Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reissue Patent
2000-10-11
2002-03-19
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S043000, C349S144000
Reissue Patent
active
RE037591
ABSTRACT:
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION:
The present invention relates to a liquid crystal display device which is used as a display of a computer, a TV set, and the like, for example, and includes switching elements such as thin film transistors (hereinbelow, referred to as TFTs) as addressing elements.
2. DESCRIPTION OF THE RELATED ART:
FIG. 11
is an exemplary equivalent circuit of a conventional TFT type liquid crystal display device using TFTs as switching elements. Pixel electrodes
6
are formed in a matrix, and TFTs
1
as switching elements are connected to the respective pixel electrodes
6
. A gate electrode of each TFT
1
is connected to a gate line
2
as a scanning line so that the TFT
1
can be driven under control of a gate signal input to the gate electrode via the gate line
2
. A source electrode of the TFT
1
is connected to a source line
3
as a signal line so that a data (display) signal can be input to the corresponding pixel electrode
6
via the TFT
1
when the TFT
1
is being driven. Such gate lines
2
and source lines
3
are provided perpendicular to each other along the peripheries of the pixel electrodes
6
arranged in a matrix. A drain electrode of each TFT
1
is connected to the corresponding pixel electrode
6
and a supplemental capacitance (C
s
). A counter electrode of the supplemental capacitance is connected to a common line
4
(hereinbelow, referred to as a C
s
line). Liquid crystal (C
1c
) interposed between the pixel electrode
6
and a counter electrode
18
is thus driven.
In order to reduce power consumption of such an active matrix liquid crystal display device, a technique of increasing the aperture ratio of a liquid crystal display device by overlapping pixel electrodes with lines via an interlayer insulating film is disclosed in Japanese Laid-Open Patent Publication No. 6-160900. According to this technique, however, the orientation of the liquid crystal is disordered due to a failure in rubbing at steps formed by lines and TFTs, and crosstalk arises due to a parasitic capacitance generated between the interlayer insulating film and the pixel electrodes. Such troubles badly influences the display. The above publication neither discloses nor suggests any means for solving these problems.
FIG. 12
is a plan view of one pixel portion of an active matrix substrate of a liquid crystal display device
600
proposed by the applicant of the present application in Japanese Patent Application No. 7-206367for solving the above problems.
Referring to
FIG. 12
, the gate lines
2
and the source lines
3
as shown in
FIG. 11
are formed on a transparent substrate to run perpendicular to each other. The TFT
1
is formed as a switching element in the vicinity of each of the crossings of the gate lines
2
and the source lines
3
, and is connected to the corresponding pixel electrode
6
through a connection line
5
via a contact hole
7
formed through an interlayer insulating film (not shown). The connection line
5
overlaps a supplemental capacitance (C
s
) line
4
via a gate insulating film (not shown), forming a supplemental capacitance. The pixel electrode
6
overlaps the adjacent gate lines
2
and source lines
3
via the interlayer insulating film. With this configuration, the aperture ratio of the resultant liquid crystal display device improves, and disclination can be suppressed since the electric field generated by the lines
2
and
3
is shielded. Moreover, when the gate and source lines
2
and
3
are formed of a conductive light-shielding material such as metal, they can be used as light-shielding films between the pixel electrodes
6
. The parasitic capacitance can be reduced by increasing the thickness of the interlayer insulating film to about 2 &mgr;m. Conventionally, the vertically adjacent pixel electrodes
6
in the source line direction are generally separated from each other along and above the corresponding gate line
2
, as shown in FIG.
12
and as disclosed in the Japanese Laid-Open Patent Publication No. 6-160900.
The active matrix substrate with the above configuration is attached to a counter substrate with a liquid crystal layer interposed therebetween, so as to form a liquid crystal display device. In order to realize a color display in such a liquid crystal display device, color filters are generally formed on the counter substrate. A black matrix is generally provided on the color filters formed on the counter substrate to prevent color mixing and light leakage. In order to reduce production cost, however, omitting such a black matrix has been proposed in the above-mentioned Japanese Patent Application No. 7-206367, for example.
When the black matrix is omitted, the gate and source lines
2
and
3
serve as the light-shielding films for spaces between the pixel electrodes
6
. However, a peripheral region surrounding a display region (the region where the plurality of pixel electrodes
6
are formed) of the liquid crystal panel is not shielded. In the peripheral region, shielding of light from a backlight is required especially in the case of a black display. Otherwise, leakage of light from the backlight from the peripheral region will lower the display quality. Moreover, this light shielding should preferably be in such a level as to obtain a light transmittance of 0.1% or less, in consideration of the contrast ratio of the resultant liquid crystal display device.
Omitting the formation of a light-shielding film on the counter substrate is an effective way of reducing production-cost of the liquid crystal display device as described above. Therefore, the development of a liquid crystal display device where light leakage from the peripheral region surrounding the display region can be prevented without forming a light-shielding film on the counter substrate has been sought.
As a solution to the above problem, the applicant of the present application has formed a light-shielding pattern in the peripheral region as shown in FIG.
13
. More specifically, a liquid crystal display device
700
has a light-shielding pattern
30
with a line width of 1 mm to several millimeters formed in the peripheral region located between the display region and external driving circuits
22
and
23
of an active matrix substrate. With this configuration, light leakage from the peripheral region can be sufficiently reduced.
FIG. 14
is a layout showing a boundary between the display region and the peripheral region. Specifically, an outermost one of the pixel electrodes
6
located lowermost of the display region shown in FIG.
13
and the portion of the light-shielding pattern
30
adjacent to the outermost pixel electrode
6
are shown. The light-shielding pattern
30
formed in the peripheral region overlaps the source lines
3
extending to the peripheral region. This increases the parasitic capacitance in the peripheral region. Moreover, when the light-shielding pattern
30
is formed so as to short-circuit with the outermost gate line
2
corresponding to the outermost pixel electrode
6
, the width of the outermost gate line
2
becomes 1 mm to several millimeters, while that of the other normal gate lines is only several micrometers to several tens of micrometers. This further increases the parasitic capacitance, and the time constant becomes so large due to the increased parasitic capacitance that the outermost gate line
2
cannot be driven sufficiently. In the case of a liquid crystal display device with a diagonal of 12 inches, for example, the parasitic capacitance may become nearly ten times as large as the supplemental capacitance of the normal gate lines
2
. This causes problems in both the output capacity of the driver and the time constant of the lines.
In order to avoid the above problems, the light-shielding pattern
30
needs to be separated from the outermost gate line
2
. For this separation, a gap
31
should be formed therebetween as shown in FIG.
14
. This causes another problem of light leakage from this gap. Such light leakage is visually recognizable enough to lo
Katayama Mikio
Kondo Naofumi
Kubo Masumi
Mitsumoto Kazuyori
Shimada Takayuki
Nguyen Dung
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Sikes William L.
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