Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
2000-03-03
2004-04-20
Eckert, George (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S059000, C257S350000
Reexamination Certificate
active
06724011
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a display device in which a thin film transistor (hereinafter, abbreviated as “TFT”) is used as a switching element.
2. Description of the Related Art
Recently, a TFT in which a polycrystalline silicon film is used as an active layer is being developed as a driver element or a pixel driver element for various kinds of display devices such as an active-matrix liquid crystal display device (hereinafter, abbreviated as “LCD”).
Hereinafter, a reflection type LCD having conventional TFTs will be described. A reflection type liquid crystal display device is a display device in which light entering from the side of the viewer is reflected by a reflective display electrode so as to enable a display to be viewed.
FIG. 8
is a plan view of a TFT of a display pixel portion of the conventional art, and
FIG. 9
is a section view of an LCD using a TFT and taken along the line E—E in FIG.
8
.
As shown in
FIG. 8
, a TFT of a display pixel portion is disposed in the vicinity of an intersection of a gate signal line
51
through which a gate signal is supplied, and a drain signal line
52
through which a video signal is supplied. The source
13
s
of the TFT is connected to a reflective display electrode
19
. The reflective display electrode
19
is not formed on the TFT.
Referring to
FIG. 9
, the structures of the TFT and the LCD will be described.
Gate electrodes
11
made of a refractory metal such as chromium (Cr) or molybdenum (Mo), a gate insulating film
12
, and an active layer
13
configured by a poly-silicon film are sequentially formed on an insulative substrate
10
which is made of quartz glass, non-alkali glass, or the like.
In the active layer
13
, disposed are channels
13
c
above the gate electrodes
11
, and a source
13
s
and a drain
13
d
which are respectively formed on both sides of the channels
13
c
by ion implantation using stopper insulating films
14
on the channels
13
c
as a mask.
A interlayer insulating film
15
in which an SiO
2
film, an SiN film, and an SiO
2
film are laminated is formed over the whole face of the gate insulating film
12
, the active layer
13
, and the stopper insulating film
14
. A drain electrode
16
is formed by filling a metal such as Al into a contact hole which is opened correspondingly with the drain
13
d
. A planarizing insulating film
17
which is made of an organic resin or the like and which planarizes the surface is formed over the whole face. In the planarizing insulating film
17
and the interlayer insulating film
15
, a contact hole is opened at a position corresponding to the source
13
s
. A reflective display electrode
19
which is made of a reflective material such as Al and contacted with the source
13
s
via the contact hole is formed on the planarizing insulating film
17
. An alignment film
20
which is made of an organic resin such as polyimide and which is used for aligning a liquid crystal
21
is formed on the reflective display electrode
19
. At this time, the reflective display electrode
19
is not formed on the TFT.
The insulative substrate
10
having TFTs which have been produced as described above, and a counter electrode substrate
30
having a counter electrode
31
and an alignment film
32
which are opposed to the substrate
10
are bonded in periphery together by a sealing adhesive agent
23
. The liquid crystal
21
is filled into a gap formed between the substrates. Polarizing plates
33
are respectively applied to the outsides of the substrates
10
and
30
, thereby completing the LCD.
FIG. 10
shows characteristics of the TFT. The abscissa indicates the gate voltage Vgs, and the ordinate indicates the drain current Ids.
Since the reflective display electrode
19
does not exist above the channels of the TFT, the TFT exhibits characteristics in which, as indicated by the solid line in
FIG. 10
, the off-state current does not flow when the gate voltage Vgs is 0 V. By contrast, in the case where the reflective display electrode is formed so as to cover also the whole face of the TFT, the characteristics are changed as indicated by the broken line in FIG.
10
.
This is caused by the following reason. A voltage is applied to the reflective display electrode
19
disposed above the channels
13
c
. Charges are generated by an electric field due to the voltage, and so-called back channels are generated with respect to the channels
13
c.
When such a TFT is used in an LCD, the reflective display electrode may be elongated to extend above the TFT in order to improve the aperture ratio. This causes the threshold voltage to be changed in a decreasing direction, and hence the off-state current is increased as shown in FIG.
10
. As a result, there arise drawbacks that a bright defect in which a pixel is always bright, or an unlit defect in which a pixel is always unlit occurs and an excellent display cannot be obtained, and that the threshold voltages is varied among TFTs and a display which is uniform in brightness over the screen cannot be obtained.
The invention has been conducted in view of the drawbacks of the conventional art. It is an object of the invention to provide a display device in which a TFT is shielded against an electric field caused by a pixel electrode above the TFT to stabilize the threshold voltage of the TFT, thereby enabling a display in which defects such as a bright defect are reduced, brightness is uniform over the screen, and the aperture ratio is improved.
SUMMARY OF THE INVENTION
In the display device of the invention, for example, a thin film transistor is disposed on an insulative substrate, the thin film transistor comprises: a first gate electrode; a first insulating film; a semiconductor film which is formed on the first gate electrode and which has a channel; a second insulating film; and a second gate electrode which is formed on the second insulating film and at least above the channel, and which is connected to the first gate electrode, wherein a display electrode connected to a source which is formed in the semiconductor film is elongated to extend above the thin film transistor.
Namely a first aspect of the device is a display device in which a thin film transistor is disposed on an insulative substrate, said thin film transistor comprising: a first gate electrode; a gate insulating film; a semiconductor film which is formed on said gate insulating film and which has a channel; a insulating film; and a display electrode connected to a source which is formed in said semiconductor film is elongated so as to extend above said channel of said thin film transistor,
wherein a second gate electrode being at a constant potential, which is formed between said first gate electrode and said display electrode.
A second aspect of the device is a display device according to the first aspect, wherein said second gate electrode is connected with said first gate electrode.
A third aspect of the device is a display device according to the second aspect, wherein said second gate electrode is formed so as to be faced with said first gate electrode through said insulating film.
A fourth aspect of the device is a display device according to the third aspect, wherein said display electrode is rectangular.
A fifth aspect of the device is a display device according to the third aspect, wherein said channel is covered with a stopper insulating film.
A sixth aspect of the device is a display device according to the fifth aspect, wherein said stopper insulating film is made of an SiO
2
film.
A seventh aspect of the device is a display device according to the fifth aspect, wherein said stopper insulating film is made of a two-layered film of SiN and organic film.
An eighth aspect of the device is a display device according to the first aspect, wherein said first gate electrode is a double gate structured electrode divided above the channel.
A ninth aspect of the device is a display device according to the eighth aspect, wherein said second gate electrode is a double gate structured e
Noritake Kazuto
Sano Keiichi
Segawa Yasuo
Eckert George
Richards N. Drew
Sanyo Electric Co,. Ltd.
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