Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-12-08
2001-05-29
Ngô ;, Ngâ ;n V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
Reexamination Certificate
active
06239468
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 1998-54096, filed on Dec. 10, 1998, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin film transistor (TFT) and, more particularly, to a sensor TFT used in an optical detecting sensor that can detect light reflected from an object.
2. Discussion of the Related Art
A thin film transistor type optical detecting sensor can be used as an image reader of an image detecting system such as a character recognition system, a fingerprint recognition system and a telecopy machine.
As shown in
FIG. 1A
, such a TFT type optical detecting sensor comprises a window through which light is transmitted, a sensor TFT (ST) for forming an optical current using light reflected from an object, a current charging part, or a storage capacitor (SC) for charging current flowing through the sensor TFT (ST), and a switching part, or a switch (SW) for selectively discharging the current charged in the current charging part (SC).
In operation, when light reflected from the object to be read is transmitted to an active area formed between drain and source electrodes of the sensor TFT (ST), an optical current flows along the active area. The optical current is then transmitted to an external circuit through the current charging part (SC) and the switching part (SW). At this point, the optical current corresponds to information on an image of the object. That is, the amount of the optical current varies according to the strength of the reflected light. In addition, the amount of the optical current further depends on the length and width of the active area to which the reflected light is introduced.
For example, when the length of the active area is fixed, the amount of the optical current is increased as the width of the active area is increased.
As is well known, as the amount of the optical current is increased, the image information becomes more accurate. Accordingly, by enlarging the width of the active area relative to the length, the amount of the optical current can be increased. However, when the width is increased, since the sensor TFT occupies much space, it is very difficult to improve the degree of integration of the sensor.
To solve the above problems, a method has been developed for increasing the current ratio as a function of light intensity by reducing an off current flowing along a semiconductor layer of the sensor TFT. To realize this, a second sensor gate electrode is provided between a first sensor gate electrode and a semiconductor layer.
FIG. 1B
shows a conventional sensor TFT.
The conventional sensor TFT comprises a first gate electrode
23
for performing an On/Off operation of a transistor by receiving a voltage from a gate wiring; second gate electrodes
27
a
and
27
b
disposed on the first gate electrode, the second gate electrodes
27
a
and
27
b
spaced away from each other in parallel; a semiconductor layer
31
formed on the second gate electrodes
27
a
and
27
b
; and source and drain electrodes
33
and
35
disposed on the second gate electrodes
27
a
and
27
b
, respectively.
An exposed portion of the semiconductor layer
31
between the source and drain electrodes
33
and
35
is an active area or conducting channel which has a length L and a width W. That is, the length L becomes a channel length of the semiconductor layer along which the optical current flows, and the width W becomes a channel width of the semiconductor layer.
FIG. 2
is a sectional view taken along line II—II of
FIG. 1B
for illustrating a manufacturing process of the sensor TFT.
A metal conductive layer is first deposited on a glass substrate
21
, then patterned into the first gate electrode
23
. A first insulating layer
25
is formed on the substrate, covering the first gate electrode
23
.
The second gate electrodes
27
a
and
27
b
are formed on the first insulating layer
25
, then a second insulating layer
29
is formed on the substrate while covering the second gate electrodes
27
a
and
27
b.
An amorphous silicon layer is deposited on the second insulating layer
29
, then patterned into the semiconductor layer
31
.
A contact hole
28
is formed on the second insulating layer
29
so that the drain electrode
35
can be electrically connected to the second gate electrode
27
a.
Next, the source and drain electrodes
33
and
35
are formed on the second insulating layer
29
while respectively covering both edges of the semiconductor layer
31
.
Finally, a protecting layer
37
is formed covering the semiconductor layer
31
, and the source and drain electrodes
33
and
35
.
In the above described sensor TFT, the first gate electrode
23
is always applied with a negative voltage as the sensor TFT operates with an optical current created by light in an Off state. The optical current created by the light reflected from an object flows along the semiconductor layer
31
. At this point, a hole is generated at a portion of the semiconductor layer
31
contacting the second insulating layer
29
by the negative voltage applied to the first gate electrode
23
. A current generated by the hole is called an Off current. In this state, when the light is radiated, electron-hole pairs are formed on the semiconductor layer
31
by the light energy.
The holes of the electron-hole pairs are directed to the source electrode
33
along the hole channel formed by a gate voltage, and the electrons are directed to the drain electrode
35
to produce optical current.
Since there is a limit to an amount of the optical current generated in a TFT having a predetermined ratio between the width and length of the channel, if the amount of the off-current is too much, the display quality will be not good.
Accordingly, to increase an optical current ratio by reducing the amount of the off-current, the second gate electrode
27
a
is provided between the semiconductor layer
31
and the first gate electrode
23
. That is, to apply a positive voltage to the source and drain electrodes
33
and
35
, the drain electrode
35
is connected to the second gate electrode
27
a
so that an equi-potential can be generated on the semiconductor layer
31
disposed between the drain electrode
35
and the second gate electrode
27
a
. The equi-potential characteristic suppresses the generation of holes, reducing the amount of the off-current flowing along the semiconductor layer.
However, in the above-described conventional sensor TFT, since an additional process for forming the second gate electrodes
27
a
and
27
b
is further required, the manufacturing process is complicated.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a thin film transistor type photo detecting sensor, a thin film transistor and a method for fabricating the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a sensor TFT that can increase the amount of the optical current flowing along the semiconductor layer without using the second gate electrodes.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve the above-need, the present invention provides a sensor TFT comprising a substrate, a gate electrode formed on the substrate; an insulating layer covering the substrate and gate electrode a semiconductor layer patterned on the insulating layer to generate an optical current using received light, source and drain electrodes formed on the semiconductor layer, the source and drain electrodes being spaced apart from each, and a conductive channel defined by a space between the source and drain e
Chang Youn Gyoung
Kim Jeong Hyun
Kim Se June
Lee Jae Kyun
Yi Jong Hoon
LG. Philips LCD Co. Ltd.
Long Aldridge & Norman LLP
Ngô ; Ngâ ;n V.
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