Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1999-09-23
2004-04-06
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S139000, C349S143000
Reexamination Certificate
active
06717630
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a liquid crystal display device and a method of fabricating the same, and more particularly to a liquid crystal display device which is capable of eliminating variance in parasitic capacitance to be generated between a pixel electrode and signal lines, and a method of fabricating such a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device has recently drawn attention as a thin and low power-consumption display to be substituted for a conventional cathode ray tube. In particular, a so-called active matrix type liquid crystal display device, which employs a non-linear device such as a thin film transistor (TFT) and a metal insulator metal (MIM) type transistor as a driver element, has drawn attention due to high quality in display.
Hereinbelow is explained an operation of a conventional liquid crystal display device with reference to
FIG. 1
which is a circuit diagram of a circuit equivalent to a pixel.
As illustrated in
FIG. 1
, a thin film transistor (TFT)
14
is comprised of a drain electrode
14
a
electrically connected to a first signal line
11
i
, a source electrode
14
b
, and a gate electrode
14
c
electrically connected to a scanning line
12
. A pixel electrode
13
is electrically connected to the source electrode
14
b
. A liquid crystal capacitance
32
having a capacity of C
LC
and including liquid crystal agent as dielectric material is formed between the pixel electrode
13
and an opposing electrode
33
formed on an opposing substrate.
A liquid crystal display device is generally designed to have a plurality of scanning lines
12
(only one of them is illustrated in
FIG. 1
) to which a scanning signal is successively applied. In a period other than a scanning period, a scanning signal is not applied to the scanning lines
12
, and hence, the drain electrode
14
a
and the source electrode
14
b
are electrically insulated from each other. In a scanning period, a scanning signal is applied to the scanning lines
12
. The scanning signal activates a channel of the thin film transistor
14
, and thus, the drain electrode
14
a
and the source electrode
14
b
are electrically connected to each other. At the same time, a signal is applied to the first signal line
11
i
in accordance with a voltage to be applied to the liquid crystal capacitance
32
, which is charged by the signal.
After a scanning period has been over, a scanning signal is no longer applied to the scanning lines
12
, resulting in that the drain electrodes
14
a
and the source electrodes
14
b
become electrically insulated from each other again. Hence, the liquid crystal capacitance
32
is kept charged, and thus, it is possible to optically control liquid crystal by means of an electric field generated between the opposing electrode
33
and the pixel electrode
13
.
In a period other than a scanning period, a quite small amount of current, which is so-called leakage current, flows between the drain electrode
14
a
and the source electrode
14
b
. The leakage current reduces a difference in voltage between the pixel electrode
13
and the opposing electrode
33
until a next scanning period. If such a difference in voltage is not reduced, there would caused degradation in contrast, resulting in reduction in display quality.
In order to avoid such degradation in contrast, an auxiliary capacitance
34
having a capacity of C
s
is additionally inserted in parallel with the liquid crystal capacitance
32
to thereby prevent reduction in voltage. In the liquid crystal display device illustrated in
FIG. 1
, the auxiliary capacitance
34
is provided in parallel with the pixel electrode
13
and the thin film transistor
14
.
The auxiliary capacitance
34
may be provided at other locations, unless the auxiliary capacitance
34
is in parallel with the liquid crystal capacitance
32
.
The liquid crystal display device is designed to include a plurality of such pixels as illustrated in
FIG. 1
, in an array.
FIG. 2
is a plan view of a pixel of the liquid crystal display device having the above-mentioned structure. In
FIG. 2
, the auxiliary capacitance
34
illustrated in
FIG. 1
is not illustrated for the sake of simplicity.
As illustrated in
FIG. 2
, the pixel electrode
13
is sandwiched between the first and second signal lines
11
i
and
11
j
. In other words, the first signal line
11
i
extends along one side of the pixel electrode
13
, and the second signal line
11
j
extends along the other side of the pixel electrode
13
. Hence, there are generated a first parasitic capacitance
16
i having a capacity of Cd-pii between the pixel electrode
13
and the first signal line
11
i
, and a second parasitic capacitance
16
j
having a capacity of Cd-pij between the pixel electrode
13
and the second signal line
11
j.
The longer the length Li and Lj along which the pixel electrode
13
is adjacent to the first and second signal lines
11
i
and
11
j
are, or the shorter the spaces di and dj between the pixel electrode
13
and the first and second signal lines
11
i
and
11
j
are, the greater the parasitic capacitances
16
i
and
16
j
are. The parasitic capacitances
16
i
and
16
j
cause a voltage of the pixel electrode
13
to be influenced by fluctuation in voltage of the first and second signal lines
11
i
and
11
j.
The fluctuation &Dgr;Vpi in voltage of the pixel electrode
13
is represented as follows.
&Dgr;
V
pi=(Cd-pii×&Dgr;
V
i+Cd-pij×&Dgr;
V
j)/(
C
LC
+C
s
+Cd-pii+Cd-pij)
In the equation, &Dgr;Vi and &Dgr;Vj represent fluctuation in voltage of the first and second signal lines
11
i
and
11
j
, respectively.
Hereinbelow is explained a method of driving the liquid crystal display device illustrated in
FIGS. 1 and 2
.
It is desirable that an orientation of an electric field to be applied to the liquid crystal capacitance
32
, that is, a polarity of the pixel electrode
13
is inverted every period for updating display. This is because if a polarity of the pixel electrode
13
is kept unchanged, there would occur phenomenon called “burning” in which display is fixed when the same image is kept displayed, and display cannot be returned back to original display. This degrades quality in display.
In addition, it is desired that polarity of the pixel electrodes is uniformly distributed in a screen of a liquid crystal display device. The reason is as follows. In an actual liquid crystal display device, there exists a slight difference in brightness in display in accordance with positive or negative polarity of the pixel electrode
13
. If the polarity of the pixel electrode
13
in an entire screen alters between positive and negative ones each time display is updated, brightness and darkness are repeated, which remarkably deteriorates visibility.
For this reason, there have been suggested a lot of arrangements of polarity of pixel electrodes in a screen and a lot of methods of driving a liquid crystal display device.
FIG. 3A
illustrates polarity of pixel electrodes in a screen in a certain display updating period, and
FIG. 3B
illustrates polarity of pixel electrodes in a screen in a next display updating period both in gate line inversion drive. Namely,
FIGS. 3A and 3B
show how polarity of pixel electrodes vary in successive display updating periods in gate line inversion drive. Similarly,
FIG. 4A
illustrates polarity of pixel electrodes in a screen in a certain display updating period, and
FIG. 4B
illustrates polarity of pixel electrodes in a screen in a next display updating period both in drain line inversion drive. Namely,
FIGS. 4A and 4B
show how polarity of pixel electrodes vary in successive display updating periods in drain line inversion drive.
FIG. 5A
illustrates polarity of pixel electrodes in a screen in a certain display updating period, and
FIG. 5B
illustrates polarity of pixel electrodes in a screen in a next display updating period both in dot inversion drive. Namely,
FIGS
Chowdhury Tarifur R.
Duong Tai
NEC LCD Technologies Ltd.
Scully Scott Murphy & Presser
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