Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-12-05
2004-09-07
Pham, Long (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S184000, C257S187000, C257S203000, C257S221000, C257S292000, C257S293000, C257S461000, C257S462000, C257S929000
Reexamination Certificate
active
06787829
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a liquid crystal display panel, and more particularly to an improvement for obtaining better display quality in a horizontal electric field liquid crystal display panel, typified by an in-plane switching (IPS) mode liquid crystal display panel.
BACKGROUND ART
A liquid crystal display panel is an electro-optical device for displaying images in which an electric field is formed in a liquid crystal layer that is sandwiched between a pair of substrates and the intensity of the light transmitting through the liquid crystal layer is controlled by varying the orientation of liquid crystal molecules in the liquid crystal layer.
Systems for applying electric field across the liquid crystal layer are classified into two types: a so-called vertical electric field system such as twisted nematic (TN) mode, wherein a pair of electrodes for applying an electric field, which drives liquid crystal molecules, are provided on different substrates; and a so-called horizontal electric field system typified by IPS mode, wherein pairs of electrodes are provided on the same substrate.
An example of an IPS mode liquid crystal display panel is shown in
FIG. 14. A
comb-shaped pixel electrode
2
and a comb-shaped common electrode
1
are disposed in a pixel region, which consists of a region surrounded by a pair of image signal lines
5
and a pair of scanning signal lines
6
. The common electrode
1
is electrically connected to a common electrode bus bar
9
. The pixel electrode
2
and the common electrode
1
are electrically insulated from each other, and when a switching element
7
electrically connects the image signal line
5
and the pixel electrode
2
in response to the scanning signal supplied via the scanning signal line
6
, an electric field is formed between the pixel electrode
2
and the common electrode
1
in the manner shown in FIG.
15
.
The IPS mode liquid crystal display panel has an advantage over vertical electric field liquid crystal display panels in that the IPS mode LCD panel shows smaller hue variation dependent on viewing angle. However, the IPS mode LCD panel has a lower aperture ratio than that of the vertical electric field system and therefore requires a high power backlight to ensure sufficient display image brightness. In order to improve pixel aperture ratio, Japanese Unexamined Patent Publication No. 9-61842 suggests that a first or a second electrode (i.e., pixel electrode
2
or common electrode
1
) be made of a transparent conductor.
Note that, as shown in
FIG. 15
, an electric field is formed between the image signal line
5
and a common electrode
1
a
disposed adjacent thereto. Accordingly, the liquid crystal molecules located in the region designated by the slanted lines in
FIG. 14
do not behave in the same fashion as do those located between the pixel electrode
2
and a common electrode
1
b
, but are driven by the electric field formed between the common electrode
1
a
and the image signal line
5
. Therefore, this region does not exhibit a brightness that is required for the image to be displayed. In the operation of a liquid crystal display panel, generally, the polarity of the potential V
S
of the image signal line
5
is inversed with respect to the potential V
C
of the common electrode bus bar
9
so that the adjacent pixels or the adjacent lines have different polarities. For this reason, in cases where the polarity of the potential V
pix
of the pixel electrode
2
differs from that of the potential V
S
, depending on various conditions such as shapes of the image signal line
5
, the common electrode
1
, and the pixel electrode
2
, the gap of the pair of electrodes, the amplitude of V
S
, and so forth, the potential V
S
affects the electric field formed between the common electrode
1
a
and the pixel electrode
2
to change the orientation state of the liquid crystal molecules located between these electrodes. Thus, brightness variation is sometimes caused within a pixel, which prevents good image display.
In particular, when the screen is viewed from an oblique direction, light transmitting through this region does not go through the black matrix disposed in the panel, and thereby displayed images are adversely affected. This phenomenon greatly reduces the advantage of having a wide viewing angle that is attained by the horizontal electric field system liquid crystal display panel. It is possible to reduce the adverse effect by using a wide black matrix, but this technique reduces aperture ratio as a function of the relationship of the widths and the positions between the black matrix and the common electrodes.
In the prior art technique of the above-described publication, if the common electrode line
1
a
that is adjacent to the image signal line
5
is made of a transparent material, displayed image quality degrades because the region that shows a brightness different from a desired brightness is made to be a display region. That is, the mere use of a transparent electrode only exacerbates image quality degradation caused by this region. In view of this problem, by increasing the width of the common electrode
1
a
adjacent to the image signal line
5
, for example, it is possible to reduce the adverse effect of the electric field formed between the image signal line
5
and the common electrode
1
a
, which influences the brightness of the pixel. However, this technique requires a large non-display region, lowering the aperture ratio, which is not particularly high in comparison with liquid crystal display panels of other display modes. To maintain brightness of the liquid crystal panel while maintaining the aperture ratio, the output power of the backlight needs to be increased. Consequently, an increase in power consumption is inevitable.
When the area per pixel is reduced to obtain higher resolution, the aperture ratio of the pixels accordingly decreases unless the widths of the pixel electrode and the counter electrode are reduced in proportion to the area. The above-mentioned prior art technique can reduce pixel size without decreasing the aperture ratio, but display quality is lowered because the region between the common electrode
1
a
and the image signal line
5
becomes a display region.
Japanese Unexamined Patent Publication No. 9-179096 proposes, as shown in
FIG. 16
, a liquid crystal display panel that has a conductive layer
50
covering portions of an image signal line
5
and a common electrode (reference electrode)
1
, the conductive layer formed thereover and over an insulating layer (not shown). According to the publication, the electric field formed in the liquid crystal layer due to the potential of the image signal line
5
is shielded by the conductive layer
50
and therefore does not reach the common electrode
1
. However, in this technique, since a capacitance is formed between the image signal line
5
and the conductive layer
50
, undercharging caused by a large wiring time constant and consequent signal waveform deterioration occurs when the panel size is large and the image signal lines are accordingly long. Moreover, in cases where the insulating film has defects, such as pinholes, a short circuit between the image signal line and the conductive layer occurs, resulting in display defects.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a liquid crystal display panel that solves the foregoing problems of the prior art, has a high aperture ratio, and controls the image quality degradation caused by the electric field generated between the image signal lines and the common electrodes adjacent thereto.
A liquid crystal display panel according to the present invention has a line-shaped pixel electrode and a line-shaped common electrode disposed in each of pixel regions of the array substrate that is surrounded by a pair of image signal lines and a pair of scanning signal lines, and of these line-shaped electrodes, the electrodes that are disposed adjacent to and parallel the signal lines are made of an opaque condu
Asada Satoshi
Fukami Tetsuo
Kimura Masanori
Kumagawa Katsuhiko
Okafuji Michiko
Louie Wai-Sing
Matsushita Electric - Industrial Co., Ltd.
Parkhurst & Wendel L.L.P.
Pham Long
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