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
1998-07-17
2001-10-16
Jackson, Jr., Jerome (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
Reexamination Certificate
active
06303946
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin-film transistor substrate, a liquid crystal display unit provided with the same and a manufacturing method of the thin-film transistor substrate. More particularly, the invention relates to a technique capable of improving transistor properties by using a special structure for the connecting portion between source and drain electrodes and a semiconductor active film.
2. Description of the Related Art
FIGS. 6 and 7
illustrate a typical structure of a thin-film transistor array substrate provided with various parts such as a gate wiring G and a source wiring S on a substrate in a conventional thin-film transistor type liquid crystal display unit. In the thin-film transistor array substrate shown in
FIGS. 6 and 7
, the gate wiring G and the source wiring S are arranged in a matrix shape on a transparent substrate
6
such as a glass one. The region surrounded by the gate wiring G and the source wiring S serves as a picture element section
1
in which a thin-film transistor
3
is provided.
The thin-film transistor
3
shown in
FIGS. 6 and 7
has a general configuration of the etch-stopper type, and comprises a gate insulating film
9
provided on a gate wiring G and an gate electrode
8
provided by extracting from the gate wiring G; a semiconductor active film
10
comprising amorphous silicon (a-Si) provided on this gate insulating film
9
oppositely to the gate electrode
8
; and a drain electrode
11
and a source electrode
12
comprising a conductive material provided opposite to each other on this semiconductor active film
10
. On the upper sides of the both sides of the semiconductor active film
10
, there are formed ohmic contact films
10
a
and
10
a
made of amorphous silicon by doping impurities serving as donors such as phosphorus at high concentrations, and an etching stopper
13
held between the drain electrode
11
and the source electrode
12
is formed thereon. A transparent picture element electrode
15
comprising a transparent electrode material is connected so as to range from above the drain electrode
11
to a side of the drain electrode
11
.
A passivation film
16
is provided so as to cover the gate insulating film
9
, the transparent picture element electrode
15
, the drain electrode
11
and the source electrode
12
. An orientation film not shown is formed on the passivation film
16
. An active matrix liquid crystal display unit is formed by providing a liquid crystal above this orientation film, so that impression of an electric field to liquid crystal molecules by means of the transparent picture element electrode
15
permits orientation control of the liquid crystal molecules.
The liquid crystal display unit of the above-mentioned construction has a configuration in which a back light is provided on the back of the transparent substrate
6
, and the user can recognize bright or dark from whether the orientation-controlled liquid crystal interrupts or allows to transmit a light emitted from the back light.
However, when a part of the light entering the transparent substrate
6
should reach the semiconductor active film
10
between the drain electrode
11
and the source electrode
12
, a charge is produced in the semiconductor active film
10
through excitation by the light, causing photocurrent to flow. When driving the thin-film transistor, therefore, leakage current would flow although the circuit is turned off. The flow of such leakage current causes an increase in turnoff current (I
OFF
) during driving of the liquid crystal, and this may adversely affect light transmissivity of the liquid crystal.
For the purpose of avoiding this inconvenience, there is proposed a structure in which the gate electrode
8
is formed of a light shielding conductive film by forming the gate electrode
8
into a size larger than the semiconductor active film
10
so as to prevent the light of the back light from reaching the semiconductor active film
10
.
FIG. 8
illustrates a typical thin-film transistor structure of this kind: the thin-film transistor
27
comprises a gate electrode
21
made of a light-shielding conductive material provided on a substrate
20
; a gate insulating film
22
covering the gate electrode
21
; a semiconductor active film
23
smaller than the gate electrode
21
provided opposite to the gate electrode
21
on the gate insulating film
22
; ohmic contact films
24
and
24
provided on the both sides of the semiconductor active film
23
; a source electrode
25
covering one of the ohmic contact films
24
; and a drain electrode
26
covering the other ohmic contact film
24
.
With the structure shown in
FIG. 8
, in which the gate electrode
21
serves also as a light shielding layer, it is possible to prevent the light from the back light from entering the semiconductor active film
23
, and ensure satisfactory electrical contact of the source electrode
25
and the drain electrode
26
with the semiconductor active film
23
under the effect of the ohmic contact films
24
and
24
.
With the structure shown in
FIG. 8
, however, measurement of OFF-current (I
OFF
) and ON-current (I
ON
) as a thin-film transistor gives a curve as shown in
FIG. 10
, suggesting a problem of impossibility to achieve a sufficiently low OFF-current. As a result of search for causes carried out by the present inventors, this is attributable to the fact that, in the structure shown in
FIG. 8
, an end of the semiconductor active film
23
imparted a strong electric field facing the source electrode
25
is in direct contact with the source electrode
25
, or an end of the semiconductor active film
23
facing the drain electrode
26
is in direct contact with the drain electrode
26
, at portion e in
FIG. 8
, and this makes it impossible to obtain a sufficient Hall blocking effect.
FIG. 9
illustrates another example of conventional structure of thin-film transistor. The thin-film transistor
28
in this example comprises an ohmic contact film
29
provided so as to cover the end of the semiconductor active film
23
and the gate insulating film
22
on the side thereof, i.e., so as to be laminated over the source electrode
25
at the bottom of the source electrode
25
, and another ohmic contact film
29
provided so as to be laminated over the drain electrode
26
at the bottom of the drain electrode
26
.
In the structure shown in
FIG. 9
, however, measurement of OFF-current (I
OFF
) and ON-current (I
ON
) as a thin-film transistor results in a curve b as shown in FIG.
10
: while the value of OFF-current can be reduced sufficiently, the value of ON-current cannot be increased.
When manufacturing the thin-film transistor
28
of the structure shown in
FIG. 9
, the semiconductor active film
23
is once formed on the entire upper surface of the gate insulating film
22
to form the semiconductor active film
23
on the gate insulating film
22
. When patterning this film to achieve the semiconductor active film
23
having an island shape with a target size, the upper surface of the semiconductor active film
23
is susceptible to easy contamination, and even formation of the ohmic contact films
29
thereafter cannot ensure sufficient electric connection between the semiconductor active film
23
and the ohmic contact films
29
. This is considered to be a cause of the above inconvenience.
SUMMARY OF THE INVENTION
The present invention was developed in view of the above-mentioned circumstances, and has an object to provide a thin-film transistor and a liquid crystal display unit provided therewith in which a source electrode and a drain electrode are connected to a semiconductor active film so as not to cause mutual contact, via a low-resistance silicon compound film, ON-current being increased by improving electric connection of these components, and OFF-current is reduced by improving electric connection between the semiconductor active film and ohmic contact films. Another object of the invention is to provide a manufacturing method
Brinks Hofer Gilson & Lione
Jackson, Jr. Jerome
LG. Philips LCD Co. Ltd.
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