Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1998-06-15
2001-03-13
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S051000, C349S139000, C345S055000
Reexamination Certificate
active
06201584
ABSTRACT:
TECHNICAL FIELD
The present invention relates to liquid-crystal display devices, and in particular, to the structure of a liquid-crystal display device including an active device for each pixel.
BACKGROUND ART
There is a conventional liquid-crystal display device including one active device connected to an interconnection layer so as to correspond to each pixel. In this liquid-crystal display device a voltage is supplied to pixel electrodes via active devices based on the potential of the interconnection layer, whereby an electric field applied to a liquid crystal layer disposed between the pixel electrodes and a counter electrode is controlled, so that the display condition of the pixels is controlled.
In this case, when the liquid-crystal display device is a so-called active matrix type, the pixel electrodes are arranged in the form of a matrix, and a plurality of scanning lines and a plurality of signal lines positioned perpendicularly to the scanning lines independently control the display condition of each pixel region.
Three-terminal devices such as thin film transistors (TFTS) and two-terminal devices such as metal-insulator-metal (MIM) devices are known as the active devices.
In
FIG. 12
, the equivalent circuit of an active-matrix liquid-crystal device with a two-terminal device as an active device is shown.
MIM devices
13
as active devices are connected to a plurality of scanning lines
11
as an interconnection layer formed on the internal surface of a device substrate, and other terminals of the MIM devices
13
are connected to pixel electrodes
15
. The pixel electrodes
15
are connected to a counter electrode formed on the internal surface of a counter substrate, with a liquid crystal layer
14
provided between them.
There are device capacitors C
MIM
in the MIM devices
13
, which are connected in series to liquid-crystal capacitors C
LC
formed by the pixel electrodes
15
, the counter electrode
16
and the liquid crystal layer
14
between them.
The MIM devices
13
are connected to the scanning lines
11
. However, in reverse, there may be a case where the liquid crystal layer
14
is connected to the scanning lines
14
.
In
FIG. 13
, a schematically perspective view of a structural example of a conventional active-matrix liquid-crystal display device is shown. In addition, in
FIG. 14
, a sectional view of an MIM device as one example of an active device is shown.
On the internal surface of one substrate
30
A is formed a base layer
30
a
composed of, e.g., Ta oxide for enhancing the adhesion between the base and an interconnection layer formed thereon.
As shown in
FIG. 14
, on the base layer
30
a
is formed a first electrode portion
31
a
integrated with a scanning line
31
composed of, e.g., tantalum. On its surface an insulating film
33
a
composed of tantalum oxide is formed by using anodic oxidation. And, a second electrode portion
33
b
composed of, e.g., chromium is formed thereon.
The first electrode portion
31
a
, the insulating film
33
a
and the second electrode portion
33
b
constitute an MIM device
33
.
In addition, as shown in
FIG. 13
, a pixel electrode
35
composed of, e.g., indium-tin-oxide (ITO) is formed to be connected to the second electrode portion
33
b
of the MIM device
33
.
On the internal surface of another opposite substrate
30
B, a counter electrode
36
composed of ITO is formed in the direction (the direction parallel to the sheet) intersecting the scanning line
31
, and the counter electrode, the pixel electrode, and a liquid crystal layer provided between the substrates
30
A and
30
B constitute a pixel for display.
However, the liquid-crystal display device including the above conventional active device has a circuit arrangement in which the device capacitors C
MIM
and the liquid-crystal capacitors C
LC
are connected in series as shown in FIG.
12
. Thus, when a potential is supplied from the scanning line
31
, the device capacitor causes a decrease in the ratio C
LC
/(C
MIM
+C
LC
) of a voltage applied to the device capacitor of the MIM device with respect to a voltage applied between the scanning line
31
and the counter electrode
36
(signal line
32
). Accordingly, problems occurs in which sufficient writing may not be performed due to the insufficient setting of the switching ratio of the MIM device, and in which voltage applied to the liquid crystal layer decreases even during a charge-holding period with the MIM device cut off after writing.
The above problems can be solved by reducing the device capacitance of the MIM device if possible. Reducing the device capacitance requires, for example, a reduction in the device area. However, it is difficult in production to form the device smaller than the conventional one.
Accordingly, the present invention solves the above problems, and provides a novel structure for ensuring sufficient a writing operation without decreasing a voltage applied to liquid crystal by different means from a method for improving the device characteristics of an active device such as an MIM device.
DISCLOSURE OF INVENTION
In order to solve the foregoing problems, according to the present invention, there is provided a liquid-crystal display device including pixel electrodes connected via active devices to an interconnection layer formed on a first substrate, and performing display by applying an electric field to a liquid crystallayer provided between the pixel electrodes and a counter electrode formed on a second substrate, in which the potential of a point at which the active device and the pixel electrode are connected is electrically connected to predetermined potential via an electrostatic capacitor.
The predetermined potential is a potential without no large potential change in most of a period in which at least the active devices operate to perform the writing to the pixel electrodes, and the new writing after the termination of the writing is performed. It may have such change that it cannot affect the operation of the active devices and liquid crystal display via the static capacitors.
According to this means, the connection of the electrostatic capacitors connected to the predetermined potential causes a condition similar to a case in which the capacitance of the liquid-crystal capacitors connected in series to the active devices having predetermined device capacitance is increased. This increases the portion of the voltage applied to the active devices in cut-off condition. Thus, the switching ratio of the active devices can be increased, which enables secure writing to the pixel electrodes, and even in the termination of writing, a decrease in voltage caused by the device capacitance of the active devices can be suppressed, which reduces a decrease in the voltage applied to the liquid crystal layer.
The interconnection layer comprises scanning lines, and preferably, the predetermined potential is the potential of the scanning line being not selected, corresponding to a pixel to which the active device belongs.
According to this means, the predetermined potential connected to the electrostatic capacitors can be obtained from the scanning line for another pixel. Thus, the means can be structured without a new interconnection pattern for supplying the predetermined potential.
Also, the means is preferably structured such that the scanning lines are arranged in parallel to form a matrix display unit to be driven by sequentially selecting the scanning lines, and the predetermined potential is obtained from the scanning line for an adjacent pixel selected just before the scanning line corresponding to the pixel to which the active device belongs is selected.
According to this means, the predetermined potential is obtained from the scanning line for an adjacent pixel selected just before the scanning line belonging to the pixel is selected. Thus, the stable potential of the scanning line being not selected, and only electrical connection to the scanning line for the adjacent pixel is required, so that the connection can be easily established.
In addition, the
Iisaka Hidehito
Sonehara Tomio
Takeuchi Tetsuhiko
Chowdhury Tarifur R.
Harnes Dickey & Pierce P.L.C.
Seiko Epson Corporation
Sikes William L.
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