Fringe field switching mode LCD

Details Fringe field switching mode LCD Fringe field switching mode LCD

2001-10-10

2003-11-11

Chowdhury, Tarifur R. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S143000

Reexamination Certificate

active

06646707

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fringe field switching mode liquid crystal display, and more particularly, to a fringe field switching mode liquid crystal display capable of preventing degradation of screen quality due to color shift and disclination line.
2. Description of the Related Art
It is well known that a Fringe Field Switching mode Liquid Crystal Display (hereinafter referred to as FFS mode LCD) has been proposed in order to improve a low aperture ratio and transmittance of In Plane Switching mode LCD.
FIG. 1
is a plane view for showing a conventional FFS mode LCD. As shown in
FIG. 1
, a plurality of gate bus lines
3
and data bus lines
7
are arranged crossing with each other on a transparent insulating substrate such as glass substrate (not shown). A counter electrode
2
made of transparent conductor such as Indium Tin Oxide (ITO) is disposed in a pixel region defined by the lines
3
,
7
crossing with each other. The counter electrode
2
generally has a plate shape and may have a comb shape.
A common electrode line
10
is disposed in order to continuously supply common signals to the counter electrode
2
. The common electrode line
10
is parallel to the gate bus line
3
, including a first part
10
a
being in contact with the upper part of the counter electrode
2
and a pair of second parts
10
b
extended from the first part
10
a
to be parallel to the data bus line
7
and to be in contact with one side and the other side of the counter electrode
2
, respectively. The common electrode line
10
is generally formed at the same time when the gate bus line
3
is formed.
A pixel electrode
9
made of transparent conductor such as ITO, is arranged in the pixel region, overlapping with the counter electrode
2
. The pixel electrode
9
and the counter electrode
2
are electrically insulated by a gate insulating layer (not shown). The pixel electrode
9
has a comb shape including a plurality of branches
9
a
arranged parallel to the data bus line
7
with the same distances and a bar
9
b
connecting one ends of each branch
9
a.
A thin film transistor TFT is formed at the intersection of the gate bus line
3
and the data bus line
7
. The thin film transistor TFT includes a part of the gate bus line
3
i.e. a gate electrode, a semiconductor layer (not shown) formed on the gate electrode with a gate insulating layer interposed, a source electrode
7
a
disposed over one side of the semiconductor layer and being in contact with the bar
9
b
of the pixel electrode
9
, and a drain electrode
7
b
extended from the data bus line
7
and disposed over the other side of the semiconductor layer.
Although it is not shown in the above, a color filter substrate is arranged opposite to the above array substrate with a distance longer than that between the counter electrode
2
and the pixel electrode
9
and then, a liquid crystal layer comprising a plurality of liquid crystal molecules is interposed between the substrates.
According to the FFS mode LCD having the above structure, when a predetermined voltage is applied in the a counter electrode
2
and the pixel electrode
9
, a fringe field is generated between the two electrodes and on the upper part thereof since the distance between the array substrate and the color filter substrate is longer than that between the electrodes
2
,
9
. The fringe field has influence on all parts including the upper parts of the counter electrode
2
and of pixel electrode
9
, thereby driving all liquid crystal molecules on the upper parts of the electrodes
2
,
9
as well as those between the electrodes
2
,
9
.
Therefore, the FFS mode LCD has a high aperture ratio since the counter electrode and the pixel electrode are made of transparent conductor. Moreover, the FFS mode LCD has an improved transmittance since liquid crystal molecules over the upper part of the electrodes as well as those between the electrodes are driven.
However, in the FFS mode LCD, when a field is generated between the counter electrode and the pixel electrode, liquid crystal molecules having refractive anisotropy are arranged in one direction, thereby generating color shift according to a viewing angle and degrading screen quality.
Therefore, it is essential to prevent the color shift in order to improve screen quality of FFS mode LCD. As an effort to obtain improved screen quality, as shown in
FIG. 2A
, a pixel electrode
19
may have fracture slits S having a shape of “<” in a pixel. Alternatively, as shown in
FIG. 2B
, it has been proposed that the pixel electrode
19
has a structure that each pixel has a slit S in a slant direction symmetrical with the adjacent pixel. In the above structures, an electric field is generated in one pixel or in two symmetrical directions between adjacent pixels, thereby compensating refractive anisotropy of liquid crystal molecules and as a result, it is possible to prevent color shift.
However, according to the FFS mode LCD having compensating electrode structure to prevent color shift, when a positive liquid crystal is applied, a disclination line is generated from the end of pixel electrode. And, it is difficult to eliminate the disclination line, thereby degrading screen quality. In particular, the disclination line is generated more, when high voltage over driving voltage is applied on panel and external pressure is applied on a voltage-applied panel.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a FFS mode LCD preventing generation of color shift and disclination line or easily removing it when the disclination line is generated.
In order to accomplish the above object, FFS mode LCD of the present invention comprises: a first and a second transparent insulating substrates arranged opposite to each other with a predetermined distance, with a liquid crystal layer including a plurality of liquid crystal molecules interposed between them; a plurality of gate bus lines and data bus lines formed on the first transparent insulating substrate and arranged in a matrix form to define a unit pixel; a thin film transistor formed at the intersection of the gate bus line and the data bus line; a counter electrode disposed in each unit pixel, made of transparent conductor; and a pixel electrode arranged in each unit pixel to generate a fringe field with the counter electrode, being insulated with the counter electrode and made of transparent conductor and including a plurality of upper slits and lower slits symmetrical each other with respect to long side of the pixel with a predetermined tilted angle.
The pixel electrode may further have a reference slit arranged parallel to the gate bus line on the center of long side of pixel and dividing an upper slit and a lower silt. The upper slit and the lower slit have a tilted angle below +45° and below −45° respectively, and desirably, of ±2~20° with respect to the axis dividing them. The slit has a structure that adjacent pixels in the same column have the same tilted angle and adjacent pixels in the same line have opposite tilted angle.
And, the FFS mode LCD of the present invention include a first and a second alignment layers respectively arranged on the top of inner sides of the first and the second transparent insulating substrates and a first and a second polarizing plates respectively arranged on outer sides of the first and the second transparent insulating substrates. The first and the second alignment layers are rubbed parallel to the gate bus line when positive liquid crystals are applied, and rubbed parallel to the data bus line when negative liquid crystals are applied. The first and the second polarizing plates have transmission axes perpendicular to each other and one of the axes has the same direction as rubbing direction of the alignment layer.
Also, the FFS mode LCD of the present invention further include a common electrode line to apply common signals to the counter electrode. The common electrode line is arranged on the edge of pixel ad

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