Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-02-12
2003-11-25
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S138000
Reexamination Certificate
active
06654087
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2000-6222, filed on Feb. 10, 2000, under 35 U.S.C. §119, the entirety of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transflective LCD device.
2. Description of Related Art
Until now, the cathode-ray tube (CRT) has been developed for and is used mainly for the display systems. However, the flat panel display is beginning to make its appearance due to the requirements of small depth dimensions, undesirably low weight and low voltage power supply. At present, the thin film transistor-liquid crystal display (TFT-LCD) with high resolution and small depth dimension has been developed.
During operation of the TFT-LCD, when the pixel is turned ON by switching elements, the pixel transmits light generated from a backlight device. The switching elements are generally amorphous silicon thin film transistors (a-Si:H TFTs) which use an amorphous silicon layer. Advantageously, the amorphous silicon TFTs can be formed on low cost glass substrates using low temperature processing.
In general, the TFT-LCD transmits an image using light from the back light device that is positioned under the TFT-LCD panel. However, the TFT-LCD only employs 3~8% of the incident light generated from the backlight device, i.e., the inefficient optical modulation.
Referring to the drawings, a TFT-LCD device that is manufactured by a conventional method will now be explained in some detail.
FIG. 1
is a graph illustrating a light transmittance respectively measured after light passes through each layer of a conventional liquid crystal display device.
The two polarizers have a transmittance of 45% and, the two substrates have a transmittance of 94%. The TFT array and the pixel electrode have a transmittance of 65%, and the color filter has a transmittance of 27%. Therefore, the typical transmissive TFT-LCD device has a transmittance of about 7.4% as seen in
FIG. 1
, which shows a transmittance after light passes through each layer of the device. For this reason, the transmissive TFT-LCD device requires a high, initial brightness, and thus electric power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device. Moreover, there still exists a problem that the battery cannot be used for a long time.
In order to overcome these problems, the reflective TFT-LCD has been developed. Since the reflective TFT-LCD device uses ambient light, it is light and easy to carry. Also, the reflective TFT-LCD device is superior in aperture ratio, compared to the transmissive TFT-LCD device. Namely, since the reflective TFT-LCD substitutes an opaque reflective electrode for a transparent electrode material in the pixel of the conventional transmissive TFT-LCD, it reflects the ambient light.
As described above, since the reflective TFT-LCD device uses ambient light other than an internal light source such as a backlight device, battery life can be increased resulting in longer use times. In other words, the reflective TFT-LCD device is driven using light reflected from the reflective electrode. Thus, only the drive circuitry that drives the liquid crystal uses the battery power in the reflective TFT-LCD device.
Additionally, the reflective TFT-LCD device has a problem that it is affected by its surroundings. For example, the brightness of indoors-ambient light differs largely from that of outdoors-ambient light. Also, even in the same location, the brightness of ambient light depends on the time of day (e.g., noon or dusk). Therefore, the reflective TFT-LCD device cannot be used at night without ambient light.
Accordingly, there is a need for a transflective TFT-LCD device that can be used during daytime hours as well as nighttime because the transflective LCD device can be changed to either a transmissive mode or a reflective mode depending on the users.
FIG. 2
is a schematic cross-sectional view illustrating one pixel of the transflective TFT-LCD device according to the conventional art. As shown, the transflective TFT-LCD device
51
includes a liquid crystal panel and a backlight device
70
. The liquid crystal display panel includes lower and upper substrates
50
and
60
and an interposed liquid crystal layer
80
. The upper substrate
60
has color filters
61
. The lower substrate
50
serves as the array substrate and includes TFTs (not shown), and transmissive and reflective electrodes
54
and
52
serve as a pixel electrode. The reflective electrode
52
surrounds the transmissive electrode
54
and has a light transmitting hole
53
having a length “&Dgr;L”. The reflective electrode
52
is also made of a conductive material such as chrome (Cr), aluminum (Al) or tantalum (Ta), which has good light reflectivity and reflects the ambient light
74
. The transmissive electrode
54
that is formed in the light transmitting hole
53
transmits the light
72
from the backlight device
70
.
The transflective LCD device
51
is operated as follows. First, in the reflective mode, the incident light
74
from the outside is reflected from the reflective electrode
52
and is directed toward the upper substrate
60
. At this time, when the electrical signals are applied to the reflective electrode
52
by the switching element (not shown), arrangement of the liquid crystal layer
80
varies and thus the reflected light of the incident light
74
is colored by the color filter
61
and is displayed in the form of colored light. Second, in the transmissive mode, light
72
emitted from the backlight device
70
passes through the transmissive electrode
54
(or transmitting hole
53
). At this time, when the electrical signals are applied to the transmissive electrode
54
by the switching element (not shown), arrangement of the liquid crystal layer
80
varies. Thus, the light
72
passing through the liquid crystal layer
80
is colored by the color filter
61
and displayed in the form of images with other colored lights.
FIG. 3
is a cross-sectional view of the conventional transflective LCD device. In
FIG. 3
, the color filter is not depicted because it does not affect the state of the light. As shown, the conventional transflective LCD device
110
includes a first substrate
106
(an array substrate) and a second substrate
204
(a color filter substrate). A liquid crystal layer
300
that affects the state of the light is interposed between the first substrate
106
and the second substrate
204
.
On the surface of the first substrate
106
that faces the second substrate
204
are a TFT (not shown) and a transparent conductive electrode
104
(i.e., a pixel electrode). On the transparent conductive electrode
104
is a lower passivation layer
107
. On the lower passivation layer
107
is a reflective electrode
108
(i.e., a pixel electrode) that has a transmitting hole
150
. On the other surface of the first substrate
106
a lower polarizer
102
. A backlight device
101
is adjacent to the lower polarizer
102
. The lower polarizer
102
, the first substrate
106
, the transparent conductive electrode
104
, the lower passivation layer
107
and the reflective electrode
108
are all together referred to as a lower substrate
100
.
On one surface of the second substrate
204
is a retardation film (Quarter Wave Plate (&lgr;/4 plate) referred to hereinafter as a QWP
206
. On the QWP
206
is an upper linear polarizer
208
. An upper passivation layer
202
that protects the color filters (not shown) is on the other surface of the second substrate
204
. The passivation layer
202
, the second substrate
204
, the QWP
206
, and the upper polarizer
208
are all together referred to as an upper substrate
200
.
The reflective electrode
108
is made of a reflective metallic material having a good light reflectivity, such as Al, Cr or Ta. The transmitting hole
150
of the reflective electrode
108
transmits the li
Chung Jae-Young
Park Sung-II
Song In-Duk
L.G. Philips LCD Co., Ltd.
Morgan & Lewis & Bockius, LLP
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