Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-06-23
2003-01-14
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S139000, C349S143000
Reexamination Certificate
active
06507375
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the circuit configuration and layout of a pixel region in a liquid crystal display device provided with switching elements.
2. Description of the Related Art
In a conventional active matrix liquid crystal display device, a liquid crystal is enclosed between alignment layers provided on the individual inner surfaces of a pair of substrates, and a plurality of gate lines, a plurality of source lines, and thin-film transistors are formed on the opposing surface of one of the substrates. In order to increase the aperture ratio of an active matrix liquid crystal display device, a structure for suppressing disclination of the liquid crystal is conventionally used, in which sections that produce an electric field unnecessary to the liquid crystal, such as gate lines and source lines, are covered with transparent pixel electrodes.
The disclination is a phenomenon in which the alignment of a TN liquid crystal, which is a helical elastic body, is disordered in the presence of an electric field depending on the field intensity, the field direction, the helical direction, and the elastic constant. The disclination causes light leakage, and so on, resulting in a decrease in contrast ratio and degradation in display quality, such as residual images.
FIG. 8
is a plan view of a pixel region of a conventional active matrix liquid crystal device in which the aperture ratio is increased, and
FIG. 9
is a sectional view taken along the line IX—IX of FIG.
8
.
In the active matrix liquid crystal device, a plurality of gate lines
201
are arrayed in parallel on a glass substrate
210
and a plurality of source lines
202
are arrayed so as to be orthogonal to the gate lines
201
with a first insulating layer
208
therebetween. In the vicinity of each of the intersections of the gate lines
201
and the source lines
202
, a thin-film transistor
211
composed of a semiconductor layer
203
composed of amorphous silicon or the like is disposed, and a drain electrode
204
thereof is connected to a transparent pixel electrode
206
through a contact hole
205
with a second insulating layer
209
therebetween.
In order to increase the aperture ratio, the transparent pixel electrode
206
is formed so as to overlap the gate lines
201
and the source lines
202
, and thus the area of the transparent pixel electrode
206
is increased as much as possible, leakage of electric fields of the gate lines
201
and the source lines
202
is suppressed, and the occurrence of disclination is restricted within the gate line and source line regions. That is, the overlapping widths between the transparent pixel electrode
206
and the gate lines
201
and between the transparent pixel electrode
206
and the source lines
202
are designed so as to block the light leakage due to disclination.
In the structure described above in which the aperture ratio is increased, each overlapping portion of the transparent pixel electrode
206
with the gate line
201
or the source line
202
corresponds to parasitic capacitance. Although the dielectric constant of the second insulating layer
209
is decreased and the thickness of the second insulating layer
209
is increased, it has been known that the source lines
202
in which signals frequently vary are capacitively coupled to the transparent pixel electrode
206
, resulting in crosstalk. In order to overcome this problem, the polarities of signals are reversed between the source lines
202
on both sides of the transparent pixel electrode
206
, and thereby, the potential variation due to the capacitive coupling is offset. In order to obtain the offset effect, overlapping widths x
1
and x
2
of two adjacent transparent pixel electrodes
206
with the source line
202
are equalized.
Although disclination occurs at a different position depending on a liquid crystal molecular alignment direction, i.e., a rubbing direction
221
of a TFT substrate and a rubbing direction
222
of a counter substrate, in general, disclination occurs asymmetrically in two regions on either side of a source line. This is because of the fact that since an electric field curves like a bow above the source line, there is a deviation in disclination because the liquid crystal molecular alignment direction differs between a side in which the anchoring direction of liquid crystal molecules, which are anchored to the surface of the TFT substrate with a predetermined angle, and the electric field direction relatively agree with each other and a side in which they greatly differ. That is, in order to merely block the disclination, the aperture ratio can be further increased by setting the overlapping width x
1
smaller than the overlapping width x
2
.
However, in the conventional structure, in view of offsetting the potential variation due to the capacitive coupling, the overlapping width x
1
must be set equal to the overlapping width x
2
, and the overlapping width must be matched to the side in which the disclination is greater. The optically unused regions along the source lines, which are produced independent of the pixel size, cause a large decrease in the aperture ratio, in particular, in a high-resolution liquid crystal display device with a small pixel size.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a liquid crystal display device in which crosstalk is prevented from occurring and the aperture ratio is improved.
In accordance with the present invention, a liquid crystal display device includes a pair of substrates, a liquid crystal enclosed between alignment layers provided on the individual inner surfaces of the pair of substrates, a plurality of gate lines and a plurality of source lines intersecting each other arrayed in a matrix with a first insulating layer therebetween on the opposing surface of one of the substrates, switching elements, each connected to a gate electrode extending from one of the gate lines and to a source electrode extending from one of the source lines in the vicinity of each of the intersections between the gate lines and the source lines, and a transparent pixel electrode for driving the liquid crystal connected to each switching element through a drain electrode. A second insulating layer is disposed between the source lines and the transparent pixel electrodes, the transparent pixel electrodes adjacent to both sides of one of the source lines are formed so as to overlap the source line with different overlapping widths, and an auxiliary capacitor is formed to cancel a difference in the amount of parasitic capacitance generated due to the overlap of the individual transparent pixel electrodes with the source line.
The liquid crystal display device of the present invention is provided with the auxiliary capacitor for canceling the difference in parasitic capacitance generated due to the overlap of the transparent pixel electrodes with the source line with different overlapping widths in adjacent two pixel regions. That is, parasitic capacitance decreases in one of the adjacent pixel regions in which the overlapping width is smaller, and parasitic capacitance increases in the other pixel region in which the overlapping width is larger, and thus, by adding the auxiliary capacitance to the region having smaller parasitic capacitance, the adjacent pixel regions have the same sum of the parasitic capacitance and the auxiliary capacitance. Consequently, even if the overlapping width of the transparent pixel electrode with the source line differs between the adjacent pixel regions, the potential variation in the source line is satisfactorily offset, and crosstalk can be avoided. In other words, in the present invention, since crosstalk is prevented from occurring and the different overlapping widths of the transparent pixel electrodes with the source line are set, the transparent pixel electrodes and the source line are required to overlap merely by the widths for blocking the disclination occurring at the asymmetrical positions c
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Schechter Andrew
Ton Toan
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