Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-05-26
2001-07-03
Malinowski, Walter J. (Department: 2164)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06256081
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a liquid crystal display, and more particularly to a liquid crystal display of wide viewing angle preventing color shift and simultaneously improving its aperture ratio and transmittance.
BACKGROUND OF THE INVENTION
Liquid crystal display devices have been used in various information display terminals and video devices. The major operating system for the liquid crystal display device is the twisted nematic(“TN”) mode and the super twisted nematic (“STN”) mode. Though they are commercially used in the market at present, the problems of narrow viewing angle remain 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. The gate bus lines
11
are parallel to each other. A plurality of data bus lines
15
are formed along an y direction which is substantially perpendicular to the x direction. Therefore a pixel region is defined. At this time, a pair of gate bus lines
11
and a pair of data bus lines
15
are shown in the drawing so as to define the 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
, for example in the form of a rectangular frame, is formed within the pixel region and is disposed at the same plane with the gate bus line
11
.
A pixel electrode
14
is formed at each pixel region where the counter electrode
12
is formed. The pixel electrode
14
consists of a web region
14
a
which divides the region surrounded by the rectangular frame shaped counter electrode
12
in the y direction, a first flange region
14
b
connected to one end 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
and is connected to the other end of the web region
14
a
. Thus, the pixel electrode
14
appears like the letter “I”. Herein, the counter electrode
12
and the pixel electrode
14
are made of opaque metal layers. To ensure an appropriate intensity of electric field, the widths of both the counter and pixel electrodes are preferably in the range of 10~20&mgr;m.
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
is disposed at the intersection of the gate bus line
11
and the data bus line
12
. This thin film transistor
16
includes a gate electrode extended from the gate bus line
11
, a drain electrode extended from the data bus line
15
, a source electrode extended from the pixel electrode
14
and a channel layer
17
formed on the upper portions of the gate electrode.
A storage capacitor(Cst) is disposed at the region where the counter electrode
12
and the pixel electrode
14
overlap.
Although not shown in
FIG. 1
, an upper substrate(not shown) equipped with a color filter(not shown) is disposed on the first substrate
10
opposite to each other with a selected distance. Herein, the distance between the upper substrate and lower substrate
10
is smaller than the distance between the counter electrode region in the y direction and the web region of the pixel electrode thereby forming an electric field which is parallel to 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, on the resultant structure of the lower substrate and on an inner surface of the upper substrate are formed homogeneous alignment layers respectively. By the homogeneous alignment layer, in the absence of electric field between the counter electrode
12
and the pixel electrode
14
, long axes of liquid crystal molecules
19
are arranged parallel to the substrate surface. Also, by the rubbing axis of the homogeneous alignment layer, the orientation direction of the molecules
19
is decided. The reference R in the drawings means 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 directions of rubbing axis R and polarizing axis 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 selected gate bus line
11
and a display signal is applied to the data bus line
15
, the thin film transistor
16
disposed adjacent to 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 thin film transistor
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, as the direction of electric field E is referenced as x direction as described in the
FIG. 1
, it has a predetermined degree of angle with the rubbing axis.
Afterward, when no electric field is 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 passing through the first polarizing plate and the liquid crystal layer is unable to pass the second polarizing plate, and the screen shows dark state.
On the other hand, when the electric field is generated, the long axes(or short axes) are rearranged parallel to the electric field. Therefore the incident light passing through the first polarizing plate and the liquid crystal layer, passes the second polarizing plate, and the screen shows white state.
At this time, the direction of the long axes of the liquid crystal molecules change according to the electric field, and the liquid crystal molecules themselves are arranged parallel to the substrate surface. Accordingly, the viewer can see the long axes of liquid crystal molecules from all directions, and the viewing angle characteristic is improved.
However, the IPS mode liquid crystal display as described above also includes the following problems.
It is well known that refractive anisotropy(or birefringence,
n) occurs due to the difference in lengths of the long and the short axes. The refractive anisotropy
also varies according to the observer's viewing directions. Therefore a selected color can be shown in the region where the polar angle is of 0 degree and the azimuth angle is in the range of degrees 0, 90, 180 and 270, even in the white state screen. This is regarded as color shift and a more detailed description thereof is attached with reference to equation 1.
T≈T
0
sin
2
(2&khgr;)·sin
2
(&pgr;·&khgr;nd/&lgr;) . . . equation 1
wherein, T: transmittance;
T
0
: 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 the viewing directions, the value of &lgr; varies so as to make
d/&lgr; to be &pgr;/2. According to this condition, the color corresponding to the varied wavelength &lgr; appears in the screen.
Accordingly,
Choi Woo Ho
Eom Tae Yong
Lee Seung Hee
Hyundai Electronics Industries Co,. Ltd.
Ladas & Parry
Malinowski Walter J.
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