Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-05-23
2004-07-06
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S062000, C349S065000
Reexamination Certificate
active
06760081
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a liquid crystal display device, and more particularly to a liquid crystal display device which has uniform feedthrough voltage components, a high image luminance, and high image display quality.
BACKGROUND OF THE INVENTION
An active matrix type liquid crystal display panel is a liquid crystal display panel in which a TFT (Thin Film Transistor) is added to each of pixel electrodes disposed in a matrix on a surface of a substrate. Recently, the active matrix type liquid crystal display panel is widely used in a display apparatus for a portable type personal computer or for a desktop type personal computer, in a projection type display apparatus, in a liquid crystal display television and the like. This is because, in the active matrix type liquid crystal display panel, a polarity inversion drive system and the like are recently adopted and thereby display quality such as an image contrast, a response speed to a moving picture signal and the like is improved. However, recently, the panel size of the liquid crystal display device becomes large, each pixel becomes minute and an aperture ratio of each pixel becomes high. For these reasons, a length of each gate wiring conductor becomes long and a width of each gate wiring conductor becomes small, so that an electrical resistance of each gate wiring conductor inevitably increases.
Also, according to an increase in the length of and a decrease in the width of each of the gate wiring conductors, a waveform of a gate pulse applied to the liquid crystal display panel is blunted, because the pixel electrodes driven by the gate pulse have capacitance as well as because the gate wiring conductor has large gate wiring resistance. Further, as a distance from an input end of the gate pulse becomes long, the resistance of the gate wiring conductor becomes large, so that degree of bluntness of the pulse waveform also becomes large as the distance from the input end of the gate pulse becomes long.
For example, in a liquid crystal display panel
1
schematically shown in
FIG. 12
, signal input portions
2
and
3
are disposed along a left side portion and a lower or bottom side portion of the panel
1
. Assuming that the signal input portion
2
is a gate signal input portion from which gate signals are supplied to TFT's in pixels disposed in a matrix, distances from the gate signal input portion
2
to the TFT's become larger in order of points (a), (b), (c), or in order of points (A), (B), (C) shown in the drawing. Therefore, gate wiring resistance becomes larger in the same order.
As a result, depending on difference of bluntness of the gate signals, i.e., gate pulses, magnitude of a shift of a potential of a pixel electrode caused when the gate pulse is turned off, that is, magnitude of a feedthrough voltage varies.
As shown by signal waveforms in
FIGS. 5A through 5D
, the feedthrough voltage becomes a voltage difference, i.e., VFDIN or VFDOUT, between the center potential of drain pulses, i.e., DPC, and the center potential of source pulses, i.e., SPC. Here, it is assumed that VFDIN designates a feedthrough voltage at an input end of the gate pulse and VFDOUT designates a feedthrough voltage at an end opposite to the input end of the gate pulse. In such case, since the feedthrough voltage becomes smaller as the distance from the input end of the gate pulse becomes larger, there is a relationship VFDIN>VFDOUT.
When the difference of the feedthrough voltages within an image display area becomes large, there occur image persistence, stain and the like and thereby display quality is deteriorated. Conventionally, in order to minimize the difference of the feedthrough voltages within the image display area, a voltage of an opposing electrode is lowered taking a voltage drop of an offset of a drain signal caused by the feedthrough at the center of the image display area into consideration. However, although an optimum condition is obtained at the center of the image display area, an optimum condition is not obtained at the peripheral portions of the image display area. Therefore, at the peripheral portions, since a DC voltage component is applied to liquid crystal, the above-mentioned image persistence, stain and the like may occur and quality of image display is deteriorated. That is, even if the voltage of the opposing electrode is adjusted to obtain an optimum condition at the center of the image display area, a DC voltage is applied to the liquid crystal at the peripheral portions and it is difficult to effectively avoid deterioration of the image display quality mentioned above.
Here, the reason why the feedthrough voltage varies depending on the blunting of the gate pulse waveform will be described.
FIG. 4
illustrates an equivalent circuit of a portion of a liquid crystal display panel. As shown in
FIG. 4
, an equivalent circuit of a pixel comprises a TFT
14
whose drain (D) is coupled to a drain signal line
15
and whose gate (G) is coupled to a gate signal line
13
, a gate-source capacitance Cgs, a storage capacitance Csc, and a liquid crystal (LC) capacitance Clc. The storage capacitance Csc exists between the source electrode (S) of the TFT
14
and an adjacent gate signal line
13
. The LC capacitance Clc exists between the source electrode of the TFT
14
, i.e., a display electrode, and an opposing electrode
21
.
By using the gate-source capacitance Cgs, the storage capacitance Csc, the liquid crystal (LC) capacitance Clc, and a gate pulse amplitude &Dgr;Vg, a feedthrough voltage Vfd is represented approximately as follows.
Vfd=[Cgs
/(
Clc+Csc+Cgs
)]*&Dgr;
Vg
(1)
On the other hand, when a falling edge of the gate pulse is blunted due to a resistance of a gate wiring conductor, a current flows from the source electrode to the drain signal line until the TFT
14
is completely turned off. A total amount of such current, i.e., a TFT leakage, becomes as follows.
∫Ids dt
Taking the total amount of such current into consideration, the feedthrough voltage Vfd becomes as follows.
Vfd
=(
Cgs*&Dgr;Vg−∫Ids dt
)/(
Clc+Csc+Cgs
) (2)
Here, the total amount of the current, i.e.,
∫Ids dt
is approximately proportional to the degree of bluntness of the gate pulse, and therefore becomes as follows at the side of the gate signal input portion
2
.
∫Ids dt≈0
Therefore, the feedthrough voltage components differ between the side of the gate signal input portion
2
and the side opposite thereto, and a feedthrough voltage difference &Dgr;Vfd in an image display area is produced which is a difference between the values of the formulas (1) and (2) and is represented as follows.
&Dgr;
Vfd=∫Ids dt
/(
Clc+Csc+Cgs
) (3)
In order to uniformalize the feedthrough voltage component within an image display area, it is possible to lower gate wiring resistance to reduce quantity of bluntness of the gate pulse. To realize this, it is possible to enlarge a width or a film thickness of the gate wiring conductor, and to change wiring material into those having lower specific resistance, for example, aluminum, gold and the like. However, when enlarging the film thickness of the wiring conductor and when changing the wiring material, it is necessary to change a manufacturing process of the liquid crystal display device. Also, when the width of the wiring conductor is enlarged, an aperture ratio of the liquid crystal display device is deteriorated.
In order to solve such problem, for example, Japanese patent laid-open publication No. 10-39328 discloses a liquid crystal display device in which feedthrough voltage components are uniformalized within an image display area, and variation of DC voltage components applied to the liquid crystal in the image display area is suppressed. Thereby, image persistence, stain and the like of a liquid crystal display panel are improved to obtain a high image display quality.
In the liquid crystal display device of the Japanese laid-open
Chowdhury Tarifur R.
NEC LCD Technologies Ltd.
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