacturing process. Like in Embodiment 9, the storage
capacitor electrodes 10b, 10c can also be formed on the common
wiring 3 out of a transparent electric conductor layer.
Embodiment 11
[0182] The present embodiment relates to color display devices with
structures as describe in the above embodiments of the present
invention. In a color display device having dots arranged in a
matrix, a black matrix and a color filter are generally formed in
an opposing substrate facing an array substrate. The color filter
is formed on an aperture of the black matrix, and each pixel
thereof has a color layer of red, green or blue so that, in the
display device as a whole, these three colors are repeated in an
array. In other words, as shown in the area enclosed by the bold
line in FIG. 27, it is common that one pixel 91 is formed out of
three dots each having one of three primary colors, i.e, red (R),
green (G) and blue (B).
[0183] As shown in FIG. 28, this color display device comprises a
scanning signal driver M1 supplying a scanning signal by applying a
prescribed voltage to a gate wiring 4 and an image signal driver M2
supplying an image signal by applying a prescribed voltage to a
source wiring 7. These drivers M1, M2 are controlled by a
controller C. In the color display device having such structure, a
bright image with a wide viewing angle and reduced flicker can be
obtained by arranging each dot shown in FIG. 27 so as to have a
structure as described in the above embodiments of the
invention.
Embodiment 12
[0184] The present embodiment relates to a color display device in
which, in the dot array on an array substrate shown in FIG. 27,
three dots (RGB) in one pixel are structured so as to have the same
structure and flicker polarities are cancelled between any two
adjacent pixels. FIG. 29 shows dot arrays in the two adjacent
pixels.
[0185] In FIG. 29, dots P and Q are structured so that they have
flicker polarities opposite of each other relative to the same
drive voltage. For example, the structure of dot D1 in Embodiments
9 and 10 corresponds to that of P and the structure of dot D2
corresponds to that of Q. The subscripts R, G and B express the
colors of each dot. According to the present embodiment, as shown
in the figure, in the two adjacent pixels 91, 91, the structure of
the dots is the same within a pixel and different from that of the
dots in the adjacent pixel. This makes it possible to readily
cancel flicker polarities between any two adjacent pixels.
Furthermore, since the dots within a pixel have the same
properties, this arrangement has an advantage that color distortion
can be prevented even in halftone display which tends to be
adversely affected by the difference between the brightness and
voltage properties.
Embodiment 13
[0186] In the color display device of Embodiment 12, the dots have
the same structure within a pixel. On the other hand, as shown in
FIG. 30, in a color display device according to the present
embodiment, any two adjacent dots are arranged so as to have
different structures. Thereby, flicker polarities can be cancelled
between more subdivided regions.
Embodiment 14
[0187] The present embodiment relates to a color display device
having the dot array as shown in FIG. 29 which employs a drive
method further enhancing the flicker reduction effect.
[0188] In order to reduce flicker, it is preferable that the
arrangement cycle of the regions showing the same flicker polarity
relative to a same voltage differ from the inversion cycle of a
driving voltage. If the two cycles are coincident with each other,
the effect of inversion is offset, adversely affecting the flicker
reduction effect.
[0189] Next, examples of the repeated patterns of dots (array of P
and Q in FIG. 29) and desirable combinations with the inversion
methods of a drive voltage will be explained. FIGS. 31(a) to 31(f)
show, when it is assumed, as shown in FIG. 29, that the dots within
a pixel 91 have the same structure as in, the polarities of the
drive wave in an odd frame, the dot structure and the patterns of
flicker polarity (odd frame) defined by their combination. Although
not shown in the figures, an even frame has a pattern of drive wave
polarity inverted from that of an odd frame, resulting in a pattern
of flicker polarity opposite to that of an odd frame.
[0190] An enhanced flicker reduction effect can be obtained in an
arrangement in which the distribution of flicker polarity of pixels
or dots is inverted every line.
[0191] Specific examples of such combinations are as follows:
[0192] FIG. 31(a): Combination of a line-inversion (row-inversion)
drive and a line-non-inversion (row-non-inversion) dot array;
[0193] FIG. 31(c): Combination of a frame-inversion drive and a
line-inversion (row-inversion) dot array; and
[0194] FIG. 31(e): Combination of a column-inversion drive and a
line-inversion (row-inversion) dot array.
[0195] On the other hand, for example, when a checkerboard pattern
appears on a computer screen as wallpaper, etc., it is preferable
that, between the pixels or dots, flicker polarity be inverted
every two lines to prevent interference between the checkerboard
pattern and the flicker pattern. Specific examples of such
combinations are as follows:
[0196] FIG. 31(b): Combination of a line-inversion (row-inversion)
drive and a two-line-inversion (two-row-inversion) dot array;
[0197] FIG. 31(d): Combination of a frame-inversion drive and a
two-line-inversion (two-row-inversion) dot array; and
[0198] FIG. 31(f): Combination of a column-inversion drive and a
two-line-inversion (two-row-inversion) dot array.
[0199] Although not shown, as a pattern in which flicker polarity
inversion between the pixels or dots is performed every two lines,
regarding drive wave polarity and dot structure, it is also
possible to switch the pattern of the drive inversion cycle and the
dot arrangement cycle shown in FIGS. 31(b) and 31(f).
Specific examples are as follows:
[0200] (b'): Combination of a two-line-inversion
(two-row-inversion) drive and a line-inversion (row-inversion) dot
array; and
[0201] (f'): Combination of a two-line-inversion
(two-row-inversion) drive and a column-inversion dot array.
[0202] Likewise, when n is 3 or greater, it is possible to invert
flicker polarities between pixels and dots every n lines. When n is
10 or smaller (preferably 5 or smaller), interference with
checkerboard patterns can be prevented while reducing flicker,
obtaining the same effect achieved by inverting flicker polarity
distribution every two lines
Embodiment 15
[0203] The present embodiment relates to a color display device
having the dot array shown in FIG. 30 in which employs a drive
method further enhancing the flicker reduction effect.
[0204] In order to reduce flicker, like in Embodiment 14, it is
preferable that the arrangement cycle of the regions showing the
same flicker polarity relative to a same voltage differ from the
inversion cycle of a driving voltage.
[0205] Next, examples of the repeated patterns of dots (array of P
and Q in FIG. 30) and desirable combinations with the inversion
methods of a drive voltage will be explained. FIGS. 32(a) to 32(d)
show, when assumed the two adjacent dots have the different
structures in within pixel 91 as shown in FIG. 30, polarities of
drive wave in an odd frame, the dot array and patterns of flicker
polarity (odd frame) defined by their combination. Although not
shown in the figure, an even frame has a pattern of drive wave
polarity inverted to that of an odd frame, resulting in having a
pattern of flicker polarity inverted to that of an odd frame.
[0206] Like in Embodiment 14, an enhanced flicker reduction effect
can be obtained in an arrangement in which the distribution of
flicker polarity of pixels or dots is inverted every line.
[0207] Specific examples of such combinations are as follows:
[0208] FIG. 32(a): Combination of a line-inversion (row-inversion)
drive and a line-non-inversion (row-non-inversion) dot array;
and
[0209] FIG. 32(c): Combination of a frame-inversion drive and a
line-inversion (row-inversion) dot array.
[0210] On the other hand, for example, when a checkerboard pattern
appears on a computer screen as a wallpaper, etc., it is preferable
that, between the pixels or dots, flicker polarity be inverted
every two lines to prevent interference between the a checkerboard
pattern and the flicker pattern. Specific examples of such
combinations are as follows:
[0211] FIG. 32(b): Combination of a line-inversion (row-inversion)
drive and a two-line-inversion (two-row-inversion) dot array;
and
[0212] FIG. 32(d): Combination of a frame-inversion drive and a
two-line-inversion (two-row-inversion) dot array.
[0213] Although not shown, as a pattern in which flicker polarity
inversion between the pixels or dots is performed every two lines,
regarding drive wave polarity and dot structure, it is also
possible to switch the pattern of the drive inversion cycle and the
dot arrangement cycle in FIG. 32(b). Specific examples are as
follows:
[0214] (b'): Combination of a two-line-inversion
(two-row-inversion) drive and a line-inversion (row-inversion) dot
array.
[0215] Likewise, when n is 3 or greater, it is possible to invert
flicker polarities between pixels and dots every n lines. When n is
10 or smaller (preferably 5 or smaller), interference with
checkered patterns can be prevented while reducing flicker,
obtaining the same effect achieved by inverting flicker polarity
distribution every two lines.
[0216] The display devices according to Embodiments 14 and 15 can
be operated in the same manner as shown in FIG. 28 described above.
This allows a display of bright images with wide viewing angles and
reduced flicker, when considering the entire screen or the regions
comprising a plurality of dots as a whole.
[0217] In Embodiments 14 and 15, the relationship between the drive
wave polarity and the dot array are explained with a dot taken as
the unit; however, the preferable combinations of the drive wave
polarity and the dot array described in the embodiments can also
achieve the same effects when a pixel is assumed to be the unit.
Furthermore, it is also possible to consider the drive wave
polarity or the dot array with a dot as the unit and the other with
a pixel as the unit.
Embodiment 16
[0218] FIG. 33(a) is a sectional view of the display device
according to Embodiment 16 and FIG. 33(b) is a plan view showing
the structure of one dot of the array substrate. FIG. 33(a) is a
view taken along the line DD-DD' in FIG. 33(b).
[0219] FIG. 34 is an expanded sectional view showing the structure
around a switching element of a display device according to the
present embodiment.
[0220] FIG. 35(a) is a plan view showing a 4.times.4 dot section of
pixels and FIGS. 35(b) and 35(c) are schematic diagrams showing the
polarities created in the pixels. In FIG. 35(a), S1, S2, etc.
indicate image signals supplied to each pixel and G1, G2, etc.
indicate scanning signals supplied to each pixel.
[0221] In FIG. 33, 201 represents an opposing substrate, 202
represents liquid crystal, 209a represents an oriented film formed
inside of the array substrate 9, 209b represents an oriented film
formed inside of the opposing substrate 201, and 210a, 210b and
210c represent color filter materials. Other elements which are
identical to the elements of Embodiment 1 are identified with the
same numerical symbols, and repetitious explanation will be
omitted. In the present embodiment, a pixel electrode 1 is formed
out of a metal material and an opposing electrode 2 is formed of a
transparent electric conductor.
[0222] In FIG. 34, 8a represents an a-Si layer, 8b represents an n+
type a-Si layer and 14 represents a contact hole formed in
insulating layers 11a, 11b.
[0223] The display device of the present embodiment is formed in
the manner described below. On the array substrate 9, a first metal
layer is formed of an opaque electric conductor made of Al, Ti or
the like. The first metal layer is patterned into predetermined
shapes to obtain a common wiring 3 and a gate wiring 4. On the thus
obtained layer, the insulating layer 11a is formed, and then, a
semiconductor switching element 5 is formed out of the a-Si layer
8a and the n+ type a-Si layer 8b on the predetermined area of the
insulating layer 11a.
[0224] Thereafter, on the predetermined areas of the insulating
layer 11a and the semiconductor switching element 5, a second metal
layer is formed out of an opaque electric conductor made of Al, Ti
or the like, and then the second metal layer is patterned into
predetermined shapes to obtain a source wiring 7, a drain electrode
6 and a pixel electrode 1. On the thus obtained layer, the
insulating layer 11b made of SiNx or the like is formed. The
insulating layer 11b also serves as an overcoat protecting the
semiconductor switching element 5.
[0225] Thereafter, the opposing electrode 2 is formed out of an ITO
film, which is a transparent electric conductor. In order to make
the common wiring 3 made of an opaque electric conductor and the
opposing electrode 2 made of a transparent electric conductor
electrically conductive, a contact hole 14 is formed in the
insulating layers 11a, 11b.
[0226] Then, on the array substrate 9 and the opposing substrate
201, oriented films 209a, 209b made of polyimide or the like are
formed to align molecules of liquid crystal 202.
[0227] The opposing substrate 201 is arranged so as to face the
array substrate 9. On the opposing substrate 201, the red color
filter material 210a, the green color filter material 210b, the
blue color filter material 210c and a black matrix 211 are formed
so as to have a predetermined pattern.
[0228] The thus obtained array substrate 9 and opposing substrate
201 have their orientation directions formed in predetermined
directions. The substrates are bonded together on the edges by a
sealer, and liquid crystal 202 is sealed therein.
[0229] Operation of the display device is described below. The
semiconductor switching element 5 has its on-and-off status
controlled by drive signals supplied from the gate wiring 4. Then,
an electric field is generated by a liquid crystal drive voltage
applied between the pixel electrode 1 and the opposing electrode 2;
which are both connected to the semiconductor switching element 5.
By varying the orientation directions of the liquid crystal 202,
the brightness (light transmittance) of each pixel is controlled to
achieve image formation.
[0230] In FIG. 33, d represents the cell gap, w1 represents the
wiring width of the opposing electrode 2, w2 represents the wiring
width of the pixel electrode 1 and 1 represents the distance
between the opposing electrode 2 and the pixel electrode 1.
[0231] In the present embodiment, as shown in FIG. 33, it is
assumed that the wiring width of the opposing electrode 2 is 5
.mu.m (w1=5 .mu.m), the wiring width of the pixel electrode 1 is 4
.mu.m (w2=4 .mu.m), the cell gap is 4 .mu.m (d=4 .mu.m) and the
distance between the electrodes is 10 .mu.m (1=10 .mu.m). In other
words, it is designed so that the wiring widths of the opposing
electrode 2 and the pixel electrode 1 (w1, w2) become approximately
the same as the distance between the array substrate 9 and the
opposing substrate 201, i.e., d (cell gap).
[0232] Regarding the shape of the electrode, for example, as shown
in FIG. 33(b), preferable is a comb-like electrode in which the
opposing electrode 2 and the pixel electrode 1 are alternately
arranged with a lateral electric field generated between the
opposing electrode 2 and the pixel electrode 1. By employing the
above described electrode arrangement, in addition to the lateral
electric field, the peripheral electric fields of the individual
electrodes 1, 2 enhance the electric field strength on the
electrodes, rotating the liquid crystal. In the present embodiment,
by forming the pixel electrode 2 out of a transparent conductive
material, the electrode transmits light.
[0233] According to such an electrode structure, for example, by
employing the liquid crystal 202 described below, it is possible to
supply sufficient electric field strength and drive the liquid
crystal using a generally applied liquid crystal drive voltage
(around 5V).
[0234] Specifically, as a liquid crystal material 202, a
cyano-based liquid crystal material containing a cyano-based
compound in the range from about 10% to about 20% is used. Here,
the optical-path difference .DELTA.n.times.d (multiply the cell gap
d by the difference in the refractive index .DELTA.n) is assumed to
be around 350 nm. It is also assumed that the liquid crystal
material used in the liquid crystal layer 2 have a splay elastic
constant K11 of 12 pN (K11=12 pN), a twist elastic constant K22 of
7 pN (K22=7 pN), a bend elastic constant K33 of 18 pN (K33=18 pN)
and a dielectric constant anisotropy .DELTA.e of +8 (.DELTA.e=+8).
The dielectric constant anisotropy .DELTA.e and the bend elastic
constant K33 are important factors in selecting the drive voltage
applied to the liquid crystal. To lower the voltage, it is
preferable that the dielectric constant anisotropy .DELTA.e be +8
or greater and the bend elastic constant K33 be 18 pN or smaller.
Cyano-based compounds are useful to prevent the localized
accumulation of electric charge in the liquid crystal; however,
having a concentration thereof exceeding 35% may lower the
reliability of the device because the ionicity is too strong.
[0235] Furthermore, since the pixel electrode 1 and the opposing
electrode 2 are crooked, the liquid crystal molecules rotate in two
directions. Therefore, differences in color observed from different
viewing angles can be eliminated, obtaining a panel structure
exhibiting little variance in color when seen from variable
directions. Not shown in the figure, however, if the source wiring
7 and the black matrix 211 are formed into crooked shapes having
the same crooking angles as the opposing electrode 2 and the pixel
electrode 1, the increase in area which blocks light caused by the
crooked shapes of the electrodes 1, 2 can be offset, obtaining a
liquid crystal display device exhibiting a further enhanced
aperture ratio.
[0236] The advantages achieved by the display device of the present
embodiment will be described below. FIGS. 36(a) and 36(b) show the
light transmittance properties of a pixel portion in a display
device according to the present embodiment. In this figure, the
pixel electrodes (opaque) 1, 1, the opposing electrode
(transparent) 2 and the relative brightness distribution
(transmittance distribution) in the aperture are shown. FIG. 36(a)
shows the case where a positive image signal is applied to the
pixel electrode 1 and FIG. 36(b) shows the case where a negative
image signal is applied to the pixel electrode 1. From the figures,
it is understood that the light transmittance properties are
changed by the polarities of the liquid crystal drive voltage,
causing flicker polarity (light or dark polarity). As described
above, the light transmittance properties are changed by the
polarities of the liquid crystal drive voltage, and therefore the
frame-inversion drive whereby the polarity is inverted every frame
causes flicker. In the H-line inversion drive in which a drive
voltage polarity is inverted every line or the V-line inversion
drive in which a drive voltage polarity is inverted every column,
when a specific pattern such as a vertical line or a horizontal
line is displayed, it appears on the screen as vertical stripes or
horizontal stripes.
[0237] Therefore, the drive method employed in the liquid crystal
display device of the present embodiment is the 1H/1V
line-inversion drive (also referred to as the dot inversion drive)
in which the polarity inversion of the pixel voltage is performed
every line and every column as shown in FIG. 35(b). As an
alternative, the 2H/1V line-inversion drive as shown in FIG. 35(c)
in which polarity inversion of the pixel voltage is performed every
two lines and every column can be employed.
[0238] In the 1H/1V line-inversion drive shown in FIG. 35(b), when
a pattern of vertical lines or horizontal lines are displayed, the
brightness difference between the positive and negative polarities
are cancelled between two adjacent pixels, and thereby apparent
flicker can be cancelled. On the other hand, when a checkerboard
patter is displayed, the 2H/1V line-inversion drive shown in FIG.
35(c) is preferable. According to this method, even in a
checkerboard pattern, the brightness difference between the
positive and negative polarities can be cancelled, and thereby
flicker does not appear on the screen. The same effect can be
achieved by employing the 1H/2V line-inversion drive.
[0239] In the present embodiment, inversion of drive voltage
polarity is performed with the dot taken as the unit; however, it
is also possible to perform inversion of the pixel voltage polarity
in the manner as shown in FIG. 35(b) or FIG. 35(c) when a pixel
composed of red, green and blue dots is assumed to be the unit.
This arrangement is advantageous in that color distortion can be
prevented even in halftone display which tends to be adversely
affected by the difference between the properties of brightness and
voltage, since the properties of each dot in a pixel can readily be
balanced.
[0240] The conventional drive frequency for a frame is 30 Hz;
however, apparent flicker can be cancelled by applying a frequency
of 60 Hz, since even if the brightness differences caused by
polarities are generated, the human eye cannot recognize them at a
frequency this high. This is also true in other embodiments.
Embodiment 17
[0241] FIG. 37(a) is a sectional view of the display device
according to Embodiment 17. FIG. 33(b) is a plan view showing the
structure of one dot of the array substrate. FIG. 37(a) is a view
taken along the line EE-EE' in FIG. 37(b).
[0242] FIG. 38 is an expanded sectional view showing the structure
around a switching element of a display device according to the
present embodiment.
[0243] FIG. 39(a) is a plan view showing a 4.times.4 dot section of
pixels and FIG. 39(b) is a schematic diagram showing the waveform
of image signal applied to each pixel shown in FIG. 39(a). In FIG.
39(a), S1, S2, . . . indicate image signals supplied to each pixel,
S1', S2', . . . indicate compensated image signals supplied to each
pixel and G1, G2, . . . indicate scanning signals supplied to each
pixel.
[0244] In the present embodiment, both the pixel electrode 1 and
the opposing electrode 2 are made of transparent electric
conductors. In other respects, the configuration thereof is the
same as that of the Embodiment 16. Therefore, the elements which
are identical to the elements of Embodiment 16 are identified with
the same numerical symbols, and repetitious explanation will be
omitted.
[0245] The display device of the present embodiment is formed in a
manner as described below. On the array substrate 9, a first metal
layer is formed out of an opaque electric conductor made of Al, Ti
or the like, and the first metal layer is patterned into
predetermined shapes to obtain a common wiring 3 and a gate wiring
4. On the thus obtained layer, the insulating layer 11a is formed
and a semiconductor switching element 5 formed out of an a-Si layer
8a and an n+ type a-Si layer 8b is obtained on the predetermined
area of the insulating layer 11a. Thereafter, on the predetermined
areas of the insulating layer 11a and the semiconductor switching
element 5, a second metal layer is formed out of an opaque electric
conductor made of Al, Ti or the like, and then the second metal
layer is patterned into predetermined shapes to obtain a source
wiring 7, a drain electrode 6 and a pixel electrode 1. On the thus
obtained layer, the insulating layer 11b made of SiNx or the like
is formed. The insulating layer 11b also serves as an overcoat
protecting the semiconductor switching element 5.
[0246] Thereafter, on the insulating layer 11b, the pixel electrode
1 and the opposing electrode 2 are formed out of an ITO film, which
is a transparent electric conductor. The opposing electrode 2 is
connected to the common wiring 3 through a contact hole 14 formed
in the insulating layers 11a, 11b. The pixel electrode 1 is
connected to the drain electrode 6 through a contact hole 13 formed
in the insulating layer 11b. Instead of forming the pixel electrode
1 and the opposing electrode 2 on the same layer as in the present
embodiment, it is also possible to provide another layer and form
the electrodes on separate layers.
[0247] Then subsequent production steps are the same as those of
Embodiment 16. In the thus obtained display device of the present
embodiment, both the pixel electrode 1 and the opposing electrode 2
are transparent, realizing a display device with an enhanced actual
aperture ratio compared to that of Embodiment 16.
[0248] The advantages achieved by the display device of the present
embodiment will be described below. FIGS. 40(a) and 40(b) show
light transmittance properties of a pixel portion in a display
device according to the present embodiment. In this figure, the
pixel electrodes (transparent) 1, 1, the opposing electrode
(transparent) 2 and the relative brightness distribution
(transmittance distribution) in the aperture are shown. FIG. 40(a)
shows the case where a positive image signal is applied to the
pixel electrode 1 and FIG. 40(b) shows the case where a negative
image signal is applied to the pixel electrode 1. From the figures,
it is understood that the light transmittance properties are
changed by the polarities of the liquid crystal drive voltage,
causing flicker polarity (light or dark polarity). In FIG. 40,
there are two pixel electrodes 1 and one opposing electrode 2.
Therefore, even if both electrodes are transparent, the displayed
images become brighter in the case (b) where the pixel electrode 1
has a relatively negative voltage. This phenomenon occurs because
of the difference in numbers and areas between the pixel electrode
1 and the opposing electrode 2.
[0249] In the present embodiment, as shown in FIG. 39, the
difference in brightness between the positive and negative
polarities can be cancelled by supplying brightness compensation
signals S1', S2, . . . in addition to general image signals S1, S2,
. . . . Specifically, when a positive liquid crystal drive voltage
is applied to the pixel electrodes 1, 1, as shown in FIG. 39(b),
compensation for the image signal is performed by adding brightness
compensation signals +S1', +S2' . . . to image signal S1. Thereby,
the variance in the electric potential of a liquid crystal drive
voltage is increased and the displayed image becomes brighter. As a
result, as shown in FIG. 40(a), the light transmittance property
changes from the condition without brightness compensation signal,
represented by the solid line, to the condition with brightness
compensation signal, represented by the broken line.
[0250] On the other hand, when a negative liquid crystal drive
voltage is applied to the pixel electrodes 1, 1, as shown in FIG.
39(b), compensation for image signal is performed by adding
brightness compensation signals -S1', -S2' . . . to image signal
S1. Thereby, the variance in the electric potential of a liquid
crystal drive voltage is decreased and the displayed image becomes
darker. As a result, as shown in FIG. 40(b), the light
transmittance property changes from the condition without
brightness compensation signal represented by the solid line to the
condition with brightness compensation signal represented by the
broken line.
[0251] By increasing or decreasing the transmittance properties by
supplying brightness compensation signals, the variances in
brightness when a positive liquid crystal drive voltage is applied
and when a negative liquid crystal drive voltage is applied can be
made approximately the same.
[0252] It is preferable that the brightness compensation signals
S1', S2' . . . be controlled so that an appropriate voltage is
supplied based on the ratio of the area of between the pixel
electrode 1 and the opposing electrode 2, which are both formed out
of transparent conductive layers. Specifically, the variance in
brightness caused by the polarity of a liquid crystal drive voltage
can be cancelled if the area SA of the transparent pixel electrode
and the area SB of the transparent opposing electrode 2 are the
same. When SA and SB are different, the variance in brightness
caused by polarity remains. The more the ratio of SA to SB moves
away from 1, the greater the variance in brightness is. Therefore,
it is preferable that the variance in brightness be cancelled by
supplying an appropriate compensation voltage obtained based on a
calculation of how far away the area ratio is from 1. Having this
arrangement allows a flicker reduction by canceling the variance in
brightness caused by polarities regardless of the number of
electrodes.
[0253] In the present embodiment, both the pixel electrode 1 and
the opposing electrode 2 are transparent; however, even if only the
opposing electrode 2 is formed out of a transparent conductive
layer like in Embodiment 16, the same effect can be achieved by
adding brightness compensation signals.
[0254] It is also true that in the present embodiment, like in
Embodiment 16, the flicker reduction effect can be enhanced by
employing the double-speed drive method which has a drive frequency
of 60 Hz or higher.
[0255] In Embodiment 16 and the present embodiment, a-Si (amorphous
silicon) is used for forming a semiconductor switching element 5;
however, use of p-Si (polysilicon) or the other semiconductor
layers can also achieve a similar result. This is true also in the
other embodiments.
[0256] In Embodiment 16 and the present embodiment, there are
explanations of cases where the pixel electrode 1 and the opposing
electrode 2 are crooked; however, the actual aperture ratio can be
enhanced regardless of the electrode shape, allowing use of linear
electrodes, U-shape electrodes or others. This is true also in the
other embodiments.
Embodiment 18
[0257] In the embodiments descried above, a rectangular dot was
taken for the example as the display unit, and the cases where the
dots are arranged in matrix are explained. However, the advantage
of the present invention is satisfactorily achieved even when a
display unit is not a rectangular dot or the display units are not
arranged in matrix.
[0258] To be more specific, the present invention can be applied to
a structure having elements whose functions are substantially the
same as those of a pixel electrode and an opposing electrode even
if the elements are not called by such names. Such examples include
a circular-graph-shaped indicator I as shown in FIG. 41 for use in
several kinds of meters and a segment display as shown in FIG. 42
for use in display of numerical characters or the like. By
structuring each display block B and each segment SG based on the
schemes described in the embodiments of the invention, it is
possible to cancel flicker in the blocks B and the segments SG. As
a result, an excellent display with a wide viewing angle,
satisfactory brightness and reduced flicker can be obtained.
[0259] Also in a liquid crystal display device having a different
arrangement, by structuring display units that perform the same
kind of display based on the schemes described in the above
embodiments, it is possible to cancel flicker within each display
unit. Thereby, an excellent display with a wide viewing angle,
satisfactory brightness and reduced flicker can be obtained.
[0260] Also in the case where the whole surface of a wide image is
controlled by a single signal such as in shutters for lighting or
blinds for windows, with assuming it to be one display unit, by
structuring the display unit so as to have two flicker polarities
based on the schemes described in the embodiments of the invention,
flicker can be cancelled in each display unit. This achieves light
control with reduced flicker and without being affected by a
variance in the observation direction.
Other Embodiments
[0261] In the display devices according to the embodiments
described above, it is preferable that, in one dot, the total
number of the pixel electrodes 1 and the opposing electrodes 2 be
an odd number and the number of intervals between the pixel
electrodes 1 and the opposing electrodes 2 be an even number. For
example, in the structure of Embodiment 1 shown in FIG. 1 and that
of Embodiment 5 shown in FIG. 5, by arranging the numbers of the
electrodes and the intervals therebetween as above, the composition
of a dot becomes almost symmetrical in the left and the right
halves, the flicker reduction effect can be enhanced. This is also
true in the arrangements of Embodiment 9 shown in FIG. 25 and
Embodiment 10 shown in FIG. 23.
[0262] In the arrangements of Embodiment 5 shown in FIG. 10 and
Embodiment 6 shown in FIG. 14, it is preferable that the number of
the opposing electrodes disposed between the two source wirings be
an odd number so that one of the opposing electrodes is situated in
the middle of the two source wirings. Thereby, two dots can be
separated by the opposing electrode, enhancing the aperture
ratio.
[0263] In the arrangements of Embodiment 7 shown in FIG. 16 and
Embodiment 8 shown in FIG. 19, it is preferable that the total
number of opposing electrodes in the dot be an odd number, and it
is more preferable that the number be 5+4n (n is an integer), i.e.,
5 or 9, etc. This allows the composition of the dot to become
almost symmetrical in the left and the right halves, enhancing the
flicker reduction effect.
[0264] In the embodiments descried above, IPS-style liquid crystal
display devices are used; however, as long as they have an
arrangement comprising a pixel electrode and an opposing electrode
on one substrate, there is no limitation on the style of the liquid
crystal display used.
[0265] Furthermore, with respect to the materials of the pixel
electrode and the opposing electrode, they are not limited to a
combination of a transparent conductive layer and a metal layer.
For example, a material, even one which is not completely
transparent, if it exhibits a transmittance at a certain level, has
the effect of improving the brightness of the display device.
Therefore, such a material and a metal layer can be used in
combination. A combination of two transparent conductive layers
having different transmittances is also possible. This arrangement
can further enhance the transmittance.
[0266] When performing reflective-type display, the present
invention can be employed in a display device comprising a
combination of two materials having different reflectances or a
display device having a reflective electrode as a back side
electrode and a transparent electrode as an observer's side
electrode.
[0267] In a liquid crystal display device, as described above,
flicker tends to occur in the case where some portion of an
electric field has a splay-shape around an electrode, causing the
flexoelectric effect, and the case where electric fields become
asymmetric in the left and right electrodes affected by a
peripheral electric potential caused by some portion of a display
unit having no electrode. The present invention exhibits remarkable
advantages compared to display devices having such structures.
[0268] An example of a display device having such a structure
includes a liquid crystal display device in which liquid crystal is
driven by an electric field substantially parallel to a substrate.
To be more specific, an IPS-style liquid crystal display device in
which liquid crystal molecules respond only in the direction
parallel to the substrate, an FFS (Fringe Field Switching) style
liquid crystal display device, and an HS (Hybrid Switching) style
liquid crystal display device are included. The above-described
structures of the embodiments of the invention can be applied to a
perpendicular-oriented type liquid crystal display device, in which
liquid crystal molecules L are perpendicularly oriented as shown in
FIG. 43(a) when a drive voltage is turned off and the orientation
angles of the liquid crystal molecules L change along the electric
field generated between electrodes 21, 22 as shown in FIG. 43(b)
when the drive voltage is turned on.
[0269] In an MVA (Multi-domain VA) style liquid crystal device, a
splay-shaped electric field is used when the orientation of the
liquid crystal is divided by the distortion of the electric field.
Therefore, the arrangements of the present invention can be
employed in a reflective-type display or the like which uses
electrodes having different optical properties.
[0270] Rod type low molecular weight compounds are generally used
as the liquid crystal materials in each embodiment of the
invention. In order to obtain the desired properties, anywhere from
a few to several dozen kinds of materials are mixed. There is no
limitation on the materials used; however, in order to reduce the
flicker polarities, a mixture containing a compound having a wider
end directed to the positive electrode side, which prevents the
flexoelectric effect, is desirable. The compounds represented by
general formulae (A) to (F) below are preferable.
##STR00001##
[0271] In the above general formulae (A) to (F), X and Y represent
cyclic hydrocarbon residues. Specific examples are aromatic
hydrocarbon residues (benzene ring), aliphatic hydrocarbon residues
(cyclohexane ring), and compounds in which some of the carbon atoms
composing aromatic hydrocarbon residues or aliphatic hydrocarbon
residues are replaced by hetero atoms such as nitrogen or
oxygen.
[0272] P represents central groups including ester groups
(--COO--), etc. P includes something which directly connects the
groups on both sides.
[0273] In the end groups, Cn can be F, CF.sub.3, CHF.sub.2 or
CH.sub.2F and CH.sub.3 can be C.sub.nH.sub.2n+1 (n is an integer
from 2 to 20). These end groups are responsible for changes in the
properties of the liquid crystal such as electric or optical
anisotropy and the temperature in which it is a range forming
liquid crystal.
[0274] The number of the cyclic groups contained in the core
section enclosed by the broken line is three or less in practical
applications; however, it can be greater than three.
[0275] As specific examples of the compounds described above, the
compounds represented the formula below are included.
##STR00002##
* * * * *