Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-07-26
2003-11-11
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S034000, C349S141000, C349S192000
Reexamination Certificate
active
06646691
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to an active-matrix in-plane switching mode LCD panel, and more particularly, to an improvement of an active-matrix LCD (liquid crystal display) panel driven by an in-plane electric field.
(b) Description of the Related Art
LCD panels are generally categorized by the mode of the liquid crystal into a plurality of types including one driven by a perpendicular electric field, such as a TN-mode (twisted nematic mode) LCD panel. In this mode of the LCD panel, the orientations of the directors (axes) of the LC molecules are changed by application of a perpendicular electric field which is normal to the substrate surface, to thereby control the transparency thereof (or transmittance of light passing therethrough) for image display on the display panel. This mode of the LCD panel, however, has the drawback of a narrow viewing angle wherein the refractive index of the LC layer largely depends on the viewing angle for the LCD panel, because the directors of the LC molecules are oriented in the direction normal to the substrate surface during application of the drive voltage. Thus, this mode of the LCD panel is not suited for a variety of applications which require a wider viewing angle.
On the other hand, another mode of the LCD panel, known as an in-plane switching mode LCD panel, has a higher viewing angle and provides a higher image quality. In this mode of the LCD panel, the directors of the LC molecules are initially oriented in the direction parallel to the substrate surface and applied with the lateral (in-plane) electric field to be rotated in a plane parallel to the substrate surface for controlling the light transmittance. Thus, the in-plane switching mode LCD panel is extensively studied and developed in recent days. It is known that the wider viewing angle and the higher image quality result from the extremely small dependency of the change in the refractive index of the LC layer on the viewing angle.
FIG. 1
shows the LCD panel (or LCD panel assembly) of an in-plane switching mode LCD device in a front view. The LCD panel assembly includes a plurality of scanning lines
502
extending in a column direction and driven by an external driver, a plurality of video signal lines
103
extending in a row direction perpendicular to the scanning lines
502
, a plurality common electrode lines
106
extending parallel to the scanning line
502
, and a plurality of pixel elements arranged in a matrix and each defined by a pair of adjacent scanning lines
502
and a pair of adjacent video signal lines
103
. Each pixel element includes a TFT (thin film transistor)
503
acting as a switching transistor and an associated pixel electrode
104
connected to the source of the TFT
503
. The common electrode line
106
has a pair of branches acting as common electrode
106
A for each pixel element and extending parallel to the pixel electrode
104
. The voltage applied between the pixel electrode
104
and the common electrode
106
A generates a lateral electric field or in-plane electric field in each pixel element parallel to the substrate surface.
Referring to Fig.
FIG. 2
taken along line II—II in
FIG. 1
, the LCD panel assembly, generally designated by numeral
300
, includes a TFT panel
100
and a counter panel
200
sandwiching therebetween a LC layer
107
. The TFT panel
100
includes a TFT glass substrate
102
, and, for each pixel, the common electrode
106
A formed thereon, the pixel electrode
104
and the video signal line
103
formed thereon with an intervention of a gate insulator film
130
. The pixel electrodes
104
and the signal lines
103
are disposed on the TFT panel
100
alternately with each other and covered with a protective insulator film
110
, on which a first orientation film
120
is formed by coating and rubbing. The first orientation film
120
has a function for determining the orientation of the LC molecules in the LC layer
107
in the vicinity of the orientation film
120
.
The counter panel
200
includes a counter glass substrate
101
, a shield film
203
formed on the inner surface of the counter glass substrate
101
and having an opening for each pixel, a color layer
142
formed on the counter glass substrate
101
at each opening, a planarizing film
202
formed on the color layer
142
, and a second orientation film
202
formed on the planarizing film
202
by coating and rubbing. The direction of the rubbing in the second orientation film
202
is opposite to the direction of the rubbing in the first orientation film
120
.
Between the TFT panel
100
and the counter panel
200
are disposed the LC layer
107
and ball spacers
302
, the ball spacers
302
having a diameter for defining the distance or gap between the TFT panel
100
and the counter panel
200
. A first polarizing film
145
is formed on the outer surface of the TFT glass substrate
102
so that the light transmission axis of the polarizing film
145
is perpendicular to the direction of the rubbing in the orientation film
120
. A second polarizing film
143
is formed on the outer surface of the counter substrate
101
so that the light transmission axis of the polarizing film
143
is perpendicular to the light transmission axis of the first polarizing film
145
.
FIG. 3
shows a schematic block diagram of the LCD device having the LCD panel (assembly)
300
of
FIG. 1
, wherein the LCD panel
300
is placed on a back light
400
and driven by a LCD driver
500
. The LCD driver
500
supplies scanning signals, video signals and a potential for the common electrode line to the LCD panel
300
.
FIG. 4
shows an equivalent circuit diagram of the LCD panel assembly of FIG.
1
. The scanning signals, the video signals and the common electrode potential are supplied from the LCD driver
500
shown in
FIG. 3
to the scanning lines G
1
to Gn, video signal lines D
1
to Dn and the common electrode line COM, respectively. The common electrode potential is controlled by a variable voltage source ECOM in the LCD driver.
When one of the scanning lines G
1
to Gn assumes a higher potential, the TFTs of the pixel elements disposed in a corresponding row are turned on, whereby electric charge flows from the video signal lines D
1
to Dn into the corresponding pixel electrodes P
1
. This generates a specific voltage between the common electrode line
106
and the corresponding pixel electrodes
104
, thereby generating an electric field between the pixel electrode
104
and the common electrode
106
A. As a result, a portion of the LCD layer
107
interposed between both the panels
100
and
200
and located between both the electrodes
104
and
106
A as viewed perpendicular to the substrate surface rotates parallel to the substrate surface, whereby the electro-optics effect of the LCD layer
107
allows image display based on the video signals.
FIG. 5
shows the distribution of the electric field generated in the pixel. The capacitance involved between the pixel electrode
104
and the common electrode
106
A is considered to include a first, LC layer capacitance C
LC
which depends on the orientation of the LC layer
107
and a second, storage capacitance C
SC
which is constant, although both the capacitances are difficult to be separated. The sum of the capacitances C
LC
and C
SC
or the total capacitance between both the electrodes
104
and
106
A can be obtained by the following formula:
∫
v
D
∫
0
E
_
·
ⅆ
D
_
ⅆ
v
=
1
2
(
C
Lc
+
C
SC
)
V
2
(
1
)
wherein the integration with respect to “v” is conducted in the entire area for the LCD panel assembly, and E, D and V denote the electric field vector, the electric displacement vector and the voltage between the pixel electrode
104
and the common electrode line
106
, respectively.
As understood from formula (1), the total capacitance depends on the medium through which the electric lines of force pass and the number of such electric lines of force. In
FIG. 5
, the electric lines of
Watanabe Makoto
Watanabe Takahiko
Kim Robert H.
McGinn & Gibb PLLC
NEC LCD Technologies Ltd.
Schechter Andrew
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