Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1997-01-22
2001-08-21
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S042000, C349S043000
Reexamination Certificate
active
06278504
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film transistor for liquid crystal display device having a structure for suppressing the deterioration of characteristics caused by a light such as a back light and the like and a liquid crystal display device provided with the thin film transistor.
2. Description of the Related Art
FIG. 5
shows an example of constitution of an equivalent circuit of an active matrix liquid crystal display device using a thin film transistor as a switch element.
In
FIG. 5
, a circuit is arranged such that a multiplicity of scan electrode wires G
1
, G
2
, . . . , G
n
and a multiplicity of signal electrode wires S
1
, S
2
, . . . , S
m
are wired in a matrix state and each of the scan electrode wires G is connected to a scan circuit
1
and each of the signal electrode wires S is connected to a signal supply circuit
2
, respectively, thin film transistors (switch elements)
3
are disposed to the vicinities of the portions where the respective wires intersect and a capacitance unit
4
serving as a capacitor and a liquid crystal element
5
are connected to the drain of each of the thin film transistors
3
.
In the circuit shown in
FIG. 5
, the scan electrode wires G
1
, G
2
, . . . , G
n
are sequentially scanned to thereby turn of all the thin film transistors
3
on one scan electrode wire at the same time and a signal charge is accumulated from the signal supply circuit
2
to the capacitance units
4
which correspond to liquid crystal elements
5
to be displayed among the capacitance units
4
connected to the turned-on thin film transistors
3
through the signal electrode wires S
1
, S
2
, . . . , S
m
in synchronism with the above scan. Since the thus accumulated signal charge continuously excites the corresponding liquid crystal elements
5
until the next scan is carried out even if the thin film transistors
3
are turned off, the liquid crystal elements
5
are controlled by a control signal and displayed. That is, the above drive permits the respective liquid crystal elements
5
to be driven statically even if they driven by external drive circuits
1
,
2
on time sharing basis.
FIG.
6
and
FIG. 7
show an example of structure in which the portions such as the scan electrode wires G, the signal electrode wires and the like are actually disposed on a substrate in the conventional active matrix liquid crystal display device shown by the equivalent circuit in FIG.
5
.
In the active matrix display device shown in FIG.
6
and
FIG. 7
, the scan electrode wires G and the signal electrode wires S are wired on a transparent substrate
6
such as a glass or the like in a matrix state through insulation layers
9
disposed to the portions where the scan electrode wires G intersect the signal electrode wires S each other. Further, the thin film transistors are
3
disposed to the vicinities of the portions where the scan electrode wires G intersect the signal electrode wires S.
Since the thin film transistor
3
shown in FIG.
6
and
FIG. 7
has a most ordinary arrangement, it includes the insulation layer
9
disposed on a gate electrode
8
provided by being drawn from the scan electrode wire G, a semiconductor layer
10
composed of amorphous silicon (a-Si), polysilicon or the like and disposed on the insulation layer
9
, an etching stopper layer
7
disposed on the semiconductor layer
10
, and further a drain electrode
11
and a source electrode
12
each composed of a conductor such as aluminum or the like and disposed so as to confront each other. Note, the semiconductor layer
10
is arranged as a channel portion where a carrier moves and the thin film transistor
3
shown in
FIG. 7
is formed to a structure generally called an etch stopper type.
The drain electrode
11
is connected to a pixel electrode
15
formed on the substrate
6
through a contact hole
13
drilled to the insulation layer
9
as well as the source electrode
12
is connected to the signal electrode wire S. Ohmic contact layers
11
a
,
12
a
are formed under the drain electrode
11
and the source electrode
12
on the semiconductor layer
10
side thereof to establish an electric contact with the semiconductor layer
10
serving as the channel portion.
The active matrix liquid crystal display device is arranged such that a passivation layer
16
is disposed on the insulation layer
9
, the drain electrode
11
, the source electrode
12
and the like so as to cover them, an oriented film
17
is formed on the passivation layer
16
, a transparent confronting substrate
19
including an oriented film
18
is disposed above the oriented film
17
shown in FIG.
7
and further a liquid crystal
20
is sealed between the oriented films
17
,
18
. Therefore, the pixel electrode
15
can control the orientation of the molecules of the liquid crystal by applying an electric field thereto. Note, numeral
22
in
FIG. 7
denotes a black mask disposed to the confronting substrate
19
on the bottom surface side thereof so that it covers and conceals the portion other than the region where the pixel electrode
15
controls the orientation of the liquid crystal.
The liquid crystal display device having the above structure is usually arranged such that a polarization plate and a back light are disposed on the back side of the transparent substrate
6
as well as a polarization plate is also disposed on the back side of the confronting substrate
19
to permit a user to recognize a bright state and a dark state depending upon whether the orientation controlled liquid crystal
20
obstructs or pass the polarized state of a light emitted from the back light. However, when, for example, a light incident on the transparent substrate
6
from an oblique direction reaches the semiconductor layer
10
between the drain electrode
11
and the source electrode
12
as shown by the arrow L
1
of
FIG. 7
, a charge is made to the semiconductor layer
10
by being excited by the light and a photoelectric current flows. This phenomenon means that a leak current flows when the thin film transistor is driven regardless of that the circuit is turned off. Since the flow of the leak current increases an off-current when the liquid crystal is driven, there is a possibility that the light permeable characteristics of the liquid crystal are adversely affected by it.
Further, when a portion of a light incident on the transparent substrate
6
from an oblique direction is reflected by the black mask
22
and reaches the semiconductor layer
10
as shown by the arrow L
2
in
FIG. 7
, a charged is made to the semiconductor layer
10
by being excited by the light and a photoelectric current flows, thus there is a possibility that the light permeable characteristics of the liquid crystal are adversely affected by it likewise the above.
To solve the above problem, there is conventionally proposed a structure in which a source electrode
31
and a drain electrode
32
are disposed on a gate electrode
30
in confrontation with each other, the structure being arranged such that the gate electrode
30
is formed wider than a conventional one, both an etching stopper layer
33
and a semiconductor layer disposed thereunder are formed to a fallen-H shape and the etching stopper layer
33
is formed to such a width as to permit both the ends of the source electrode
31
and the drain electrode
32
to reach the half portions of the projections
33
a
of the etching stopper layer
33
, as a plan structure is shown in FIG.
8
A.
According to the structure of the example, since it is found that when the above photoelectric light flows, it flows through the side portion of the semiconductor layer located under the fallen-H-shaped projections
33
a
of the etching stopper layer
33
, the photoelectric current is suppressed by increasing the length of the conductor bus of the photoelectric current at the side portion in the structure of the example.
Further, as shown in
FIG. 8B
, there is proposed a structure in which a source electrode
36
and an L-
Brinks Hofer Gilson & Lione
LG. Philips LCD Co. Ltd.
Ngo Julie
Sikes William L.
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