Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1999-06-10
2001-10-30
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
Reexamination Certificate
active
06310668
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particular to a liquid crystal display device of an active matrix type, which uses a TFT (i.e., Thin Film Transistor) as a switching element of the device.
2. Description of the Related Art
In general, the liquid crystal display device is advantageous in that reduction of the device is possible in its thickness, weight and also in its power consumption. Consequently, in recent years, the liquid crystal display device is extensively used in information equipment, AV (i.e., Audio Visual) equipment and like equipment. Of these liquid crystal display devices, particularly, the liquid crystal display device of the active matrix type has a construction in which a plurality of pixels are driven by their corresponding switching elements. Further, of these active matrix type liquid crystal display devices, particularly, one using the TFT as its switching element is in principle not dependent upon the number of scanning cycles in sharpening an image being displayed. In addition to such sharp contrast, the active matrix type of liquid crystal display device having the TFT is advantageous in a high speed display mode. Consequently, the active matrix type of liquid crystal display device is advantageously used in large display screens and also in high definition motion display units.
Conventionally, the TFT has been produced so as to have a reversed stagger construction, in which a channel region is formed over a gate electrode by applying both a thin-film forming technology and a lithography technology in a transparent insulation substrate made of glass and like materials.
FIGS. 9 and 10
show in construction a plan view and a cross-sectional view of one of pixels of the conventional active matrix type of liquid crystal display device, respectively, wherein: the cross-sectional view is taken along the line C—C of
FIG. 9
; and, the active matrix type of liquid crystal display device is provided with the TFT having such reversed stagger construction, and is hereinafter referred to simply as the liquid crystal display device.
In a construction shown as in FIGS.
9
and
10
: a gate electrode
32
is formed on a transparent insulation substrate
31
; connected with this gate electrode
32
is a gate bus wiring
33
; the gate electrode
32
is covered with a gate insulation film
35
; a semiconductor layer
36
assuming a longitudinally elongated shape is formed in a portion of the gate insulation film
35
; and, each of a drain electrode
39
and a source electrode
40
is formed in each of opposite ends of this semiconductor layer
36
through a contact layer
37
. Formed in the source electrode
40
is a transparent pixel electrode
42
. Extending from the drain electrode
39
is a drain bus wiring
38
which extends in a direction substantially perpendicular to a longitudinal direction of the gate bus wiring
33
. Formed under the transparent pixel electrode
42
are an auxiliary capacitance wiring
44
and an auxiliary capacitance electrode
45
.
A TFT
50
having the reversed stagger construction is constructed of: the gate electrode
32
; a semiconductor layer
36
; a contact layer
37
; a drain electrode
39
; and, the source electrode
40
. In the TFT
50
, a channel region formed on a surface of the semiconductor layer
36
disposed over the gate electrode
32
is placed under the control of the gate electrode
32
. Further, all the semiconductor layer
36
, drain electrode
39
, source electrode
40
and the transparent pixel electrode
42
are covered with a protective film
43
.
On the other hand, a transparent insulation substrate
51
corresponding to the transparent insulation substrate
31
is formed. A portion of this transparent insulation substrate
51
is formed into a colored layer
52
. Further, a light shield layer
53
is formed in a position corresponding to that of the TFT
50
. A transparent common electrode
54
is so formed as to cover both the light shield layer
53
and the colored layer
52
.
In the above construction, a liquid crystal layer
55
is sealed in between: the transparent insulation substrate
31
on which the TFT
50
is formed; and, the transparent insulation substrate
51
formed over the transparent common electrode
54
, so that the conventional liquid crystal display device is formed.
In the conventional liquid crystal display device having the above construction, the gate bus wiring
33
is connected with scan lines (not shown) through which scanning signals are applied to the gate bus wiring
33
. On the other hand, connected with the drain bus wiring
38
are signal lines (not shown), to which video signals are applied to operate the liquid crystal display device.
Next, a method for driving the conventional liquid crystal display device will be described.
FIG. 7
shows a circuit equivalent in function to one of pixels of the above conventional liquid crystal display device, in which: the reference character C
GS
denotes a parasitic capacitance between the gate electrode
32
and the source electrode
40
; the reference character C
LC
denotes a capacitance of the liquid crystal; and, the reference character C
SC
denotes a holding capacitance.
Further,
FIG. 8
shows timing charts of various electric signals in the drive mode of the conventional liquid crystal display device.
In operation, in a condition in which a predetermined signal voltage V
D
is applied to the source electrode
40
from the signal lines, a fist row of the matrix is scanned at first, so that a gate voltage V
GON
serving as a selecting signal is applied to the gate electrode
32
through the gate bus wiring
33
, whereby all the TFTs of the pixels connected with the first row of the matrix are turned ON, which causes a potential level V
of the transparent pixel electrode
42
of each of the TFTs
50
to be equal to the signal voltage V
D
.
Next, when a second row is scanned, the gate voltage of the gate electrode
32
of each of the first row's pixels decreases to a voltage of V
GOFF
, which turns OFF the TFTs
50
of the first row's pixels. However, since each of the TFTs
50
functions as a memory element because of the presence of: the parasitic capacitance C
GS
between the gate and the source electrode; a liquid crystal capacitance C
LC
and the holding capacitance C
SC
, the above-mentioned potential level V
P1
of the transparent pixel electrode
42
of each of the TFTs
50
remains at the same potential level as that V
D
of the signal line. Consequently, the liquid crystal
55
has its molecules vary in their orientation on the basis of such potential level V
P1
. As a result, due to rotary polarization of the liquid crystal, each of the pixels of the display unit is modified in its transmission light, which enables to display an image on the display unit as a whole.
On the other hand, the potential level V
P1
of the transparent pixel electrode
42
is held at the same potential level as that of the signal potential level V
D
, but actually the potential level V
P1
of the transparent pixel electrode
42
reduces from the potential level V
D
by the amount of a potential of &Dgr;V under the influence of the presence of: the parasitic capacitance C
GS
; liquid crystal capacitance C
GL
and the holding capacitance C
SC
at a time when the TFT
50
is turned OFF, i.e., when the gate voltage is reduced from a voltage of V
GON
to a voltage of V
GOFF
. In the above, the potential of &Dgr;V is called the feed-through voltage which is given by the following equation (1):
&Dgr;
V=&Dgr;V
G
(
C
GS
/(
C
GS
+C
LC
+C
SC
)) (1)
Where: &Dgr;V
G
=(V
GON
−V
GOFF
)
As is clear from the above equation (1), the feed-through voltage &Dgr;V is proportional to the parasitic capacitance C
GS
between the gate and the source electrode. Consequently, it is preferable to reduce the parasitic capacitance C
GS
.
Here, as shown in
FIG. 8
, with respect to the signal potential level V
D
which is applied from
Dudek James A.
NEC Corporation
Schechter Andrew
Scully Scott Murphy & Presser
LandOfFree
LCD wherein opening in source electrode overlaps gate... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with LCD wherein opening in source electrode overlaps gate..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and LCD wherein opening in source electrode overlaps gate... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2569743