Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-12-03
2001-06-05
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S059000, C257S072000, C257S292000, C257S294000, C257S296000, C257S350000
Reexamination Certificate
active
06242769
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 1998-53122, filed on Dec. 4, 1998, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical detecting sensor and, more particularly, to a thin film transistor (TFT) type optical detecting sensor.
2. Description of the Related Art
Generally, optical detecting sensors are used in facsimile and digital copying machines, and in fingerprint recognition systems as an image reader. In recent years, a TFT type optical detecting sensor has been suggested. The TFT changes its electrical characteristics in response to incident light. A TFT type optical detecting sensor is a system using such a TFT having such a characteristic.
FIG. 1
shows a plan view of a conventional TFT type optical detecting sensor and
FIG. 2
shows a sectional view taken along line II—II of FIG.
1
.
As shown in the drawings, an optical detecting sensor
100
comprises a window
8
through which light generated from a light source
102
passes and a sensor TFT
6
for generating optical current by detecting the light which is transmitted through the window
8
and then reflected from an object
12
.
Since the optical detecting sensor
100
is designed to detect the light passing through the window and reflected from the object
12
, it is essential that the window
8
has a sufficient light passing area. In addition, since a storage capacitor
4
for storing charges generated by the reflected light has to maintain a predetermined capacity, it is also essential to provide a sufficient storage area to the storage capacitor
4
.
As shown in
FIG. 1
, a pixel of the optical detecting sensor
100
is comprised of a storage capacitor
4
and a switching TFT
2
in addition to the window
8
and the sensor TFT
6
. Generally, an area of one pixel defined by A
1
*(B
1
+2B
1
′), where A
1
is defined by a sum of C
1
, D
1
, E
1
, and F
1
. Accordingly, an area of the window
8
can be defined by (B
1
+2B
1
′)*F
1
, and an area of the storage capacitor
4
can be defined by (B
1
+2B
1
′)*D
1
.
The optical detecting sensor
100
will be described more in detail with reference to FIG.
2
.
In
FIG. 2
, the switching TFT, the storage capacitor, the sensor TFT, and window are defined by regions
2
,
4
,
6
and
8
, respectively.
A first metal layer is formed on a substrate
1
. The first metal layer comprises a gate electrode
20
of the switching TFT
2
, a first storage electrode
30
of the storage capacitor
4
, and a gate electrode of the sensor TFT
6
. The first metal layer is made of a material selected from the group consisting of W, Mo, Cr and Al.
In addition, a gate insulating layer
14
is disposed on the first metal layer, and a semiconductor layer is deposited on the gate insulating layer
14
. The semiconductor layer is patterned such that semiconductor elements
26
and
46
are formed to act as an active layer of the switching TFT
2
and the sensor TFT
6
, respectively.
A second metal layer is deposited and patterned to form drain electrode
24
and source electrode
22
on the active layer of the switching TFT
2
, a second storage electrode
34
for the storage capacitor
4
, and drain electrode
44
and source electrode
42
on the active layer of the sensor TFT
6
.
An insulating layer
16
is formed to protect the switching TFT
2
, the storage capacitor
4
and the sensor TFT
6
. A light interrupting layer
18
is formed on a portion of the protecting insulating layer
16
corresponding to the switching TFT
2
to block light scattered from the object
12
, and a protecting layer
10
is deposited on the insulating layer
16
and covers the light interrupting layer
18
.
Generally, an active layer
46
of the sensor TFT
6
is made of a-Si:H which has a low dark conductivity and a high optical conductivity.
Since the sensor TFT
6
is operated by optical current in accordance with the intensity of incident light in an off-state, negative voltage is always applied to the gate electrode
40
to maintain the off-state. An optical current is generated in proportion to the intensity of the incident light, and is directed to the second storage electrode
34
of the storage capacitor
4
through the source electrode
42
, and then stored in the storage capacitor
4
as charges.
Furthermore, when a bias voltage is applied to the gate electrode
20
of the switching TFT, the charges stored in the storage capacitor
4
are conducted to the source electrode
22
through the drain electrode
24
of the switching TFT
2
.
In the above-described sensor, in order to receive more light, the area of the window
8
should be maximized.
It is also necessary to increase the capacity of the storage capacitor in order to store more charges.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to thin film transistor (TFT) type photo sensor that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
Therefore, it is an object of the present invention to provide a TFT type optical detecting sensor in which the areas of a storage capacitor and a window are maximized, thereby improving the signal to noise ratio (S/N).
To achieve the above object, the present invention provides a TFT type optical detecting sensor which can read an image of an object by reflected light from the object, comprising a light source, a sensor TFT for generating optical current by detecting light reflected from the object, a storage capacitor for transmitting light from the light source to the object and for storing charges of the optical current, and a switching TFT for controlling the release of the charges stored in the storage capacitor, wherein the storage capacitor is made of a transparent conductive material.
Preferably, the transparent conductive material is selected from the group consisting of ITO, TiO and SnO
2
.
The sensor TFT is preferably disposed on a central portion of the storage capacitor.
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.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
REFERENCES:
patent: 5998841 (1999-12-01), Suzawa
Chang Youn Gyoung
Kim Jeong Hyun
Kim Se June
Lee Jae Kyun
Yi Jong Hoon
L. G. Philips LCD Co., Ltd.
Long Aldridge & Norman LLP
Ngo Ngan V.
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