Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-05-26
2002-09-24
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S142000, C349S146000
Reexamination Certificate
active
06456351
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid crystal display, more particularly to a liquid crystal display having an improved picture quality.
BACKGROUND OF THE INVENTION
The In-Plane Switching (hereinafter IPS) mode liquid crystal display has been suggested to complement the viewing angle characteristic of Twisted Nematic (hereinafter TN) mode liquid crystal display. (Principle and Characteristic of Electro-optical behavior with in-plane switching mode, Asia Display 1995, p. 577-580).
The IPS mode liquid crystal display is illustrated in FIG.
1
. Electrodes
2
a
,
2
b
for driving a liquid crystal molecule
3
a
are disposed on a lower substrate
1
. At that time, a distance between the electrodes
2
a
,
2
b
is larger than a distance between the lower substrate
1
and an upper substrate (not shown) thereby forming an electric field parallel to a surface of the lower substrate
1
. Herein, the electrodes
2
a
,
2
b
are made of opaque material, such as Mow and Al. A reference symbol “R” stands for a rubbing direction of an alignment layer.
As known in the art, the refractive anisotropy (or birefringence, &Dgr;n) is occurred due to the difference of the lengths of the long and the short axes. The refractive anisotropy &Dgr;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 phenomenon is regarded 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;·&Dgr;
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;
&Dgr;n: 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 &Dgr;nd/&lgr; should be &pgr;/2 according to the equation 1. As the &Dgr;nd varies with the birefringence difference of the liquid crystal molecules from viewing directions, the value of &lgr; is varied in order to satisfy &pgr;/2. According to this condition, the color corresponding to the varied wavelength &lgr; appears.
Accordingly, as the value of &Dgr;n relatively decreases at viewing direction “&agr;” toward the short axes of the liquid crystal molecules, the wavelength of the incident light for obtaining the maximum transmittance relatively decreases also. Consequently a color of blue having a shorter wavelength than that of a color of white appears.
On the other hand, as the value of &Dgr;n relatively increases at a viewing direction “&bgr;” toward the short axes of the liquid crystal molecules, the wavelength of an incident light relatively increases also.
Consequently a color of yellow having a longer wavelength than that of the color of white appears.
Deterioration is caused in the picture quality of IPS mode liquid crystal display.
Furthermore, since the electrodes
2
a
,
2
b
are made of opaque material, the aperture area in the IPS mode liquid crystal display is reduced and the transmittance thereof is also degraded. As a result, so as to obtain a proper brightness, an intensified backlight is required which incurs too much power consumption.
So as to solve the foregoing problems, a method to form the electrodes
2
a
,
2
b
with a transparent material has been proposed. In such a liquid crystal liquid display, the aperture ratio is often increased, however the transmittance is often not improved. That is to say, to produce an in-plane electric field, a distance l between the electrodes
2
a
,
2
b
must be set to be greater than a distance between upper and lower substrates. To obtain a suitable intensity of the electric field, the electrodes
2
a
and
2
b
have relatively large dimension of width, for example, 10 to
20 &mgr;m.
However, if the electrodes have such a large dimension of width, the liquid crystal molecules positioned right above the upper surfaces of the electrodes
12
and
14
do not move thereby forming equipotential lines. As the result, since the liquid crystal molecules positioned right above the upper surfaces of the electrodes continue to hold an initial configuration even in the presence of the electric field, the transmittance is little increased.
SUMMARY OF THE INVENTION
The present invention is directed to provide a liquid crystal display having an improved picture quality.
Further, the present invention is directed to improve transmittance and aperture ratio of the liquid crystal display.
So as to accomplish foregoing objects of this invention, the liquid crystal display having high transmittance and high aperture ratio comprising:
an upper and a lower substrate opposed each other to be spaced apart;
a liquid crystal layer interposed between inner surfaces of the upper and lower substrates, the liquid crystal layer including a plurality of liquid crystal molecules;
a gate bus line and a data bus line formed in the lower substrate in a matrix configuration and defining sub-pixel regions;
a counter electrode formed on each sub-pixel region of the lower substrate;
a pixel electrode formed on each sub-pixel region of the lower substrate, wherein the counter electrode and the pixel electrode form an electric field;
a thin film transistor formed at an intersection of the gate bus line and the data bus line, and switching a signal transmitted from the data bus line into the pixel electrode when the gate bus line is selected; and
a homogeneous alignment layer formed on inner surfaces of the upper and lower substrates,
wherein a first electric field is formed between the counter and pixel electrodes of a selected sub-pixel among the sub-pixel regions, and the electric field is formed as a diagonal line with respect to the gate bus line and the data bus line,
wherein a second electric field is formed between the counter electrode and the pixel electrode of another sub-pixel adjacent to said selected sub-pixel, and the second electric field is formed as a diagonal line to make a symmetry with the first electric field.
The liquid crystal display further comprises:
an upper and a lower substrate opposed to be spaced apart;
a liquid crystal layer interposed between inner surfaces of the upper and lower substrates, the liquid crystal layer including a plurality of liquid crystal molecules;
a gate bus line and a data bus line formed in the lower substrate in a matrix configuration and defining sub-pixel regions;
a counter electrode including a body of a rectangular frame shape formed at each sub-pixel region and at least a diagonal branch dividing a region surrounded by the body;
a pixel electrode including a first electrode which is formed to be overlapped with a selected portion of the body of the counter electrode, and second electrodes being extended from the first electrode and disposed parallel to diagonal branches being formed at their corresponding sub-pixels;
a thin film transistor formed at an intersection of the gate bus line and the data bus line, and switching a signal transmitted from the data bus line into the pixel electrode when the gate bus line is selected; and
a homogeneous alignment layer formed on inner surfaces of the upper and lower substrates,
wherein the diagonal branches and the second electrodes at a selected sub-pixel region are extended as diagonal lines with respect to the gate bus line and the data bus line,
wherein other diagonal branches and second electrodes at other sub-pixel regions adjoining in all directions to said selected sub-pixel region are arranged to make a symmetry together with the diagonal branches and the second electrodes of the selected sub-pixel region,
wherein the counter and pixel electrodes are made of a transparent conductor, and widths of the diagonal branches of the counter electrode and
Kim Hyang Yul
Lee Seung Hee
Hyundai Display Technology Inc.
Ladas & Parry
Qi Mike
Ton Toan
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