Active matrix liquid crystal display device

Details Active matrix liquid crystal display device Active matrix liquid crystal display device

2001-06-01

2004-11-23

Ton, Toan (Department: 2871)

Liquid crystal cells, elements and systems

Particular excitation of liquid crystal

Electrical excitation of liquid crystal

C349S138000

Reexamination Certificate

active

06822704

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device.
2. Description of the Related Art
Heretofore, active matrix liquid crystal display devices capable of displaying color images have been of a structure including a TFT (Thin-Film Transistor) substrate with TFTs and pixel electrodes disposed thereon in association with respective pixels, an opposing substrate with color filters and a common electrode disposed thereon, and a liquid crystal layer sealed between the TFT substrate and the opposing substrate. In this structure, the color filters and the pixel electrodes need to be positioned accurately in alignment with each other. In order to prevent an unwanted leakage of light, a light shielding layer referred to as a black matrix is required to be positioned between the color filters which are combined with the respective pixels on the opposing substrate. In view of these requirements, it has been proposed to fabricate color filters on a TFT substrate. With color filters fabricated on a TFT substrate, an opposing substrate can be constructed of a transparent substrate and a transparent common electrode fabricated uniformly over the transparent substrate. Therefore, the process of manufacturing active matrix liquid crystal display devices is simplified, and it is relatively easy to achieve precise alignment between the opposing substrate and the TFT substrate. In addition, various interconnections on the TFT substrate can be used as a light shielding layer.
FIG. 1
shows in schematic cross section of a conventional active matrix liquid crystal display device with color filters mounted on a TFT substrate.
As shown in
FIG. 1
, TFT substrate
10
comprises transparent glass substrate
11
which supports on one major surface thereof a plurality of patterned data lines
12
extending parallel to each other, color layers
13
of color filters and transparent overcoat layer
14
which are successively deposited on the major surface of transparent glass substrate
11
, and transparent pixel electrodes
15
disposed on the surface of overcoat layer
14
in association with the respective pixels. Data lines
12
are covered with color layers
13
, and extend in a direction normal to the sheet of FIG.
1
. Opposing substrate
20
comprises glass substrate
21
supporting on a transparent uniform common electrode
22
on one major surface thereof. TFT substrate
10
and opposing substrate
20
are spaced a given distance from each other with pixel electrodes
15
and common electrode
22
confronting each other. A liquid crystal layer
30
is sealed between TFT substrate
10
and opposing substrate
20
. Each of data lines
12
is made of an opaque conductive material and serves to block gaps between two adjacent pixels against the entry of light. As well known to those skilled in the art, TFT substrate
10
also supports gate lines and TFTs associated with the respective pixels. The data lines are also referred to as video signal lines or drain lines and source lines, and the gate lines as scanning lines.
FIG. 2
shows an equivalent circuit of such an active matrix liquid crystal display device.
As shown in
FIG. 2
, pixel electrodes
15
and TFTs
41
which are associated with the respective pixels are arranged in a matrix form on TFT substrate
10
. TFTs
41
, which operate as switching elements, have gates connected to gate lines
42
, drains connected to data lines
12
, and sources connected to pixel electrodes
15
. However, the sources of TFTs
41
may be connected to data lines
12
, and the drains thereof to pixel electrodes
15
. Common electrode
22
is grounded, and a liquid crystal layer sandwiched between common electrode
22
and one pixel electrode
15
serves as one pixel portion
40
. On TFT substrate
10
, gate lines
42
extend parallel to each other and perpendicularly to data lines
12
. Equivalent pixel capacitors
43
are connected parallel to the respective pixel portions
40
. Data lines
12
and gate lines
42
are driven respectively by drivers
44
and drivers
45
.
It has been pointed out that the above conventional active matrix liquid crystal display device with the color filters on the TFT substrate has a smaller viewing angle than the active matrix liquid crystal display device with the color filters on the opposing substrate, even if it is provided with a phase difference compensation plate. Table 1 given below shows measured viewing angles in vertical and horizontal directions of active matrix liquid crystal display devices with color filters on TFT substrates and an active matrix liquid crystal display device with color filters on an opposing substrate. The values set forth in Table 1 were obtained with phase difference compensation plates used on these display devices.
TABLE 1
Type
9.4″ UXGA
12.1″ SVGA
12.1″ SVGA
Pixel pitch
120 &mgr;m
300 &mgr;m
300 &mgr;m
Color filter
TFT substrate
TFT substrate
Opposing
position
substrate
Viewing angle
90 degrees
 92 degrees
 90 degrees
(Vertical)
Viewing angle
90 degrees
105 degrees
110 degrees
(Horizontal)
The viewing angle referred to above is an angle in which the ratio of contrast between white and black display images is 10% or higher. As can be seen from Table 1, the vertical viewing angle remains substantially the same irrespective of whether the color filters are disposed on the opposing substrate or the TFT substrate. However, the horizontal viewing angle is much smaller with the color filters disposed on the TFT substrate than with the color filters disposed on the opposing substrate. This tendency manifests itself if the pixels are smaller.
The above phenomenon will be described in detail below with reference to FIG.
1
.
It is assumed that the conventional active matrix liquid crystal display device shown in
FIG. 1
is used in a normally white mode. If pixels disposed one on each side of data line
12
displays a black image, then when the liquid crystal display device is driven by a dot inversion driving process, since a voltage of +5 V is applied to one of the pixel electrodes and a voltage of −5 V is applied to the other pixel electrode, a strong lateral electric field is generated in a region above data line
12
of liquid crystal layer
30
, causing directors (liquid crystal molecules)
31
to fall thereby to substantially display a white image in that region. Specifically, as indicated by A in
FIG. 1
, a white image is displayed in the region of the gap between pixel electrodes
15
and a region slightly extending from the gap into the pixel electrodes. These regions are combined as a region where light leaks. In the other region, directors
31
are erected parallel to the direction from pixel electrodes
15
to common electrode
22
, and a black image is displayed. When the white image region is viewed from the front of the active matrix liquid crystal display device, it is visually recognized as a black region because light is blocked by data line
12
. When the white image region is obliquely viewed, as indicated by the arrow B, light is not blocked by data line
12
, and liquid crystal layer
30
is affected by light that passes only through light leakage region A. While the region should be visually recognized as the black region, since there is light passing through liquid crystal layer
30
as indicated by the arrow B, the contrast in the black region is lowered, resulting in a reduction in the intensity of black in the black region.
If the liquid crystal display device is a highly fine display panel with small pixel pitches, then because the ratio of light leakage regions to ordinary pixel regions tends to be larger than a display panel with greater pixel pitches, the contrast in the black region as obliquely viewed is reduced, resulting in a smaller viewing angle. The ordinary pixel regions are referred to as normal regions where liquid crystal molecules are vertically oriented to display a black image.
The above phenomenon can occur with respect to the gate lines.

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