Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-02-25
2004-03-23
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S043000, C349S110000, C349S187000
Reexamination Certificate
active
06710836
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2001-9748, filed on Feb. 26, 2001, which is hereby incorporated by reference as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to liquid crystal is generated in a plane parallel to a substrate.
2. Discussion of Related Art
A liquid crystal display (LCD) device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce images. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling an applied electric field, a desired light image can be produced.
While various types of liquid crystal display devices are known, active matrix LCDs having thin film transistors and pixel electrodes arranged in a matrix are probably the most common. This is because such active matrix LCDs can produce high quality images at reasonable cost.
Recently, liquid crystal display devices with light, thin, and low power consumption characteristics are used in office automation equipment and video units and the like. Driving methods for such LCDs typically include a twisted nematic (TN) mode and a super twisted nematic (STN) mode. Although TN-LCDs and STN-LCDs have been put to practical use, they have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and liquid crystals interposed between the upper and lower substrates.
A detailed explanation about operation modes of a typical IPS-LCD panel will be provided referring to
FIGS. 1
,
2
A and
2
B.
As shown in
FIG. 1
, upper and lower substrates
1
and
2
are spaced apart from each other, and a liquid crystal layer
3
is interposed therebetween. The upper and lower substrates
1
and
2
are called color filter substrate and array substrate, respectively. Pixel and common electrodes
4
and
5
are disposed on the lower substrate
2
. The pixel and common electrodes
4
and
5
are parallel with and spaced apart from each other. The pixel and common electrodes
4
and
5
apply an electric field
6
horizontal to the liquid crystal layer
3
. The liquid crystal layer
3
has a negative or positive dielectric anisotropy, and thus it is aligned parallel with or perpendicular to the horizontal electric field
6
, respectively.
FIGS. 2A and 2B
conceptually illustrate operation modes of a conventional IPS-LCD device. When there is no electric field between the pixel and common electrodes
4
and
5
, as shown in
FIG. 2A
, the long axes of the liquid crystal molecules maintain an angle from a line perpendicular to the parallel pixel and common electrodes
4
and
5
. Herein, the angle may be 45 degrees, for example.
On the contrary, when there is an electric field between the pixel and common electrodes
4
and
5
, as shown
FIG. 2B
, there is an in-plane horizontal electric field
6
parallel with the surface of the lower substrate
2
between the pixel and common electrodes
4
and
5
. The in-plane horizontal electric field
6
is parallel with the surface of the lower substrate
2
because the pixel and common electrodes
4
and
5
are formed on the lower substrate
2
. Accordingly, the liquid crystal molecules are twisted so as to align, for example, the long axes thereof with the direction of the horizontal electric field
6
, thereby the liquid crystal molecules are aligned such that the long axes thereof are parallel with the line perpendicular to the pixel and common electrodes
4
and
5
.
FIG. 3
is a plan view of a lower substrate of the IPS-LCD device according to a related art. Gate lines
21
and a common line
51
are arranged parallel to each other, and data lines
31
are arranged perpendicular to the gate and common lines
21
and
51
. A pair of gate and data lines
21
and
31
define a pixel region. At a crossing portion of the gate and data lines
21
and
31
, a thin film transistor (TFT)
41
that is connected to the gate and data lines
21
and
31
is disposed. The common line
51
transversely crosses the pixel region, and a plurality of common electrodes
52
are disposed perpendicular to the common line
51
and connected thereto at a center of the pixel region. The plurality of common electrodes
52
are spaced apart from each other with a predetermined interval therebetween.
A plurality of pixel electrodes
62
are disposed parallel to the data line
31
and connected to a pixel connecting line
61
, which is disposed above the gate line
21
. Since the pixel connecting line
61
overlaps a portion of the gate line
21
, the pixel connecting line
61
and the portion of the gate line
21
constitute a storage capacitor
64
. Namely, the pixel connecting line
61
acts as a first electrode of the storage capacitor
64
, while the portion of the gate line
21
acts as a second electrode of the storage capacitor
64
.
Furthermore, one of the pixel electrodes
62
is electrically connected with the TFT
41
. The plurality of common electrodes
52
and the pixel electrodes
62
are spaced apart from each other with a predetermined interval therebetween and arranged in an alternating pattern. Therefore, each common electrode
52
is parallel to an adjacent pixel electrode
62
.
By the above-described structure and with additional parts such as polarizers and alignment layers, the IPS-LCD device displays images. The IPS-LCD device has wide viewing angles since the pixel and common electrodes are both placed on the lower substrate, as shown in FIG.
3
. Namely, the in-plane horizontal electric field generated by the common and pixel electrodes makes it possible to provide the wide viewing angles.
However, in the IPS-LCD device, a color-shift, which depends on the viewing angle, still remains. It is already known that this color-shift is not acceptable for full color-image display. This color-shift is related to a rotational direction of the liquid crystal molecules under application of electric field when the applied voltage is greater than a threshold voltage. Moreover, this color-shift is caused by increasing or decreasing of a retardation (&Dgr;n·d) of the liquid crystal layer with viewing angle.
To overcome the problem of color-shift, for example, U.S. Pat. No. 5,745,207 discloses new type IPS-LCD as shown in FIG.
4
.
FIG. 4
is a plan view of an exemplary array substrate for the IPS-LCD according to the conventional art. Since the array substrate shown in
FIG. 4
is similar to or somewhat the same as the substrate shown in
FIG. 3
, some explanation is omitted hereinafter.
Compared with the substrate of
FIG. 3
, although the array substrate of
FIG. 4
has similar structure and configuration to that of
FIG. 3
, the plurality of common and pixel electrodes
52
and
62
have a herringbone shape. The plurality of common electrodes
52
are respectively angled with respect to the common line
51
and connected to each other by the common line
51
. The plurality of pixel electrodes
62
are also bent at the central portion of the pixel region that is defined by the pair of the gate and data lines
21
and
31
. Therefore, the pixel region is divided into two domains A and B.
From the structure and configuration shown in
FIG. 4
, liquid crystal molecules
71
and
72
, which are positioned in the first domain A
Chowdhury Tarifur R.
LG. Philips LCD Co. Ltd.
McKenna Long & Aldridge LLP
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