Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1998-11-04
2001-06-12
Pham, Long (Department: 2823)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S007000, C438S016000, C257S448000
Reexamination Certificate
active
06245601
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photoelectric converter, a method for driving the converter and a system thereof, for example, to a one-dimensional or two-dimensional photoelectric converter capable of reading data of a facsimile machine, a digital copying machine, an X-ray photographing device or the like, a method for driving the converter and a system thereof.
2. Related Background Art
Conventionally, a reading system using a reducing optical system and a CCD sensor has been used as a reading system for a facsimile machine, a digital copying machine, an X-ray photographing device or the like. In recent years, with the development of photoelectric conversion semiconductor materials represented by hydrogenated amorphous silicon (hereinafter represented as “a-Si”), the development of so-called closely laminated type sensors for reading data with an optical system of the same size as the information source by forming a photoelectric conversion element and a signal processing part on a large-area substrate is being targeted. In particular, since a-Si can be used not only as a photoelectric conversion material but also as a thin-film electric field effect type transistor (hereinafter described as a “TFT”), there is an advantage that a photoelectric conversion semiconductor layer and a semiconductor layer for the TFT can be formed at the same time.
FIGS. 1 and 2
are schematic sectional views showing one example of a structure of an optical sensor, respectively. FIG.
1
and
FIG. 2
are views schematically showing a structure of a layer of the optical sensor.
FIG. 3
is a view showing one example of a representative driving method which is common among optical sensors. FIG.
1
and
FIG. 2
are views both showing a photo-diode type optical sensor.
FIG. 1
is a view showing a so-called PIN type optical sensor while
FIG. 2
is a view showing a so-called Schottky type optical sensor. In
FIGS. 1 and 2
, reference numeral
1
denotes a substrate having at least an insulated surface,
2
a lower electrode,
3
a p-type semiconductor layer (hereinafter referred to as a “p layer”),
4
an intrinsic semiconductor layer (hereinafter referred to as an “i layer”),
5
an n-type semiconductor layer (hereinafter referred to as an “n layer”), and
6
a transparent electrode. In a Schottky type optical sensor shown in
FIG. 2
, the material of a lower electrode
2
is appropriately selected so that a Schottky barrier layer is formed to inhibit the injection of electrons from the lower electrode
2
into the i layer
4
. In
FIG. 3
, reference numeral
10
shows an optical sensor which is represented by symbolizing the above optical sensor. Reference numeral
11
denotes a power source. Reference numeral
12
denotes a detecting element of a current amplifier or the like. The direction shown by symbol C in the optical sensor
10
is directed toward the side of a transparent electrode
6
in
FIGS. 1 and 2
while the direction denoted by symbol A in the optical sensor
10
is directed to the side of the lower electrode
2
, and the power source
11
is set up so that a positive voltage is applied to side C with respect to side A.
Operation of the optical sensor will be briefly explained hereinafter. As shown in
FIGS. 1 and 2
, light is entered from the direction shown by an arrow. When the light reaches the i layer
4
, the light is absorbed and electrons and holes are generated. Since an electric field is applied to the i layer
4
by the power source
11
, electrons are moved to the side C, namely to the transparent electrode
6
via the n layer
5
while holes move to side A, namely, to the lower electrode
2
. Consequently, a photoelectric current flows through the optical sensor
10
. When light is not entered, electrons and holes are not generated in the i layer
4
. Furthermore, since an n layer
5
serves as a hole injection blocking layer, the p layer
3
of the PIN type shown in FIG.
1
and the Schottky barrier layer in the Schottky type shown in
FIG. 2
serves as an electron injection blocking layer, so that the holes in the transparent electrode
6
and the electrons in the lower electrode
2
cannot be moved, respectively and current does not flow. Consequently, the current is changed depending on the presence or absence of light entrance. When a change in current is detected by the detecting element
12
of
FIG. 3
, the optical sensor is operated.
However, it is not easy to actually produce at a low cost a photoelectric converter having a sufficient SN ratio using the conventional optical sensor. The reasons will be explained hereinafter.
A first reason is that both the PIN type shown in FIG.
1
and the Schottky type shown in
FIG. 2
require an injection blocking layer at two places.
In the PIN type sensor, the n layer
5
, which is an injection blocking layer, blocks the movement of electrons to the transparent electrode
6
. At the same time, a property to block the injection of holes into the i layer
4
is needed. Furthermore, in the Schottky type sensor, the Schottky barrier layer requires properties for blocking electrons from the lower electrode and for blocking holes from the n layer
5
. When either of these properties is lost, the photoelectric current is lowered, and current generated without incident light (hereinafter referred to as “dark current”) increases, which causes a decrease in the SN ratio. This dark current itself is considered to be noise. At the same time, a fluctuation referred to as shot noise, or so-called quantum noise, is included. Even when the detecting element
12
performs a process to subtract the dark current, the quantum noise that accompanies the dark current cannot be reduced. Normally, in order to improve this property, it is required that the film formation conditions for i layer
4
and n layer
5
and the annealing conditions after the preparation of the film be optimized. However, with respect to the p layer
3
, which is another injection blocking layer, the same property is required even though the electrons and holes are the opposite. In the same manner, optimization of each condition is required. Normally, the conditions for the optimization of the former n layer and for the optimization of the latter p layer are not the same. It is not easy to satisfy both conditions at the same time. In other words, injection blocking layers having two properties opposite to each other at two places are needed in the same optical sensor, making it difficult to form an optical sensor having a high SN ratio. This is applied to the Schottky type shown in FIG.
2
. In the Schottky type shown in
FIG. 2
, the Schottky barrier layer is used as one of the injection blocking layers. This utilizes the difference between the work functions of the lower electrode
2
and the i layer
4
. The material of the lower electrode
2
is restricted, and the influence of the localized level of the interface largely affects the properties. Consequently, it is not easy to satisfy all the conditions in an ideal manner. Furthermore, in order to improve the properties of the Schottky barrier layer, it has been reported that a thin oxide film of silicon or a metal or a nitride film having a thickness of about 100 angstroms or the like may be formed between the lower electrode
2
and the i layer
4
. This is intended to use a tunnel effect, introduce holes to the lower electrode
2
, and block the injection of electrons to the i layer
4
. The difference in work functions is also used here. For this purpose, limitation of the material of the lower electrode
2
is required. Furthermore, the thicknesses of the oxide film and nitride film are restricted to a very thin level of about 100 angstroms because of the use of opposite properties such as the block of the injection of electrons and hole movement caused by the tunnel effect. Furthermore, it is difficult to control the thickness or the film quality, and it is not easy to raise productivity.
In addition, the fact that an injection blocking layer is needed at
Kaifu Noriyuki
Kobayashi Isao
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Pham Long
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