Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-07-19
2002-06-11
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S039000
Reexamination Certificate
active
06404474
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display device in a horizontal electric field driving scheme.
2. Description of the Related Art
Conventionally, common liquid crystal display devices have been of a type in which an electric field acting in a direction perpendicular to a substrate surface changes orientation of the director (molecular axis) of liquid crystal molecules, thereby controlling transmittance of light to achieve display of an image on a panel (hereinafter referred to as vertical electric field driving type). A TN (Twisted Nematic) mode is a representative mode of the vertical electric field driving type.
In liquid crystal display devices of the vertical electric field driving type, however, the director is oriented perpendicularly to the substrate surface at the application of an electric field. As a result, a refractive index varies depending on a viewing direction causing high dependency upon a viewing angle, so that display devices of this type are inappropriate for applications requiring a wide viewing angle.
To address this, in recent years, research and development have been advanced for liquid crystal display devices of a type in which the director of liquid crystal molecules is oriented parallel to a substrate surface and an electric field acts in a direction parallel to the substrate surface to rotate the director within a plane parallel to the substrate, thereby controlling transmittance of light to achieve display of an image (hereinafter referred to as horizontal electric field driving type). Since liquid crystal display devices of the horizontal electric field driving type exhibit substantially reduced variations in a refractive index depending on a viewing direction, display performance can be obtained with high image quality and a wide field of view.
A prior art active matrix liquid crystal display device of the horizontal electric field driving type will be hereinafter described with reference to FIG.
1
through FIG.
9
.
Referring to
FIG. 1
, there is shown a display pixel which comprises scanning line
502
for connection to an external driving circuit, signal line
103
, common electrode
106
, thin film transistor
503
serving as a switching element, and pixel electrode
104
.
As shown in
FIG. 2
, on TFT side glass substrate
102
, common electrode
106
is formed with pixel electrode
104
and signal line
103
formed thereon interposed by interlayer insulating film
130
. At the time of formation, pixel electrode
104
and common electrode
106
are disposed alternately. These electrodes are covered with protective insulating film
110
on which TFT side alignment film
120
required for aligning liquid crystal
107
is applied and subjected to a rubbing treatment. In this manner, TFT side substrate
100
is formed.
On opposite side glass substrate
101
, light shield film
203
is provided in matrix form on which color layer
142
required for displaying colors is formed. Additionally provided on color layer
142
is planarization film
202
for planarizing the surface of the opposite side substrate on which opposite side alignment film
122
required for aligning liquid crystal
107
is applied and subjected to the rubbing treatment. The direction of the rubbing treatment is opposite to that of TFT side substrate
100
. In this manner, opposite side substrate
200
is formed.
Liquid crystal
107
and spacer
302
are filled between TFT side substrate
100
and opposite side substrate
200
. The gap between both substrates is determined by the diameter of spacer
302
. Finally, TFT side polarizer
145
is stuck on the surface of TFT side glass substrate
102
which has no electric pattern formed thereon such that the transmission axis thereof is orthogonal to the rubbing direction. Opposite side polarizer
143
is also stuck on the surface of opposite side glass substrate
101
on which no patterns are formed such that the transmission axis thereof is orthogonal to the transmission axis direction of TFT side polarizing sheet
145
. With the aforementioned process, liquid crystal display panel
300
is completed.
Thereafter, liquid crystal display panel
300
is disposed above backlight
400
and connected to driving circuit
500
as shown in FIG.
3
.
Next, the operation of the liquid crystal display device will be described with reference to FIG.
4
and FIG.
5
.
FIG. 4
is a circuit diagram showing an equivalent circuit of the prior art liquid crystal display device, while
FIG. 5
is a graph showing waveforms of voltages applied to a scanning line, a signal line, and a common electrode, and a waveform of a pixel electrode voltage. It should be noted that Vfd in
FIG. 5
is referred to as a feedthrough voltage. The voltage applied to the common electrode is set such that &Dgr;V+ and &Dgr;V−, which represent amplitudes in positive and negative frames of the pixel electrode voltage when an amplitude of a video signal corresponds to a halftone, are equal to each other.
Description is made of the flow of electric charge in a unit element and light switching of the liquid crystal. An ON/OFF signal on scanning line
502
provided in the same layer as common electrode
106
in
FIG. 1
causes thin film transistor
503
to switch. When thin film transistor
503
is ON, electric charge flows from signal line
103
into pixel electrode
104
. A constant direct-current voltage is always applied to common electrode
106
as described with reference to FIG.
5
. In terms of an electric circuit, pixel electrode
104
and common electrode
106
form capacitances C
LC
, C
GL
, and C
SC
across liquid crystal
107
, TFT side glass substrate
102
, and interlayer insulating film
130
, respectively, as shown in FIG.
4
.
Thereafter, the charge is held by the capacitances even after thin film transistor
503
turns OFF. The held charge generates a potential difference between pixel electrode
104
and common electrode
106
to create a horizontal electric field parallel to the glass substrate which rotates the director of liquid crystal
107
to change retardation for liquid crystal display panel
300
. The changed retardation causes a change in panel transmittance of incident light emitted from backlight
400
shown in
FIG. 3
in portions which are not provided with light shield film
203
, pixel electrode
104
, common electrode
106
, scanning line
502
, and thin film transistor
503
.
FIG. 6
shows a relationship between a potential difference between the common electrode and the pixel electrode and the panel transmittance.
The aforementioned prior art liquid crystal display device suffers two disadvantages described below.
A first disadvantage is that the panel transmittance is lowered and uneven display is generated as a charge holding time is reduced. The reason thereof will be described in the following.
Specifically, in the aforementioned prior art liquid crystal display device, although it is desirable that electric charge held by capacitances C
LC
, C
GL
, and C
SC
is completely held when thin film transistor
503
turns OFF, the amount of the charge is actually diminished with a certain time constant in terms of an electric circuit. The time constant &tgr;off is approximately represented as equation (1):
&tgr;off≅Roff(C
LC
+C
GL
+C
SC
+C
GS
) (1)
where Roff represents a resistance of thin film transistor
503
at OFF and C
GS
represents a gate-source capacitance of thin film transistor
503
shown in FIG.
7
.
In a liquid crystal display device of the horizontal electric field scheme, C
LC
and C
GL
are smaller than those in a liquid crystal display device of the vertical electric field scheme since they are fringe capacitances. Roff is a constant value determined by a process limit of the thin film transistor and C
GS
is determined by a size of the thin film transistor, both of which have a low degree of flexibility. Additionally, since C
SC
corresponds to an overlapping portion between pixel electrode
Watanabe Makoto
Watanabe Takahiko
NEC Corporation
Schechter Andrew
Scully Scott Murphy & Presser
Ton Toan
LandOfFree
Horizontal electric field LCD with increased capacitance... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Horizontal electric field LCD with increased capacitance..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Horizontal electric field LCD with increased capacitance... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2923265