Liquid crystal display

Details Liquid crystal display Liquid crystal display

1999-12-07

2003-12-02

Dudek, James (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

Reexamination Certificate

active

06657693

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a liquid crystal display, and more particularly to a liquid crystal display capable of preventing color shift and having enhanced response time.
BACKGROUND OF THE INVENTION
A liquid crystal display device has been used in various information display terminals. The major operating system for the liquid crystal display device is the twisted nematic(“TN”) and the super twisted nematic (“STN”). Though they are presently commercially used in the market, the problems of narrow viewing angle are still remained unsolved.
An In-Plane Switching (“IPS”) mode liquid crystal display has been suggested to solve foregoing problems.
As described in
FIG. 1
, a plurality of gate bus lines
11
are formed on a lower insulating substrate
10
along an x direction shown in the drawings and the gate bus lines
11
are parallel to each other. A plurality of data bus lines
15
are formed along a y direction which is substantially perpendicular to the x direction.
At this time, a pair of gate bus lines
11
and a pair of data bus lines
15
are formed for defining the sub-pixel region. The gate bus line
11
and the data bus line
15
are insulated by a gate insulating layer(not shown).
A counter electrode
12
is formed, for example in a rectangular frame shape, within a sub-pixel region and it is disposed at the same plane with the gate bus line
11
.
A pixel electrode
14
is formed at each sub-pixel region where the counter electrode
12
is formed. The pixel electrode
14
is composed of a web region
14
a
which divides the region surrounded by the rectangular frame type counter electrode
12
with a y direction, a first flange region
14
b
connected to a portion of the web region
14
a
and simultaneously overlapped with the counter electrode
12
of the x direction, and a second flange region
14
c
which is parallel to the first flange region
14
b
and is connected to the other portion of the web region
14
a
. That is to say, the pixel electrode
14
seems to be the letter “I”. Herein, the counter electrode
12
and the pixel electrode
14
are made of opaque metal layers.
The pixel electrode
14
and the counter electrode
12
are insulated from each other by a gate insulating layer (not shown).
A thin film transistor
16
(“TFT”) is disposed at the intersection of the gate bus line
11
and the data bus line
15
. This TFT
16
is composed of a gate electrode being extended from the gate bus line
11
, a drain electrode being extended from the data bus line
15
, a source electrode being extended from the pixel electrode
14
and a channel layer
17
formed on upper of the gate electrode.
A storage capacitor Cst is disposed at the region where the counter electrode
12
and the pixel electrode
14
are overlapped.
Although not shown in
FIG. 1
, an upper substrate(not shown) equipped with a color filter(not shown) is disposed with a predetermined distance opposite to a lower substrate
10
. Herein, the distance between the upper substrate and the lower substrate
10
is smaller than that between a region of the counter electrode toward the y direction and the web region of the pixel electrode thereby forming a parallel field which is parallel with the substrate surface. Further a liquid crystal layer(not shown) having a plurality of liquid crystal molecules is interposed between the upper substrate (not shown) and the lower substrate
10
.
Also, onto the resultant structure of the lower substrate and onto an inner surface of the upper substrate are formed homogeneous alignment layers respectively. By the homogeneous alignment layer, before forming an electric field between the counter electrode
12
and the pixel electrode
14
, long axes of liquid crystal molecules
19
are arranged parallel to the surface of the substrate
10
. Also, by the rubbing axis of the homogeneous alignment layer, the orientation direction of the molecules
19
is decided. The R direction in the drawings is the direction of rubbing axis for the homogeneous alignment layer formed on the lower substrate
10
.
A first polarizing plate(not shown) is formed on the outer surface of the lower substrate
10
and a second polarizing plate(not shown) is formed on the outer surface of the upper substrate(not shown). Herein, the first polarizing plate is disposed to make its polarizing axis to be parallel to the P direction of the FIG.
1
. That means, the rubbing axis direction R and the polarizing axis direction P are parallel each other. On the other hand, the polarizing axis of the second polarizing plate is substantially perpendicular to that of the first polarizing plate.
When a scanning signal is applied to the gate bus line
11
and a display signal is applied to the data bus line
15
, the TFT
16
disposed at the intersection of the gate bus line
11
and the data bus line
15
is turned on. Then the display signal of the data bus line
15
is transmitted to the pixel electrode
14
through the TFT
16
. Consequently, an electric field E is generated between the counter electrode
12
where a common signal is inputted and the pixel electrode
14
. At this time, the direction of electric field E is referenced as to x direction as described in the
FIG. 1
, it has a selected degree of angle with the rubbing axis.
Afterwards, before the electric field is not generated, the long axes of the liquid crystal molecules are arranged parallel to the substrate surface and parallel to the rubbing direction R. Therefore the light passed through the first polarizing plate and the liquid crystal layer is unable to pass the second polarizing plate, the screen has dark state.
When the electric field is generated, the long axes(or optical axes) are rearranged parallel to the electric field, and therefore the incident light passed through the first polarizing plate and the liquid crystal layer passes the second polarizing plate and the screen has white state.
At that time, the direction of the long axes of the liquid crystal molecules changes according to the presence of the electric field, and the liquid crystal molecules are arranged parallel to the substrate surface. Accordingly, a viewer can see the long axes of liquid crystal molecules at all points in the screen, and the viewing angle characteristic is improved.
However, the IPS mode liquid crystal display as described above also includes following problems.
As well known, the refractive anisotropy(or birefringence,
n) is occurred due to the difference in the lengths of the long and the short axes of the liquid crystal molecules. The refractive anisotropy
n is also varied from the viewer's viewing directions. Therefore a predetermined color is appeared on the region where the polar angle is of 0 degree and the azimuth angle range of degrees 0, 90, 180 and 270 in spite of the white state. This regards as color shift and more detailed description thereof is attached with reference to the equation 1.
T≈T
0
sin
2
(2&khgr;)·sin
2
(&pgr;·
nd
/&lgr;)  equation 1
wherein,
T: transmittance;
T
o
: transmittance to the reference light;
&khgr;: angle between an optical axis of liquid crystal molecule and a polarizing axis of the polarizing plate;
: birefringence;
d: distance or gap between the upper and lower substrates(thickness of the liquid crystal layer); and
&lgr;: wavelength of the incident light.
So as to obtain the maximum transmittance T, the &khgr;should be &pgr;/4 or the
nd/&lgr; should be &pgr;/2 according to the equation 1. As the
nd varies with the birefringence difference of the liquid crystal molecules depending on viewing directions, the value of &lgr; is varied in order to make
nd/&lgr; to be &pgr;/2. According to this condition, the color corresponding to the varied wavelength &lgr; appears.
Accordingly, as the value of
n relatively decreases at the viewing directions “a” and “c” toward the short axes of the liquid crystal molecules, the wavelength of the incident light for obtaining the maximum transmittance relatively decreases. Consequently a blue color having shorter wave

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