Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-11-30
2001-08-21
Jankus, Almis R. (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S087000, C345S094000
Reexamination Certificate
active
06278427
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an active matrix liquid-crystal display device intended to prevent the image-sticking defect that a residual image occurs when displaying another image after displaying the same image for a long time.
BACKGROUND OF THE INVENTION
Recently, an IPS (in-plane switching) mode (or horizontal electric field drive type) liquid-crystal display device, whose displaying is conducted by rotating the molecular axis direction (hereinafter referred to as ‘director’) of oriented liquid-crystal (hereinafter also referred to as ‘LC’) molecule in parallel direction to the substrate, has been researched and developed.
Such an IPS-mode LC display device does not have view-angle dependency to ‘standing direction’ of LC molecule because only the short-axis direction is constantly viewed even when shifting its viewpoint. Therefore, it can obtain a wide view angle, compared with an LC display device, such as conventional TN (twist nematic) mode, where electric field is generated in the perpendicular direction of substrates to sandwich an LC layer between the substrates (hereinafter referred to as ‘vertical electric field drive type).
In the research and development of the IPS-mode LC display device, various techniques developed in the vertical electric field drive type devices are diverted and applied. However, the techniques of vertical electric field drive type devices cannot be unalteredly diverted thereto, particularly, as to view-angle characteristics and reliability, by the following reasons.
For example, with reference to normally-black LC display device, the comparison between TN mode as an example of the vertical electric field drive type and IPS mode intended for this invention will be explained.
In TN mode, directors are generally twisted by 90° between two substrates in zero field, but they are existing in a plane parallel to the plane of substrate, like the case of IPS mode. However, in applying electric field, all directors are normally oriented in a plane parallel to the surface of substrate in IPS mode whereas all directors are normally oriented perpendicularly to the surface of substrate in TN mode.
Therefore, in IPS mode, the display appears to be white even when viewing from any viewpoint, but, in TN mode, it appears to be white or gray, as neutral color, depending on the viewing directions due to the refractive-index difference between the short axis and long axis of LC molecule. As understood from this, IPS mode and TN mode have no view-angle characteristic obtained resultantly in common, due to the difference in driving system.
Also, with respect to unit pixel composition, in TN mode, electrodes to compose a pixel are formed on two substrates, respectively because the electric field is generated perpendicularly to the plane of substrate, but, in IPS mode, all electrodes to compose a pixel are formed only on one substrate because the electric field is generated parallel to the plane of substrate. Namely, in TN mode, electric flux line to drive LC does not penetrate through the color layer of color filter, but, in IPS mode, electric flux line to drive LC penetrates through the color layer of color filter. Viewing from this, it is obvious that the degree of the influence of color layer of color filter to LC panel characteristics is different between TN mode and IPS mode.
From the differences described above, it is evident that the conventional techniques of TN cannot be unalteredly diverted to IPS.
FIGS. 1 and 2
are a cross sectional view and a top view, respectively, showing illustratively the basic composition of a conventional active matrix LC display device using horizontal electric field driving. Referring to
FIG. 1
, in this conventional LC display device, electrodes to compose a pixel electrode are formed only on one substrate
112
of two substrates sandwiching an LC layer
101
confined, on an opposing substrate
102
no electrode is formed and only a color filter to color light transmitted therethrough is formed.
Namely, on the electrode-forming substrate
112
, one active element (not shown in FIG.
1
), one drain signal electrode
103
, one gate signal electrode (not shown in FIG.
1
), and pairs of pixel electrodes (pixel electrodes
104
and common electrodes
105
) are disposed in unit pixel. All formed on the color filter forming substrate
102
are color filter layers (Red
109
, Green
110
, Blue
111
) to color light transmitted through LC into a specific color, generally red, green or blue, and a black matrix layer
108
to shield leakage light from the neighborhood of the drain signal electrode
103
on the electrode-forming substrate
112
or the gate signal electrode.
The color layers (R, G, B)
109
,
110
,
111
are formed considering the color purity and chromaticity level of light transmitted through panel. The color layers (R, G, B)
109
,
110
,
111
are produced by coloring an organic polymer material, such as polyvinyl alcohol or acryl resin, by using a dyestuff or pigment. Therefore, when producing it by, e.g., pigment scattering, the dielectric constants of the color layers (R, G, B)
109
,
110
,
111
vary, depending on the kind of pigment or the scattering density. Though the thickness H of color layer is set to be greater than 1 &mgr;m so as to enhance the color purity, the respective thicknesses of the color layers (R, G, B)
109
,
110
,
111
are also different each other because the respective transmissivities of the color layers (R, G, B)
109
,
110
,
111
are different each other. Accordingly, the color-layer static capacitance of color layer represented by the product of color-layer dielectric constant and color-layer thickness H is not constant in the color layers (R, G, B)
109
,
110
,
111
each.
In the arrangement of respective electrodes within unit pixel, the common electrode
105
is located just nearby the drain signal electrode
103
, the pixel electrodes
104
are disposed at certain intervals, the common electrode
105
and pixel electrode
104
are alternately disposed at equal intervals or unequal intervals, the common electrode
105
is further located in a layer covered with interlayer insulating film
106
nearby the substrate, different from the scanning signal electrode
103
and pixel electrode
104
(the common electrode
105
and pixel electrode
104
each are located in the separate layer).
Also, the active matrix LC display device uses AC drive to prevent the deterioration of panel members such as LC. For example, the polarity of signal is inverted every one field with a reference level (opposing electrode's level) at the center.
Also, in the active matrix LC display device, a drain voltage applied when the TFT element is turned on shifts by V
P
in the minus-potential direction of gate voltage when the TFT element is turned off, thereby causing a certain amount of potential fall. Namely, being up and down asymmetrical to the reference level and opposing electrode's level, resultantly V
P
(hereinafter referred to as ‘fieldthrough’) is applied to the LC drive voltage as a DC component. When the DC component is applied to the LC drive voltage, the accumulation of charge occurs, therefore causing an image-sticking defect etc.
Such a phenomenon that the DC component is applied to the LC drive voltage may occur in TN type where electric field is applied perpendicularly to the substrate. Its solution is disclosed in Japanese patent application laid-open No. 61-116392 (1986).
In this application, it is proposed that DC voltage applied to LC is corrected by adding a predetermined potential difference (V
T
) to a reference level of AC drive signal. Namely, V
P
−V
T
is given to be up and down symmetrical to the reference level.
However, LC capacitance C
LC
, which varies due to the orientation state of LC (degree of inclination of LC molecule to pixel electrode), generally differs in each pixel. The relationship between this capacitance C
LC
and a potential fall different &Dgr;V
P
of LC charging voltage when gate voltage is turned off is given by expre
Matsumoto Kimikazu
Nishida Shin-ichi
Shibahara Hideo
Jankus Almis R.
NEC Corporation
Sughrue Mion Zinn Macpeak & Seas, PLLC
Tran Henry N.
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