Photosensor and photosensor system

Details Photosensor and photosensor system Photosensor and photosensor system

2000-07-31

2003-12-30

Glick, Edward J. (Department: 2882)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C257S291000

Reexamination Certificate

active

06670595

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-218316, filed Aug. 2, 1999; No. 2000-107468, filed Apr. 10, 2000; No. 2000-122157, filed Apr. 24, 2000; and No. 2000-163303, filed May 31, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to a photosensor for sensing light or an image according to the sensed light and a photosensor system.
One known two-dimensional image reading device for reading printed matter, photographs, or fingerprints by very small irregularities in the finger has a photosensor array composed of photoelectric conversion elements (photosensors) arranged in a matrix. Generally, a solid-state imaging device, such as CCD (Charge-Coupled Device), made of single crystal silicon has been used as a photosensor array. Use of single crystal silicon causes the problem of increasing the manufacturing cost seriously.
It is well known that the CCD has a structure where photodiodes or photosensors are arranged in a matrix, causes a horizontal scanning circuit and a vertical scanning circuit to detect the charges generated according to the amount of light projected on the light-receiving section of each photosensor, and senses the luminance of the projected light. In a photosensor system using such a CCD, because select transistors for respectively bringing the scanned photosensors into the selected state have to be provided independently, an increase in the number of pixels causes the problem of making the overall system larger.
To overcome this problem, an attempt has been recently made to make the system smaller and reduce the manufacturing cost by applying a thin-film transistor with a so-called double-gate structure (hereinafter, referred to as a double-gate photosensor) to an image reading device. The double-gate photosensor is such that a photosensor has a photo sense function and a select transistor function.
The plane structure of a photosensor array composed of such double-gate photosensors PS is so designed, for example, as shown in
FIG. 30
that double-gate photosensors PS are arranged with a specific pitch of Psp in a lattice-like form (or in a matrix) in the directions of x and y crossing at right angles and that light from the insulating substrate (or glass substrate) side is projected through the element-to-element region Rp in the lattice onto the subject. Therefore, to project sufficient light on the subject to improve the light-receiving sensitivity, it is necessary to make the element-to-element region Rp as large as possible.
FIG. 31
is a sectional view showing the structure of a double-gate photosensor PS taken along line XXXI—XXXI in FIG.
30
. The double-gate photosensor PS comprises a semiconductor layer
1
where electron-hole pairs are generated by incident light, n
+
silicon layers
7
provided at both ends of the semiconductor layer
1
, a source electrode
2
and a drain electrode
3
which are formed on the n
+
silicon layers
7
and shut off light exciting the semiconductor layer
1
, a block insulating film
4
provided on the semiconductor layer
1
, an upper gate insulating film
5
covering the source electrode
2
and drain electrode
3
, a top gate electrode TG formed on the upper gate insulating film
5
, a lower gate insulating film
6
below the semiconductor layer
1
, a bottom gate electrode BG which is formed below the lower gate insulating film
6
and shuts off light exciting the semiconductor layer, and a transparent substrate
9
.
Specifically, the double-gate photosensor PS is such that a combination of two MOS transistors, of an upper MOS transistor composed of the semiconductor layer
1
, source electrode
2
, drain electrode
3
, and top gate electrode TG; and a lower MOS transistor composed of the semiconductor layer
1
, source electrode
2
, and drain electrode
3
, and bottom gate electrode BG, is formed on the transparent insulating substrate
9
, such as a glass substrate using the semiconductor layer as a common channel region.
Then, light h&ngr; emitted from above the double-gate photosensor PS advances in the direction of the arrow, passes through the top gate electrode TG and transparent insulating films
4
,
5
, and enters the semiconductor layer
1
. In the semiconductor layer
1
, electron-hole pairs are generated according to the amount of incident light. By sensing the voltage signal corresponding to the charges, the light-and-shade information on the subject is read.
A photosensor system applied to the aforementioned two-dimensional image reading device has the following problems.
(a) The semiconductor layer
1
in a double-gate photosensor PS is set on the basis of various dimensions determining the channel region, that is, of the ratio of the channel length L
0
to channel width W
0
in the semiconductor layer
1
. The channel length L
0
coincides with the length of the block insulating film in the direction of channel length.
The transistor characteristic of the double-gate photosensor PS is generally expressed by the following expression (1):
 
Ids∝W
0
/L
0
  (1)
where Ids is a source-drain current value.
The double-gate photosensor system recognizes an image by reading the voltage at the drain electrode
3
that varies with the drain current Ids flowing on the basis of the charges generated in the semiconductor layer
1
according to the amount of incident light. Therefore, to clearly recognize the image of the subject in a high contrast ratio, the difference between the drain current Ids of a double-gate photosensor PS positioned in a dark portion of the subject and the drain current Ids of a double-gate photosensor PS positioned in a bright portion of the subject has to be made larger. Since the source-drain current value Ids that determines the transistor sensitivity of the double-gate photosensor PS is determined on the basis of the ratio of the channel width W
0
to channel length L
0
in the semiconductor layer
1
, it is desirable from the viewpoint of improvement in the transistor sensitivity of the double-gate photosensor PS that the design value of the ratio W
0
/L
0
should be made as large as possible.
On the other hand, if the ratio W
0
/L
0
it set to be so that the double-gate photosensor PS is set to a high transistor sensitivity, the plane structure of the semiconductor layer
1
inevitably takes the form of a rectangular shape with a relatively large channel width W
0
and a relatively small channel length L
0
. Because the double-gate photosensor PS senses only the light caused to enter the semiconductor layer
1
, only the part not covered by the shade source electrode
2
and drain electrode
3
senses the light entering from above. As shown in
FIG. 30
, the area in which the light from the semiconductor layer
1
is allowed to enter takes a form of a near rectangle Ip
0
with the length of the shorter side being K
0
and the length of the longer side being about W
0
. Since the short-side length K
0
basically depends largely on the channel length L
0
, when the light entering the semiconductor layer
1
is perfect diffuse light or almost perfect diffuse light, the amount of light entering the semiconductor layer
1
in the direction of x is smaller than the amount of light entering the semiconductor layer
1
in the direction of Y, resulting in a noticeable deviation of the incident light in the direction in which it travels.
Specifically, in such a double-gate photosensor PS, because the area of the semiconductor layer
1
constituting the channel region which light is allowed to enter is designed to take the form of a single rectangle Ip
0
, the light transmitting area at the surface of a protective insulating film that a single double-gate photosensor PS can basically sense is a lengthwise area Ep
0
(the area shaded with slanted lines in
FIG. 30
) substantially similar in shape to a near rectangle Ip
0
, which narrows, in the sidewise direction (or the di

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