Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Amorphous semiconductor material
Reexamination Certificate
1999-06-07
2001-06-26
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Amorphous semiconductor material
C257S072000, C257S350000
Reexamination Certificate
active
06252248
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a thin film transistor (TFT) having an insulating film and to a display having such a TFT as a switching element.
2. Description of the Prior Art
In recent years TFTs having a poly silicon film as an active layer have been developed as a driver element or a pixel driving element for use in various displays, such as an active-matrix liquid crystal display (LCD).
An LCD having a conventional TFT will be described below.
FIG. 1
is a plan view of a conventional TFT used in a display pixel section;
FIG. 2
is a cross-sectional view along E—E line in
FIG. 1
of an LCD using such a TFT.
As shown in
FIG. 1
, the TFT in the pixel section is formed near the intersection of gate signal line G for providing gate signals and drain signal line D for providing image signals, and its source is connected to a display electrode.
The structure of such a TFT will be described with reference to FIG.
2
.
A gate electrode
2
made of a refractory metal (high melting point metal), such as chromium (Cr) and molybdenum (Mo), a gate insulating film
3
, and an active layer
4
made of a poly silicon film are formed in that order on an insulating substrate
1
made of quartz glass, non-alkaline glass, or the like.
In the active layer
4
, a channel
7
is formed over the gate electrode
2
and a source
5
and a drain
6
are formed with the channel
7
between by ion doping with a stopper insulating film
8
on the channel
7
as a mask.
An interlayer insulating film
9
consisting of an SiO
2
film, an SiN film, and an SiO
2
film layered is formed over the whole of the gate insulating film
3
, active layer
4
, and stopper insulating film
8
. Furthermore, a drain electrode
10
is formed by filling a contact hole made in the interlayer insulating film
9
at a position corresponding to the drain
6
, with a metal, such as Al. In addition, a planarization film
12
made of, for example, an organic resin for planarization the surface is formed entirely over the interlayer insulating film
9
. A contact hole, at a position corresponding to the source
5
, is made in the planarization film
12
. Furthermore, a display electrode
13
, which is a transparent electrode and also serves as a source electrode, made of ITO (indium tin oxide) being in contact with the source
5
via this contact hole is formed on the planarization film
12
. An alignment film
14
made of an organic resin, such as polyimide, for aligning liquid crystal
24
is formed over the display electrode
13
.
The insulating substrate
1
on which the TFT is formed, and an opposite substrate
20
having an opposite electrode
21
and an alignment film
22
opposite to the insulating substrate
1
, are bonded together at their edges with a sealing adhesive
23
and a vacant space formed is filled with liquid crystal
24
.
In the structure of such a conventional TFT, however, impurities or impurity ions generated when the sealing adhesive hardens, water or impurity ions included in the liquid crystal
24
, water permeating from the outside through a gap
25
occurring when the sealing adhesive
23
comes off, water included in the air coming in contact with the planarization film
12
(alignment film
14
for a display portion in an LCD), or other substances become attached to the surface of the planarization film
12
, so that electric charge occurs on the surface of the planarization film
12
and polarization occurs between the top and the bottom of the planarization film
12
or each film of the interlayer insulating film
9
.
Therefore, a back channel is formed in a TFT, resulting in the disadvantage of a change in a threshold voltage of the TFT.
Furthermore, if such a TFT is used in an LCD, an increase in a threshold voltage of the TFT leads to a decrease in an ON-state current of the TFT and, conversely, a decrease in a threshold voltage of the TFT leads to an increase in an OFF-state current of the TFT. If such a TFT is used as a pixel switching element in, for example, a normally-white mode LCD, a bright point defect, in which some pixels remain permanantly bright, may occur in these cases. As a result, image display is of poor quality. In addition, if a threshold voltage varies from TFT to TFT, there exists the disadvantage of being unable to obtain display having uniform brightness all over.
SUMMARY OF THE INVENTION
The present invention was made to overcome the above conventional disadvantages. That is, an object of the present invention is to provide an LCD being capable of displaying images with uniform brightness all over by preventing polarization in a planarization film or an interlayer insulating film of a TFT, stabilizing a threshold voltage of the TFT, and improving the bright point defect and others.
A TFT according to the present invention is a thin film transistor in which a gate electrode, a gate insulating film, a semiconductor film having a channel, an interlayer insulating film, and a planarization insulation film are formed on an insulating substrate, a conductive layer is formed on one of the interlayer insulating film and the planarization insulation film over the channel, the width of the conductive layer along the length of the channel is narrower than the channel length, and the edge along the channel length of the conductive layer does not overlap with the edge along the channel length of the gate electrode or the edge along the channel length of the channel.
A TFT according to another aspect of the present invention is a thin film transistor in which a gate electrode, a gate insulating film, a semiconductor film having a channel, and an interlayer insulating film are formed on an insulating substrate, a conductive layer is formed opposite to the channel in the semiconductor film with, at least, the interlayer insulating film between, and the width of the conductive layer along the length of the channel is narrower than the channel length and than the width along the channel length of the gate electrode.
In a semiconductor device according to the present invention, a gate signal line, a data signal line, and a thin film transistor near the intersection of the gate signal line and the data signal line are formed on an insulating substrate. The thin film transistor comprises a plurality of gate electrodes extending from or electrically connected to the gate signal line, a gate insulating film, a semiconductor film extending so as to overlap with the plurality of gate electrodes and having a channel in each of overlaps with the plurality of gate electrodes, an interlayer insulating film, a planarization insulation film, and a conductive layer formed opposite to at least one of the channels with, at least, the interlayer insulating film between, and the width along the channel length of the conductive layer is narrower than the length of the at least one channel opposite to the conductive layer and narrower than the width along the channel length of the gate electrode corresponding to the channel.
According to another aspect of the present invention, the conductive layer in the above thin film transistor is connected to the gate electrode.
According to another aspect of the present invention, the potential of the conductive layer in the above thin film transistor is set to floating or constant potential.
According to another aspect of the present invention, in the above thin film transistor, an insulating film including the interlayer insulating film is formed between the conductive layer and the semiconductor film and the thickness of, at least, a region in the insulating film corresponding to the channel is 5,000 angstroms or more. In addition, the insulating film can be made of a single layer of a silicon oxide film, a silicon nitride film, or a film made of an organic material, or made of a laminate of any two or more of these films.
According to another aspect of the present invention, such a thin film transistor as has been described above is used in a display.
According to still another aspect of the present invention,
Sano Keiichi
Segawa Yasuo
Tabuchi Norio
Yamada Tsutomu
Cantor & Colburn LLP
Ngo Ngan V.
Sanyo Electric Co,. Ltd.
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