Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-06-26
2004-12-14
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06831719
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a transmissive and reflective type liquid crystal display in which the display operation is carried out in reflection mode and transmission mode.
2. Description of the Related Art
Liquid crystal displays (LCDs) have become displays of choice among the various developed flat panel type displays because they are much slimmer and lighter than other types of displays. They also require lower driving voltage and lower power consumption.
LCD displays are classified as transmission type which display images using an external light source such as a backlight, as reflection type which display an image using natural light, and transmissive and reflective type which display in a transmission mode using an internal light source provided in the display itself at indoors or a dark place where an external light source does not exist and the display operates in a reflection mode to display images by reflecting external incident light in a high brightness environment such as at outdoors.
LCDs can also be classified depending on the way they are driven. For example, In the passive matrix type, pixels in the LCDs are driven using a root-mean-square (rms) of a difference between voltages applied to signal lines and scanning lines, while a line addressing in which a signal voltage is applied to all of the pixels at the same time is carried out. In the active matrix type, pixels are driven by a switching element such as a MIM (Metal-insulator-metal) device or a thin film transistor.
FIG. 1
is a sectional view of a conventional transmissive and reflective type LCD, and shows an active matrix type LCD using the thin film transistor.
Referring to
FIG. 1
, the conventional transmissive and reflective type LCD includes a first substrate
10
, a second substrate
40
arranged facing the first substrate
10
, a liquid crystal layer
50
formed between the first substrate
10
and the second substrate
40
, and a light source, i.e., a backlight assembly
60
disposed at a rear side of the first substrate
10
.
The first substrate
10
includes a first insulating substrate
11
, a thin film transistor
25
formed on the first insulating substrate
11
, a passivation film
30
having a contact hole
32
for exposing a part of the thin film transistor
25
, a transparent electrode
34
, and a reflection electrode
36
. The thin film transistor
25
includes a gate electrode
12
, a gate insulating film
14
, an active pattern
16
, an ohmic contact pattern
18
, a source electrode
20
, and a drain electrode
22
. The transparent electrode
34
functions as a pixel electrode for transmitting light that is generated from the backlight
60
and is then incident through the first substrate
10
. The transparent electrode
34
is connected to the thin film transistor
25
formed on every unit pixel region on the first substrate
10
. The reflection electrode
36
reflects external light that is incident through the second substrate
40
and at the same time functions as another pixel electrode. The transparent electrodes
34
include regions of a transmission part T and a reflection part R for reflecting the external light incident through the second substrate
40
.
The second substrate
40
includes a second insulating substrate
42
, a color filter
44
comprised of RGB pixels for displaying colors while light is transmitted therethrough, a black matrix
46
for preventing the light from being leaked between the pixels, and a transparent common electrode
48
.
The liquid crystal layer
50
is made of 90° twisted nematic (TN) liquid crystal, and has an approximately 0.24 of &Dgr;nd which is a product of anisotropy &Dgr;n in refractive index and thickness d of the liquid crystal layer
50
.
Also, according to an alignment direction of the liquid crystal molecules, a first polarizing plate
54
and a second polarizing plate
58
are respectively attached to external surfaces of the first and second substrates
10
and
40
so as to transmit only polarized light in a specific direction. The first and second polarizing plates
54
and
58
are all linear polarizers in which each polarizing axis of the first and second polarizing plates
54
and
58
is orthogonal to each other.
Between the first substrate
10
and the first polarizing plate
54
, and between the second substrate
40
and the second polarizing plate
58
, there are respectively arranged a first ¼ wavelength phase difference plate
52
and a second ¼ wavelength phase difference plate
56
. Each of the ¼ wavelength phase difference plates
52
and
56
functions to convert linearly polarized light to circularly polarized light; or vice versa by causing a phase difference of ¼ wavelength between two polarization components that are orthogonal to each other and are parallel to the optical axes of the ¼ wavelength phase difference plates
52
and
56
.
Hereinafter, there are respectively described operations in the reflection mode and the transmission mode in the conventional transmissive and reflective type LCD shown in FIG.
1
.
FIGS. 2A and 2B
are schematic views for illustrating an operation of the conventional LCD in the reflection mode.
First, when a pixel voltage is not applied (OFF), as shown in
FIG. 2A
, light that is incident from an outside is transmitted through the second polarizing plate
58
, so that the light is linearly polarized in a direction parallel to the polarizing axis of the second polarizing plate
58
. The linearly polarized light is transmitted through the second ¼ wavelength phase difference plate
56
, so that the linearly polarized light is converted onto left-handed circularly polarized light. The left-handed circularly polarized light is transmitted through the liquid crystal layer
50
, so that the left-handed circularly polarized light is linearly polarized in a direction vertical to the polarizing axis of the second polarizing plate
58
, and is then incident onto the reflection electrode
36
. The linearly polarized light, which is reflected by the reflection electrode
36
, is transmitted through the liquid crystal layer
50
, so that the linearly polarized light is converted onto the left-handed circularly polarized light. The left-handed circularly polarized light is transmitted through the second ¼ wavelength phase difference plate
56
, so that the left-handed circularly polarized light is linearly polarized in a direction parallel to the polarizing axis of the second polarizing plate
58
. And then, the linearly polarized light is transmitted through the second polarizing plate
58
, so that a white image is displayed.
When a maximum pixel voltage is applied (ON), as shown in
FIG. 2B
, light that is incident externally is transmitted through the second polarizing plate
58
, so that it is linearly polarized in a direction parallel to the polarizing axis of the second polarizing plate
58
. The linearly polarized light is transmitted through the second ¼ wavelength phase difference plate
56
, so that it is converted onto left-handed circularly polarized light. The left-handed circularly polarized light is transmitted through the liquid crystal layer
50
without variation in the polarization state, and is then incident onto the reflection electrode
36
. The light, which is incident onto the reflection electrode
36
, is reflected by the reflection electrode
36
, so that it is converted to right-handed circularly polarized light and the converted right-handed circularly polarized light is transmitted through the liquid crystal layer
50
. Thus, the right-handed circularly polarized light, which has been passed through the liquid crystal layer
50
, is transmitted through the second ¼ wavelength phase difference plate
56
, so that it is linearly polarized in a direction perpendicular to the polarizing axis of the second polarizing plate
58
. The linearly polarized light is shielded by the second polarizing
Jang Yong-Kyu
Kim Hyung-Guel
F. Chau & Associates LLC
Kim Richard H
Kim Robert H.
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