Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-04-11
2004-08-24
Chowdhuri, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S141000
Reexamination Certificate
active
06781645
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to an active matrix liquid crystal display (LCD) device, and more particularly, to an active matrix LCD device wherein liquid crystal is driven by an electric field acting in the direction substantially parallel to the substrates.
(b) Description of the Related Art
LCD devices are generally categorized into two types including a passive matrix LCD device and an active matrix LCD device based on the driving system therefor. The active matrix LCD device includes an active drive element such as thin film transistor (TFT) or diode in each pixel element, for charging a capacitor thereof with a signal voltage while selecting the pixel element for an on-state in a time-division scheme. The capacitor holds the signal voltage during a subsequent off-state of the drive element for displaying an image for the pixel. Compared to the passive matrix LCD device wherein the signal voltage is applied to the liquid crystal (LC) by using a time-division matrix-drive scheme, the active matrix LCD features a higher contrast and a larger screen.
A twisted-nematic mode (referred to as TN mode, hereinafter) has been generally used as the operational mode of the LC in the active matrix LCD device, wherein the aligned direction (referred to as director, hereinafter) of the longer axes of the LC molecules is twisted by about 90 degrees between the transparent substrates, using an electric field in the direction perpendicular to the substrates for rotating the director in the vertical direction.
The TN mode LCD device generally has a defect wherein the view angle for the LCD panel is narrow, that is, the image on the LCD panel has a large view angle dependency, especially in the case of a large screen LCD panel.
For solving the problem view angle dependency, an in-plane switching mode (referred to as IPS mode, hereinafter) has been developed for generating an electric field in the direction parallel to the substrates for rotating the director within the horizontal plane. In the proposed IPS mode LCD, the horizontal alignment of the LC orientation effected by the electric field acting parallel to the substrates affords an advantage in that the double refraction characteristic of the LC is scarcely changed even if the viewpoint is moved, thereby achieving a wider view angle compared to the TN mode LCD device.
FIG. 1
shows a top plan view of a first conventional example of IPS mode active matrix LCD devices,
FIGS. 2 and 3
show sectional views taken along line II—II and III—III, respectively, in
FIG. 1
, and
FIG. 4
shows a timing chart of the potentials of electrodes and lines in the first conventional active matrix LCD device.
The first conventional LCD device has a plurality of pixel elements each including a TFT
16
and a pixel electrode
17
, a plurality of scanning lines
13
disposed for respective rows of the pixel elements, a plurality of signal lines
14
disposed for respective columns of the pixel elements, and a common electrode
15
, which are formed on a transparent insulating substrate
11
(referred to as TFT substrate, hereinafter). Each pixel electrode
17
and a corresponding portion the common electrode
15
extend parallel to each other to generate an electric field
100
having a main component extending substantially perpendicular to the stripe electrodes
17
and
15
.
As shown in
FIG. 3
, the active matrix LCD device further includes a transparent counter substrate
12
disposed in opposed relationship with the TFT substrate
11
with an intervention of a LC layer
18
. The counter substrate
12
mounts thereon a black matrix
19
, a color filter
20
and a LC orientation layer
21
on respective sides thereof.
In the IPS mode LCD device of
FIG. 1
, the electric field generated therein includes unnecessary components between the signal line
14
and adjacent electrodes, which necessitates a light shield for covering the unnecessary image component. In the illustrated LCD device, the black mat
19
formed on the counter substrate
12
acts as the shield layer for the space between the signal line
14
and the adjacent electrodes (such as common electrode
15
). The arrangement of the TFT substrate
11
with respect to the counter substrate
12
generally involves an alignment error of about 7 to 10 &mgr;m after bonding thereof. For assuring the effective light shield, the edge of the black matrix
19
should be located with a margin, such as designated by reference numeral
200
in
FIG. 3
, from the edge of the common electrode
15
. This causes a larger area of the black matrix
19
and a smaller opening rate of the pixel area in the LCD device. Patent publication JP-A-9-80415 proposes for solving the problem low opening rate.
FIG. 5
shows a top plan view of the proposed LCD device, or second conventional LCD device,
FIG. 6
shows a sectional view taken along line VI—VI in
FIG. 5
, and
FIG. 7
shows a timing chart in the second conventional LCD device.
In the second conventional LCD device shown in
FIG. 5
, the signal line
14
partly overlaps with the common electrode
15
as viewed in the direction perpendicular to the substrates. This affords an advantage in that the portions of the black matrix
19
extending in the direction parallel to the signal lines
14
can be omitted together with their margins, and it is sufficient that the black matrix
19
has a portion extending parallel to the scanning lines
13
in the display panel. This achieves a larger opening rate of the pixel area in the LCD device.
In the second conventional LCD devices, however, the black matrix
19
formed on the counter substrate
12
involves another problem, as described hereinafter.
In case of the TN mode active matrix LCD device, since the electric field from the black matrix
19
is shielded by the transparent common electrode
15
formed on the substantially entire surface of the counter substrate
12
, the electric potential of the black matrix
19
does not affect the image quality on the LCD panel. However, in case of the IPS mode active matrix LCD device, since the black matrix
19
does not have a shield electrode such as the common electrode between the black matrix
19
and the LC layer
18
in the TN mode active matrix LCD device, the potential of the black matrix
19
fluctuates and thereby affects the image quality on the LCD panel.
It is first noted that the electric potential of the black matrix
19
is not fixed in the IPS mode active matrix LCD device, whereas the black matrix
19
is implemented by materials of a high electric conductivity, such as black resist wherein a metal or carbon black is dispersed. Thus, the potential of the black matrix
19
is generally determined based on the capacitive coupling acting between the same and the signal lines
14
, the scanning lines
13
and the common electrode
15
Assuming that the potential of the black matrix
19
, the voltages of the signal lines
14
, the scanning lines
13
, and the common electrode
15
, the coupling capacitances between the black matrix
19
and the signal line
14
, between the black matrix
19
and the scanning line
13
, between the black matrix
19
and the common electrode
15
are represented by Vbm, Vd, Vg, Vcom, Cbm-d, Cbm-g, Cbm-com, respectively, the potential Vbm of the black matrix is expressed by equations 1 and 2:
Vbm=Vd×Cbm
-
d/C
total+
Vg×Cbm
-
g/C
total+
Vcom×Cbm
-
com/C
total (1)
C
total=
Cbm
-
d+Cbm
-
g+Cbm
-
com
(2)
Voltage Vg of the scanning line
13
is higher than signal voltage Vd of the signal line
14
and the voltage of the pixel electrode
17
during the selected small time interval when the TFT is turned on by voltage Vg of the scanning line
13
, and is lower than voltage Vd and the voltage of the pixel electrode
17
during the remaining time interval.
Voltage Vd of the signal line
14
changes at an interval of the horizontal scanning cycle to charge the pixel electrodes selected in succession to a desired voltag
Chowdhuri Tarifur R.
Duong Thoi V.
NEC LCD Technologies Ltd.
Sughrue & Mion, PLLC
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