Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-06-26
2002-03-12
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S043000, C349S138000
Reexamination Certificate
active
06356318
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display having a display screen provided with a plurality of pixel electrodes, thin-film transistors (hereinafter referred to as “TFTs”) as switching elements respectively corresponding to the pixel electrodes, and storage capacitors respectively corresponding to the pixel electrodes.
2. Description of the Related Art
The configuration of a conventional active-matrix liquid crystal display will be described with reference to
FIGS. 7 and 8
.
FIG. 8
is a plan view of an essential part of an array substrate underlying a liquid crystal included in the active-matrix liquid crystal display, and
FIG. 7
is a sectional view taken on line
7
—
7
in FIG.
8
and showing the essential part of the array substrate, a liquid crystal layer and an arrangement overlying the liquid crystal layer. As shown in
FIG. 7
, a liquid crystal layer
31
is filled and sealed in a space between a transparent lower substrate
32
and a transparent upper substrate
33
disposed opposite to the lower substrate
32
. Polarizing plates
34
and
35
are attached to the respective outer surfaces of the lower substrate
32
and the upper substrate
33
, respectively. Transparent pixel electrodes
36
are formed on the inner surface of the lower substrate
32
, and common electrodes
37
are formed on the inner surface of the upper substrate
33
opposite to the pixel electrodes
36
.
Referring to
FIG. 8
, formed on the lower substrate
32
are a plurality of parallel gatelines
42
, i.e., scanning lines, and a plurality of parallel source lines
40
, i.e., signal lines, extending perpendicularly to the gate lines
42
.
The transparent pixel electrodes
36
are formed in rectangular regions surrounded by the gate lines
42
and the source lines
40
, respectively. A TFT
38
, i.e., a switching element, formed near the intersection of each source line
40
and each gate line
42
. The TFT
38
turns on to apply a data signal voltage to the corresponding pixel electrode
36
and turns off to shut the data signal voltage to the same pixel electrode
36
. Thin-film storage capacitors Cs for holding charges on the pixel electrodes
36
are formed on the gate lines
42
.
Each of the TFTs
38
includes a source electrode
40
a
extending from the source line
40
, a drain electrode
41
, a gate electrode
42
a
extending from the gate line
42
and a gate insulating film
43
. The drain electrode
41
is connected through a contact hole
45
formed in a layer insulating film
44
to the pixel electrode
36
. The pixel electrode
36
is connected through a contact hole
46
to the upper electrode
47
of the storage capacitor Cs. The gate line
42
formed on the lower substrate
32
serves as the lower electrode of the storage capacitor Cs. The upper electrode
47
is disposed opposite to the gate line
42
with the gate insulating film
43
interposed therebetween.
In this conventional active-matrix liquid crystal display, a data signal voltage is applied to a selected one of the plurality of source lines
40
and a control signal is applied to a selected one of the plurality of gate lines
42
to drive the TFT
38
connected to the selected source line
40
and the selected gate line
42
, whereby the data signal voltage is applied to the pixel electrode
36
connected to the drain electrode
41
of the TFT
38
. The TFTs
38
respectively connected to the pixel electrodes
36
arranged in a matrix are thus driven to display a desired pattern on the screen.
As shown in
FIG. 7
, the gate insulating film
43
of the TFT
38
serves also as an insulating film for the storage capacitor Cs. Use of a single film formed by a single process as both the gate insulating film
43
and the insulating film for the storage capacitor Cs is favorable in view of simplifying a liquid crystal display fabricating process. The thickness of the film is determined so that the gate insulating film
43
of the TFT
38
has a sufficient dielectric strength.
Although dependent on the configuration of the storage capacitor Cs, a voltage to be applied to the storage capacitor Cs is in the range of ¼ to ½ of a voltage to be applied to the TFT
38
. Therefore, a thickness of the insulating film for the storage capacitor Cs having a sufficient dielectric strength may be smaller than that of the gate insulating film
43
of the TFT
38
. Thus, the thickness of the gate insulating film
43
of the TFT
38
is excessively great for the insulating film for the storage capacitor Cs.
The area of the storage capacitor Cs is calculated by dividing a predetermined charge storage capacity necessary for driving the pixel electrode
36
by the storage capacity per unit area of the thin-film storage capacitor Cs. Since the storage capacity per unit area is uniquely dependent on the dielectric constant and the thickness of the gate insulating film
43
, the area of the storage capacitor Cs is determined on the basis of the dielectric constant and the thickness of the gate insulting film
43
.
An active-matrix liquid crystal display must have a high optical transmittance, i.e., a large aperture ratio, to display pictures in a high picture quality. When the size of pixels is reduced for high-definition displaying, an area occupied by the storage capacitors Cs increases relatively and hence the aperture ratio is reduced. If the brightness of back light is increased to compensate the reduction of the aperture ratio, the power consumption of the active-matrix liquid crystal display increases.
The area of the storage capacitors Cs needs to be reduced to increase the aperture ratio. However, since the insulating film for the storage capacitors Cs is part of the gate insulating film
43
of the TFTs
38
and the thickness of the gate insulating film
43
is determined on the basis of the dielectric strength of the TFTs
38
, the capacitance per unit area of the storage capacitor Cs cannot be individually increased. Consequently, the area of the storage capacitor Cs cannot be reduced securing the predetermined capacitance of the storage capacitor Cs and the aperture ratio decreases inevitably when the size of the pixels is reduced for high-definition displaying.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an active-matrix liquid crystal display having a large aperture ratio achieved by forming storage capacitors each having a small area relative to that of pixel electrodes.
According to a first aspect of the present invention, an active-matrix liquid crystal display comprises: a transparent upper substrate; a transparent lower substrate disposed opposite to the upper substrate; a liquid crystal filled and sealed in a space between the upper and the lower substrate; a plurality of parallel gate lines formed on the lower substrate; a plurality of parallel source lines formed on the lower substrate so as to extend perpendicularly to the gate lines; TFTs formed at intersections of the gate lines and the source lines, respectively; pixel electrodes connected to the TFTs, respectively; and storage capacitors connected to the pixel electrodes, respectively; wherein separate films are used as an insulating film included in the storage capacitors and a gate insulating film included in the TFTs, respectively, and each of the storage capacitors has an upper electrode connected to the pixel electrode and a lower electrode disposed opposite to the upper electrode with the insulating film sandwiched between the upper and the lower electrode. The capacitance per unit area of the storage capacitors can be determined independently of the thickness of the gate insulating film for the TFTs. Therefore, the area of each storage capacitor can be reduced relative to the area of each pixel electrode by increasing the capacitance per unit area of the storage capacitors.
Preferably, the upper electrodes are formed only in flat regions on the insulating film overlying the lower electrodes. When the upper electrodes
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Ton Toan
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