Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1998-08-12
2001-02-06
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S039000
Reexamination Certificate
active
06184945
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display apparatus comprising thin film transistors. More particularly, the invention relates to a liquid crystal display apparatus having a structure with electrodes which are faced with pixel electrodes and which generate cumulative capacity.
2. Description of the Related Art
FIGS. 10 and 11
show a typical constitution of a thin film transistor array substrate used by a conventional thin film transistor liquid crystal display apparatus. The substrate carries on its surface gate wires G and source wires S.
The thin film transistor array substrate of
FIGS. 10 and 11
is a transparent substrate
13
illustratively made of glass having gate wires G and source wires S deposited thereon in matrix fashion. Each of the areas surrounded by the gate wires C and source wires S serves as one pixel
1
. Each pixel is furnished with a thin film transistor T
100
.
The thin film transistors T
100
shown in
FIGS. 10 and 11
are of common, inversely staggered type. Each gate wire G and a gate electrode
2
forming part of the gate wire G in question are covered with a gate insulating film
3
. On the gate insulating film
3
over the gate electrode
2
is a semiconductor active film
4
which is made of amorphous silicon (a-Si) and which faces the gate electrode
2
. A drain electrode
6
and a source electrode
7
, both made of a conductive material, are positioned face to face on two edges of the semiconductor active film
4
. The upper portions of the two edges of the active film
4
are covered with ohmic contact films
8
,
8
formed illustratively of amorphous silicon doped with impurities (i.e., donors) such as phosphorus of a high density.
A passivation film
10
made of an insulating film is deposited over the gate insulating film
3
, source electrodes
6
and drain electrodes
7
. On the passivation film
10
are pixel electrodes
11
which, constituted by a transparent conductive material such as ITO (indium tin oxide), cover almost all pixels
1
ranging from top to side of the drain electrodes
6
. The pixel electrodes
11
and the passivation film
10
are covered with an oriented film, not shown. Above the oriented film are liquid crystal and an opposite substrate having common electrodes. The whole structure constitutes an active matrix liquid crystal display apparatus. When a transparent pixel electrode
11
applies an electric field to liquid crystal molecules, the orientation of the molecules is controlled as desired.
In the liquid crystal display apparatus having the constitution of
FIGS. 10 and 11
, auxiliary electrodes
12
formed simultaneously with the gate electrodes
2
on the substrate
13
are installed opposite to the pixel electrodes
11
. As illustrated in
FIG. 11
, the auxiliary electrodes
12
are furnished so as to surround the contour of each pixel
1
corresponding to the circumference of each pixel electrode
11
. Each pixel electrode
11
and its corresponding auxiliary electrode
12
sandwich the passivation film
10
to constitute a capacitor providing a cumulative capacity that is used to inhibit the adverse effects of a parasitic capacity generated naturally upon liquid crystal activation.
In the liquid crystal display apparatus of the above constitution, the transparent substrate
13
is usually backlighted. The backlight is shielded by, or allowed to transmit, the orientation-controlled liquid crystal to let the user recognize the contrast on display.
In the constitution shown in
FIGS. 10 and 11
, each pixel electrode
11
and its corresponding auxiliary electrode
12
sandwich the gate insulating film
3
and passivation film
10
to form the cumulative capacity. While advantageous in driving the liquid crystal, the above constitution is known to have the following major deficiencies:
The gate insulating film
3
is interposed between the gate electrodes
2
and the semiconductor active film
4
for insulation purposes. To attain the good insulating property of the gate insulating film
3
requires strictly managing conditions for forming the film. On the other hand, the passivation film
10
is generally formed under less severe conditions than the gate insulating film
3
because the quality demanded of the passivation film is not so high.
In the conventional structure of
FIGS. 10 and 11
where each pixel electrode
11
and its corresponding auxiliary electrode
12
sandwich the gate insulating film
3
and passivation film
10
to form cumulative capacity, two insulating films constitute the cumulative capacity. This makes it difficult to secure appreciable quantities of cumulative capacity. It is also difficult to control the cumulative capacity level to a desired value.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a liquid crystal display apparatus for allowing its cumulative capacity to be set more easily than before to ensure signal stabilization and numerical aperture improvement, the apparatus being fabricated with a fewer number of masks in a more simplified fabrication setup than before.
In carrying out the invention and according to one aspect thereof, there is provided a liquid crystal display apparatus comprising: a substrate; gate wires and source wires formed in a matrix fashion on the substrate; gate electrodes formed close to points of intersection between the gate wires and the source wires, the gate electrodes being connected electrically to the gate wires; a first insulating film formed on the gate wires and the gate electrodes, the first insulating film further carrying the source wires thereon; a semiconductor active film formed over the gate electrodes with the first insulating film interposed therebetween; source electrodes formed on the semiconductor active film and connected to the source wires; drain electrodes formed on the semiconductor active film and isolated from the source electrodes; an electrode film intended to generate capacity and formed on the first insulating film, the electrode film being close to and in parallel with at least the source wires; a second insulating film formed on the first insulating film which carries the electrode film, the source wires, the source electrodes, the drain electrodes and the semiconductor active film thereon; and pixel electrodes connected to the drain electrodes and formed on the second insulating film in order to generate capacity in cooperation with the electrode film.
With the above structure, the cumulative capacity is formed not by a plurality of insulating films as is the case with conventional structures but by a single insulating film. This means that the inventive structure using a single film may have a reduced facing electrode area to provide the same cumulative capacity as that of the conventional structure utilizing a plurality of insulating films. Hence an improved numeral aperture for the liquid crystal display apparatus of the invention can be obtained.
In the above liquid crystal display apparatus, the single-film structure, if arranged to have the same facing electrode area as that of the conventional multiple-film structure, offers a greater cumulative capacity than the latter. The cumulative capacity thus enhanced ensures better signal stabilization.
In the inventive structure above, cumulative capacity is formed by the electrode film and pixel electrodes sandwiching the second insulating film which acts as a dielectric film. Because there is only one dielectric film for generating cumulative capacity, the inventive liquid crystal display apparatus may set the cumulative capacity level more accurately than the conventional structure that has a plurality of insulating films.
Furthermore, where the second insulating film is sandwiched by the edges of each pixel electrode and by the electrode film in the inventive structure, lines of electric force generated by the edge portion of the pixel electrode are different from those created in the middle portion of the pixel electrode. This permits slightly differe
Brinks Hofer Gilson & Lione
Frontec Incorporated
Parker Kenneth
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