Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-10-12
2003-03-04
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S043000, C359S016000, C345S052000, C345S088000
Reexamination Certificate
active
06529257
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to active-matrix liquid-crystal display apparatuses, and more particularly, to the structure of opposed electrodes used in an active-matrix liquid-crystal display apparatus which is provided with a main display area and a sub display area having pixel zones different in size from each other.
2. Description of the Related Art
FIG. 6
shows a conventional active-matrix liquid-crystal display apparatus. More specifically,
FIG. 6
is a plan view of a thin film-transistor (hereinafter called TFT) array substrate
140
.
The TFT array substrate
140
of the conventional active-matrix liquid-crystal display apparatus is formed of a display area
130
where pixel zones
132
constituting pixels are disposed in a matrix manner, scanning-line leads
134
and scanning-line terminals
136
used for connecting scanning lines
101
disposed in the display area
130
to a gate-driver IC provided outside, and signal-line leads
135
and signal-line terminals
137
used for connecting signal lines
119
disposed in the display area
130
to a source-driver IC provided outside.
In addition to a display area
30
(hereinafter called a main display area), another display area
31
(hereinafter called a sub display area) has been proposed for displaying character information, for example, as shown in FIG.
1
. This increases the complexity of the conventional LCD apparatus, but adds functionality as well.
In this case, although the size of each pixel zone
32
is decreased because high resolution is required for the main display area
30
, it is not necessary for each pixel zone
33
to have the-same size as each pixel zone
32
in the main display area due to the display purpose of pixels in the sub display area. Rather, each pixel zone
33
in the sub display area
31
is designed such that it is larger than each pixel zone
32
in the main display area
30
since it is required that, for example, characters be displayed large for easy recognition.
In a conventional liquid-crystal display apparatus having a sub display area, one opposed electrode
113
common to the whole surface of an opposed substrate
141
is formed, as shown in FIG.
7
.
In each of active-matrix liquid-crystal display apparatuses, a liquid-crystal layer is sandwiched by a pair of substrates disposed oppositely and used as a display medium. An AC voltage on which a DC voltage is not superposed is applied to the liquid-crystal layer to prevent image sticking on the liquid-crystal layer. The AC voltage is used as a display voltage, and is applied to pixel electrodes mainly constituting pixel zones from signal lines through TFTs that have been turned on by gate voltages applied from scanning lines. A constant DC voltage is applied to an opposed electrode disposed oppositely to the pixel electrodes through the liquid-crystal layer. With this operation, an electric field is applied to the liquid-crystal layer to change its refractive index, and thus the liquid-crystal layer can be used as a display medium.
A dynamic voltage drop occurs in the potential Vp of the pixel electrodes when the gate voltages are changed in order to turn off the TFTs because the dielectric constant of the liquid crystal changes according to the electric-field strength, a parasitic capacitance is formed between the gate electrode and the drain electrode of each TFT, and a parasitic capacitance is formed between a scanning line and the pixel electrode.
FIG. 5
is an outlined view of driving voltages for the liquid-crystal display apparatus. In
FIG. 5
, (a) shows a voltage Vg applied to the gate electrode of a TFT, (b) shows a voltage Vs applied to the source electrode of the TFT, and (c) shows the voltage Vp of the drain electrode of the TFT, namely, the pixel electrode. In (c) of
FIG. 5
, Vsc indicates the center voltage of an AC voltage applied to the source electrode, and Vcom indicates a voltage applied to the opposed electrode. Since the voltages Vcom and Vp are applied to the opposed electrode and the pixel electrode, respectively, an effective potential is given to the liquid-crystal layer and the liquid-crystal layer operates as a display medium. The horizontal axis indicates time in
FIG. 5
to show the Vg, Vs, and Vp timing. The TFT is “on” while the voltage shown in (a) of
FIG. 5
is high, and the TFT is “off” while the voltage is low.
When the gate voltage Vg is changed in order to turn off the TFT, a dynamic voltage drop &Dgr;Vp occurs at the potential Vp of the pixel electrode as shown in (c) of FIG.
5
. This is because, when the gate voltage Vg is changed in order to turn off the TFT, charges are distributed among the capacitor formed by the liquid-crystal layer between the pair of substrates; a storage capacitor formed by a scanning line, and a gate insulating film and a capacitor electrode disposed thereabove; and the above-described parasitic capacitors to generate the voltage drop &Dgr;Vp at the potential Vp of the pixel electrode.
The voltage drop &Dgr;Vp generated at the potential of the pixel electrode
11
is shown by the following expression (1).
&Dgr;
Vp
=(
Vgh
×(
Cgd
on+
Cgp
)−
Vgl
×(
Cgd
off+
Cgp
)−
Vs
(
Cgd
on−
Cgd
off))/(
Cs+Clc+Cgd
off+
Cgp
) (1)
where,
&Dgr;Vp: Voltage drop at the potential of the pixel electrode
Vgh: High potential of the gate voltage
Cgdon: Parasitic capacitance obtained when the TFT is “on”
Cgp: Parasitic capacitance obtained between the scanning line and the pixel electrode
Vgl: Low potential of the gate voltage
Cgdoff: Parasitic capacitance obtained when the TFT is “off”
Vs: Potential of the signal voltage
Cs: Storage capacitance
Clc: Capacitance of the liquid-crystal layer
The factors which cause the voltage drop &Dgr;Vp at the potential of the pixel electrode includes the capacitance Clc of the liquid-crystal layer, the parasitic capacitance Cgd of the thin-film transistor, and the storage capacitance Cs, as shown in the expression (1).
The dielectric constant of the liquid crystal, one factor causing the voltage drop &Dgr;Vp, changes according to the electric-field strength. This change relates to the characteristics of the liquid crystal. In the two parasitic capacitances, that formed between the TFT gate electrode and the TFT drain electrode and that formed between a scanning line and the pixel electrode, which are other factors causing the voltage drop &Dgr;Vp, the parasitic capacitance formed between the TFT gate electrode and the TFT drain electrode is a capacitance generated by the gate insulating film formed between the electrodes, and originates from the structure of current active-matrix liquid-crystal display apparatuses.
When the voltage drop &Dgr;Vp occurs at the potential Vp of the pixel electrode as described above, the positive and negative voltage amplitudes of the potential Vp of the pixel electrode differ. When an identical-amplitude voltage is applied irrespective of its polarity, liquid crystal shows an identical transmittance. Therefore, in a normally-white active-matrix liquid-crystal display apparatus which has a high transmittance when a voltage is not applied, for example, the transmittance is lower at a polarity where the voltage amplitude is larger, and the transmittance is higher at a polarity where the voltage amplitude is smaller. Consequently, the repetition of brightness and darkness occurs according to the transmittances, and this pattern is seen as flicker.
When voltage amplitudes are not symmetrical for the positive and negative polarities, a DC voltage superposed on an AC voltage is always applied to any of pixel electrodes, and an image remains on the screen, which is so-called image sticking.
Therefore, flicker and image sticking are conventionally avoided by adequately adjusting the potential of the opposed electrode such that the voltage amplitudes of the AC voltage driving the liquid crystal are equal at the positive and negative sides and by forming storage capacitors in parallel to the capacitor gener
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Duong Thoi V
Sikes William L.
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