Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-08-28
2003-08-05
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S298000, C349S038000, C349S043000
Reexamination Certificate
active
06603160
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to liquid crystal display devices, and more particularly to a liquid crystal display device having a MOS (metal-oxide-semiconductor) capacitor and a fabrication process thereof. Further, the present invention relates to such a MOS capacitor, a semiconductor device having such a MOS capacitor, and a fabrication process of those.
Conventionally, liquid crystal display devices have been used widely in portable information processing apparatuses such as so-called notebook computer, as a low-power-consuming compact information display device.
On the other hand, application of a liquid crystal display device is by no means limited to such a portable information processing apparatus. Today, liquid crystal display devices are used also in desk-top type information processing apparatuses as replacement of conventional CRT display device. Further, liquid crystal display devices are attractive as a display device of high-definition televisions (HDTV). Particularly, application to a projection-type HDTV display device is studied.
In the case of such high-performance, large-area liquid crystal display devices, a simple matrix driving construction used conventionally cannot provide satisfy the required specification in terms of response speed, contrast ratio, color purity, and the like. Thus, in such high-performance, large-area liquid crystal display devices, an active-matrix driving method is used in which each pixel is driven by a corresponding thin-film transistor (TFT). In the liquid crystal display device that employs an active-matrix driving method, it has been practiced to use an amorphous silicon liquid crystal display device in which amorphous silicon is used for the active region of the TFT. On the other hand, amorphous silicon has a drawback of small electron mobility and cannot satisfy the specification required for such a high-performance liquid crystal display device. Thus, there is a need of using a polysilicon TFT as the TFT of a high-performance liquid crystal display device.
Generally, a liquid crystal display device that uses the active matrix driving uses a capacitor for each TFT for retaining a driving voltage applied to a liquid crystal layer. Such a capacitor may be formed by a dielectric film sandwiched by a pair of metal electrodes similarly to ordinary capacitors. On the other hand, in view of the fact that the capacitor is used in cooperation with a highly miniaturized TFT, it is advantageous to construct the capacitor to have a so-called MOS structure.
BACKGROUND ART
FIG. 1
shows the general construction of a conventional active-matrix driven liquid crystal display device.
Referring to
FIG. 1
, the liquid crystal display device includes a TFT glass substrate
1
A carrying thereon a number of TFTs and transparent pixel electrodes cooperating thereto and an opposing glass substrate
1
B formed on the TFT substrate
1
A. Between the substrate
1
A and the substrate
1
B, a liquid crystal layer
1
is confined by means of a seal member
1
C. In the illustrated liquid crystal display device, the transparent pixel electrodes are selectively driven via a corresponding TFT and the orientation of liquid crystal molecules is changed selectively in the liquid crystal layer in correspondence to the selected pixel electrode. Further, polarizers not illustrated are disposed at respective outer sides of the glass substrates
1
A and
1
B. Further, a molecular alignment film not illustrated is formed on the inner sides of the glass substrates
1
A and
1
B in contact with the liquid crystal layer
1
. The molecular alignment film thereby restricts the orientation of the liquid crystal molecules.
FIG. 2
shows a part of the TFT glass substrate
1
A in an enlarged scale.
Referring to
FIG. 2
, the glass substrate
1
A carries thereon a number of pad electrodes
13
A, to which a scanning signal is supplied, and a number of scanning electrodes
13
extend therefrom, wherein the glass substrate
1
A further carries thereon a number of pad electrodes
12
A, to which a video signal is supplied, and a number of signal electrodes
12
extend therefrom. The scanning electrodes
13
and the signal electrodes
12
extend in such a manner that an elongating direction of a scanning electrode
13
intersects generally perpendicularly to an elongating direction of a signal electrode
12
. Further, TFTs
11
are formed at the intersections of the scanning electrodes
13
and the signal electrodes
12
. Further, the substrate
1
A carries transparent pixel electrodes
14
thereon such that a pixel electrode
14
corresponds to each of the TFTs
11
, and each TFT
11
is selected by a scanning signal on a corresponding scanning electrode
13
. Thereby, the selected TFT
11
drives the cooperating transparent pixel electrode
14
by a video signal on the corresponding signal electrode
12
.
FIG. 3
shows the construction of a liquid crystal cell driving circuit for driving one pixel of the liquid crystal display device of FIG.
2
.
Referring to
FIG. 3
, a number of liquid crystal cells
15
are formed in the liquid crystal layer
1
of
FIG. 1
in correspondence to the plurality of pixels, and it can be seen that a number of the TFTs
11
are formed on the TFT substrate, which corresponds with the glass substrate
1
A of
FIG. 1
, in a row and column formation in correspondence to the liquid crystal cells
15
. Further, it can be seen that the signal lines
12
supplying the video signals to the TFTs
11
extend on the TFT substrate
1
A in a column direction in a substantially parallel relationship with each other. Further, it can be seen that the gate electrodes (scanning electrodes)
13
controlling the TFTs
11
extend substantially parallel with each other. In the illustrated example, a TFT
11
is formed of a pair of serially connected TFTs
11
A and
11
B and drives the corresponding liquid crystal cell
15
via the pixel electrode
14
. Further, a capacitor
16
is connected to the TFT
11
parallel to the liquid crystal cell
15
. The capacitor
16
thereby constitutes an accumulating capacitance holding the driving voltage applied to the liquid crystal cell
15
. In the construction, the capacitor
16
is connected between the pixel electrode
14
and a capacitance line
17
.
As explained before, the accumulating capacitance
16
may be constructed by sandwiching a dielectric film between a pair of metal electrode patterns. In the case of an active-matrix driven liquid crystal display device, however, it is more advantageous to construct the same in the form of a MOS capacitor.
FIG. 4
shows the circuit construction of a conventional liquid display device that has such a MOS capacitor.
Referring to
FIG. 4
, the liquid crystal cell is formed of a glass substrate
10
A corresponding to the foregoing TFT substrate
1
A, a polysilicon pattern
10
B formed on the glass substrate
10
A, and an oxide film
10
C formed on the glass substrate
10
A so as to cover the polysilicon pattern
10
B. The TFT
11
is formed of n
+
-type diffusion regions
10
a
,
10
b
and
10
c
formed in the foregoing polysilicon pattern
10
B, a gate electrode
11
a
of Al or polysilicon formed on the oxide film
10
C between the foregoing diffusion regions
10
a
and
10
b
, and a gate electrode
11
b
formed similarly of Al or polysilicon on the oxide film
10
C between the diffusion regions
10
b
and
10
c
. It should be noted that the gate electrode
11
a
corresponds to the foregoing TFT
11
A and the gate electrode
11
b
corresponds to the foregoing TFT
11
B. Further, the oxide film
10
C constitutes a gate insulation film underneath the gate electrodes
11
a
and
11
b
. Further, the signal line
12
is connected to the diffusion region
10
a
and the gate control line
13
is connected to the gate electrodes
11
a
and
11
b.
In the construction of
FIG. 4
, it can be seen that the diffusion region
11
c
extends in the right direction in the drawing and forms an n
+
-type diffusion region
10
d
. Further, an electrode
11
c
of Al or p
Dickey Thomas L.
Fujitsu Display Technologies Corporation
Greer Burns & Crain Ltd.
Tran Minh Loan
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