Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1996-09-23
2002-11-05
Christensen, Andrew B. (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C348S304000, C348S211130, C378S098800
Reexamination Certificate
active
06476867
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates a photoelectric conversion apparatus, a driving method therefor, and an X-ray system having the apparatus and, more particularly, to a one- or two-dimensional photoelectric conversion apparatus capable of performing a one-to-one read operation in a facsimile apparatus, a digital copying machine, an X-ray image pickup apparatus, or the like, a driving method for the apparatus, and a system having the apparatus.
2. Related Background Art
Conventionally, as the read system of a facsimile apparatus, a digital copying machine, an X-ray image pickup apparatus, or the like, a read system using a reducing optical system and a CCD type sensor has been used. With the recent development in photoelectric conversion semiconductor materials typified by hydrogenated amorphous silicon (to be referred to as a-Si hereinafter), there has been a remarkable development in a so-called contact type sensor, which is obtained by forming photoelectric conversion elements and a signal processing unit on a substrate having a large area, and designed to read an image of an information source through a one-to-one optical system. Since a-Si can be used not only as a photoelectric conversion material but also as a thin-film field effect transistor (to be referred to as a TFT hereinafter), a photoelectric conversion semiconductor layer and a TFT semiconductor layer can be formed at the same time.
FIGS. 20A
to
20
C show the structures of conventional optical sensors.
FIGS. 20A and 20B
show the layer structures of two types of optical sensors.
FIG. 20C
shows a typical driving method common to these sensors. Both the sensors in
FIGS. 20A and 20B
are photodiode type optical sensors. The sensor in
FIG. 20A
is of a PIN type. The sensor in
FIG. 20B
is of a Schottky type. The sensor in
FIG. 20A
includes an insulating substrate
1
, a lower electrode
2
, a p-type semiconductor layer (p-layer)
3
, an intrinsic semiconductor layer (i-layer)
4
, an n-type semiconductor layer (n-layer)
5
, and a transparent electrode
6
. The sensor in
FIG. 20B
includes the same components as those of the sensor in
FIG. 20A
except for the p-layer
3
. In the Schottky type sensor in
FIG. 20B
, a proper material is selected for the lower electrode
2
to form a Schottky barrier such that no electrons are injected from the lower electrode
2
into the i-layer
4
. The arrangement in
FIG. 20C
includes an optical sensor
10
as a symbol representing the above optical sensor, a power supply
11
, and a detection unit
12
such as a current amplifier. The direction indicated by “C” in the optical sensor
10
corresponds to the transparent electrode
6
side in
FIGS. 20A and 20B
, whereas the direction indicated by “A” corresponds to the lower electrode
2
side. The power supply
11
is set to apply a positive voltage to the “C” side opposing the “A” side. The operation of this arrangement will be briefly described. When light is incident from the direction indicated by the arrow in each of
FIGS. 20A and 20B
, and reaches the i-layer
4
, the light is absorbed to generate electrons and holes. Since an electric field has been applied from the power supply
11
to the i-layer
4
, the electrons pass through the “C” side, i.e., the n-layer
5
, and move to the transparent electrode
6
, while the holes move to the “A” side, i.e., the lower electrode
2
. That is, a photocurrent flows in the optical sensor
10
. When no light is incident, neither electrons nor holes are generated in the i-layer
4
. In addition, the n-layer
5
serves as a hole injection inhibiting layer, and the p-layer
3
in the PIN type sensor in FIG.
20
A and the Schottky barrier layer in the Schottky type sensor in
FIG. 20B
serve as electron injection inhibiting layers. For this reason, the holes in the transparent electrode
6
and the electrons in the lower electrode
2
cannot move, and hence no current flows. Therefore, a change in current occurs in accordance with the presence/absence of incident light. If this change is detected by the detection unit
12
in
FIG. 20C
, this arrangement operates as an optical sensor.
It is, however, difficult to manufacture a photoelectric conversion apparatus with a high S/N ratio at a low cost by using the above conventional optical sensor. The following are the reasons.
The first reason is that the PIN type sensor in FIG.
20
A and the Schottky type sensor in
FIG. 20B
each require injection inhibiting layers at two portions. In the PIN type sensor in
FIG. 20A
, the n-layer
5
as an injection inhibiting layer needs to have both the characteristic that guides electrons to the transparent electrode
6
and the characteristic that inhibits injection of holes into the i-layer
4
. Lack of one of the characteristics leads to a decrease in photocurrent, or generation of or an increase in a current (dark current) without incident light, resulting in a decrease in S/N ratio. This dark current itself is regarded as noise and includes a fluctuation called shot noise, i.e., quantum noise. Even if such a dark current is removed by the detection unit
12
, quantum noise accompanying the dark current cannot be reduced. In general, in order to improve the above characteristics, the conditions for formation of the i-layer
4
and the n-layer
5
and the conditions for annealing after formation of such layers must be optimized. However, the p-layer
3
which is another injection inhibiting layer needs to have the same characteristics as those described above, although the relationship between the electrons and holes is opposite to the above relationship. The respective conditions for the p-layer
3
must also be optimized. In general, the optimal conditions for the former n-layer are not the same as those for the latter p-layer, and it is difficult to satisfy the conditions for both the layers. That is, requiring injection inhibiting layers at two portions in a single optical sensor makes it difficult to form an optical sensor with a high S/N ratio. This applies to the Schottky type sensor in
FIG. 20B
as well. The Schottky type sensor in
FIG. 20B
uses a Schottky barrier layer as one injection inhibiting layer. This layer uses the difference in work function between the lower electrode
2
and the i-layer
4
. For this reason, materials for the lower electrode
2
are limited, and the localized level of the interface greatly influences the characteristics of the Schottky barrier layer, making it more difficult to satisfy the conditions for the layer. It is also reported that a thin silicon or metal oxide or nitride film having a thickness of about 100 Å is formed between the lower electrode
2
and the i-layer
4
to improve the characteristics of the Schottky barrier layer. This structure uses a tunnel effect to guide holes to the lower electrode
2
and improve the effect of inhibiting injection of electrons into the i-layer
4
. The structure also uses the difference in work function, and hence materials for the lower electrode
2
are limited. Furthermore, since the structure uses opposite properties, i.e., inhibition of injection of electrons and movement of holes by means of the tunnel effect, the oxide or nitride film is limited to a very thin film having a thickness of about 100 Å. Moreover, since it is difficult to control thickness and film quality, the productivity decreases.
Requiring injection inhibiting layers at two portions leads to low productivity and high cost for the following reason. These two injection inhibiting layers are important in terms of characteristics. If, therefore, a defect is produced in one of the two layers by dust or the like, characteristics necessary for an optical sensor cannot be obtained.
The second reason will be described with reference to FIG.
21
.
FIG. 21
shows the layer structure of a field effect transistor (TFT) made of thin semiconductor layers. A TFT is sometimes used as part of a control unit in forming a photoelectric conversion apparatus. The same reference numera
Endo Tadao
Kaifu Noriyuki
Kobayashi Isao
Morishita Masakazu
Takeda Shinichi
Christensen Andrew B.
Wilson Jacqueline
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