Electric conversion device

A photoelectric conversion device using an amorphous material composed mainly of tetrahedral elements including at least an element of hydrogen and halogens as semiconductor material is disclosed. When a strong electric field is applied to a layer formed by using this amorphous semiconductor, a charge multiplication effect is produced mainly in the interior of the amorphous semiconductor and thus it is possible to obtain a thermally stable photoelectric conversion device having a high sensitivity while keeping a good photoresponse.

CROSS-REFERENCES TO RELATED APPLICATIONS 
This application relates to U.S. application Ser. No. 69156, filed July 2, 
1987, which is based on Japanese patent applications; No. 61-156317 filed 
July 4, 1986; No. 61-255671 filed Oct. 29, 1986; No. 61-255672 filed Oct. 
29, 1986; No. 61-278635 filed Nov. 25, 1986; No. 62-4865 filed Jan. 14, 
1987; No. 62-4867 filed Jan. 14, 1987; No. 62-4869 filed Jan. 14, 1987; 
No. 62-4871 filed Jan. 14, 1987; No. 62-4872 filed Jan. 14, 1987; No. 
62-4873 filed Jan. 14, 1987; No. 62-4875 filed Jan. 14, 1987; and No. 
62-149023 filed June 17, 1987. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a photoelectric conversion device for converting 
light into electric signal and in particular to a photoelectric conversion 
device having a high sensitivity, utilizing the charge multiplication 
effect. Such photoelectric conversion device includes e.g. a photocell, a 
one-dimensional image sensor, a two-dimensional image sensor, an image 
pick-up tube, etc. 
2. Description of the Related Arts 
Heretofore, as photoelectric conversion elements, whose principal component 
is an amorphous semiconductor, there are known a photocell, a 
one-dimensional image sensor (e.g. JP-A- No. 52-144992), a two-dimensional 
image sensor combining a solid state drive circuit with an amorphous 
semiconductor (e.g. JP-B- No. 59-26154), a photoconductive image pick-up 
tube (e.g. JP-A- No. 49-24619), etc. Some of these photoconversion devices 
adopt a blocking type structure having a junction characteristic of 
preventing charge injection from the signal electrodes to the 
photoconductive layer and some others adopt a structure, by which charge 
is injected from one or both of the electrodes, a so-called injection type 
structure. 
In an injection type element, since it is inherently possible to take-out 
charge carriers which are larger in number than the incident photons, a 
high sensitivity with a gain greater than 1 can be realized. In order to 
increase the sensitivity of the photoconversion element stated above, an 
imaging device has been proposed, in which a reading-out circuit and a 
photoconductive layer having e.g. phototransistor characteristics are 
overlaid upon each other (JP-A- No. 61-222383). 
In order to achieve a similar object, a method utilizing an electrostatic 
induction type transistor as a device having a multiplication effect in 
its photoelectric converting portion itself has been proposed (JP-A- No. 
57-21876) (IEEE Transactions on Electron Devices, Vol. ED 22, (1975) pages 
185-197). There has been proposed also a method, by which a p.sup.+ .pi. p 
n n.sup.+ structure is formed using an amorphous semiconductor, whose 
principal component is Si containing hydrogen and/or halogen (e.g. 
fluorine, chlorine, etc.), which structure is similar to that formed using 
crystalline Si, in which avalanche multiplication takes place in the 
depletion layer of its p-n junction portion in order to amplify signals. 
On the other hand, in the case where the blocking type structure having a 
characteristic of preventing charge injection from the exterior of the 
photoconductive layer is adopted, since only the portion of the incident 
light, which is converted into electric charge within the photoconductive 
layer, generates a signal current, the gain of the photoelectric 
conversion is always smaller than 1. However, it has been proposed by the 
inventors of this application that even a device of blocking type 
structure can have a photoelectric conversion efficiency greater than 1, 
if a method is adopted, by which a blocking type structure is formed by an 
amorphous semiconductor layer, whose principal component is Se and in 
which avalanche multiplication is made to occur in order to amplify 
signals (U.S. patent application Ser. No. 69156). 
As described above, when the injection type structure is adopted for a 
photoelectric conversion device such as a photocell, a one-dimensional 
image sensor, a photoconductive layer piled-up type solid state 
photosensitive device, etc., since it is inherently possible to take-out 
charge carriers larger in number than the incident photons, a high 
sensitivity with a gain greater than 1 can be realized. However, by this 
method, by which a part of electric charge is injected in the interior of 
the sensor, the photoresponse is significantly deteriorated. 
Further, in the case of the electrostatic induction type transistor, it was 
difficult to have uniform multiplication factors at a same value for 
different pixels, because an amplifying portion was integrated in each of 
the pixels. 
On the other hand, in the example in which an amorphous semiconductor is 
used, since it is possible to form a homogeneous layer at a relatively low 
temperature and in addition the layer has a high resistivity, advantages 
can be obtained that no complicated pixel separation process as for 
crystalline Si is needed to realize a high resolution characteristic. 
However, for a photosensitive element, to which the avalanche 
multiplication phenomena in amorphous semiconductor are applied, there 
still remain several problematical points. 
That is, by the method by which a p.sup.+ .pi. p n n.sup.+ structure 
identical to that adopted for an avalanche diode made of a crystalline 
semiconductor is formed using amorphous Si in order to amplify signals, a 
signal light is projected through the p.sup.+ region in the .pi. region, 
where it is absorbed and converted into electric charge, which is in turn 
led to the p-n junction portion, and the avalanche multiplication takes 
place in the depletion layer of the p-n junction portion. In order to 
cause the avalanche multiplication, it is necessary that electric charge 
travels over a distance longer than a certain value. The present inventors 
test-fabricated the structure stated above using amorphous Si, and 
confirmed that since localized states existing in the forbidden band were 
more numerous for amorphous Si than for crystalline Si, the depletion 
layer in the p-n junction portion did not satisfactorily extend, resulting 
in insufficient avalanche multiplication effect. Further, it was 
recognized that when the operating temperature exceeded room temperature, 
dark current was increased, and it was not possible to apply an electric 
field thereon, which was so high that a sufficient avalanche 
multiplication effect could be obtained. These results indicate that there 
was a problem that no satisfactorily high amplification factor could be 
obtained only by forming an avalanche diode structure similar to that in 
crystalline Si by using amorphous Si. 
Furthermore, in the case of the avalanche multiplication method using 
amorphous Se having a blocking contact structure (i.e., structure which 
blocks carrier injunction from the associated electrode), although a large 
multiplication factor and a good photoresponse can be obtained, because of 
restrictions due to the material itself, e.g. in a high temperature 
environment over 80.degree. C., there is a fear that the layer is altered 
during use and in particular there is a problem that element 
characteristics are unsatisfactory at high temperature operation. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a photoelectric conversion device 
having a good photoresponse, resolving the problematical points of the 
various techniques described previously. 
Another object of this invention is to provide a photoelectric conversion 
device made of amorphous semiconductor, which has a good thermal stability 
and whose photoelectric conversion efficiency is greater than 1. 
Still another object of this invention is to provide a photoelectric 
conversion device having low dark current. 
Still another object of this invention is to provide a photoelectric 
conversion device, for which a uniform photoelectric converting portion 
having a large area can be easily formed. 
Still another object of this invention is to provide a photoelectric 
conversion device, which can be fabricated by a simple process. 
In order to achieve these objects a photoelectric conversion device 
according to this invention is characterized in that it comprises a 
substrate; a first electrode formed on said substrate; a photoconductive 
layer for converting incident light into signal carriers, being formed on 
said first electrode, and having an amorphous semiconductor layer which is 
made mainly of at least one first element belonging to tetrahedral system 
and contains at least one second element selected from element group 
consisting of hydrogen and halogens; a means for applying electric field 
to said photoconductive layer, making said carriers run through said 
photoconductive layer and multiplying said carriers in said amorphous 
semiconductor layer. 
According to this invention it is possible to obtain a photoelectric 
conversion device having a high sensitivity with a photoconductive gain 
greater than 1 and good thermal stability without reducing the excellent 
photoresponse of a photosensitive element using a photoconductive layer of 
blocking type structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The inventors of this invention had already discovered that a charge 
multiplication takes place in an amorphous semiconductor layer made mainly 
of Se, when a strong electric field is applied to the amorphous 
semiconductor layer. Heretofore, it is generally thought that such 
phenomena hardly occur in an amorphous semiconductor layer itself, because 
there are a number of internal defects in the layer, and it has been 
believed that the amorphous Se is an exceptional material. 
This time, the inventors have found that the material, in which the 
phenomena described above takes place, are not restricted to amorphous Se, 
but the blocking contact structure can be formed by using a tetrahedral 
amorphous material stated above containing hydrogen or halogens and 
further that the charge multiplication can be produced also mainly in the 
interior of a tetrahedral amorphous semiconductor layer just as in Se by 
adopting the blocking type structure described above and driving it while 
applying a high voltage to an inner region of the amorphous layer having 
no junction depletion region, contrarily to the prior art method by which 
a p-n junction is formed in such an amorphous silicon and the avalanche 
multiplication is made to occur in the depletion region of the junction 
portion. As an element of the tetrahedral system, carbon, silicon, 
germanium and tin can be used. By using this method, by which the charge 
multiplication action is made to take place in an amorphous semiconductor 
layer while applying a strong electric field to the amorphous 
semiconductor layer, it is possible to obtain a photoelectric conversion 
device having a high sensitivity with a photoconductive gain greater than 
1 without reducing the excellent photoresponse of the photosensitive 
element having the blocking type structure. 
As a result of examining these phenomena more in detail, it has been found 
that the characteristics are not deteriorated even for the temperatures 
higher than 80.degree. C. and that the stability at the high temperature 
operation is especially excellent, if a material having a forbidden band 
width greater than 1.85 eV is used as the tetrahedral amorphous 
semiconductor material forming the blocking type structure described 
above. 
Further, it has been found that a satisfactory multiplication factor can be 
obtained, if the amorphous layer is about 0.5 to 10 .mu.m thick. 
FIG. 1A shows an example of the basic construction of a photoelectric 
conversion device for realizing this invention. A transparent substrate 
11; a transparent electrode 16, whose thickness is not greater than 300 
nm; a blocking layer 15 about 5 to 500 nm thick; a photoconductive layer 
14 about 0.5 to 10 .mu.m thick containing amorphous semiconductor having 
the charge multiplication effect; a blocking layer 13 about 5 to 500 nm 
thick, which prevents injection of charge carriers, whose polarity is 
opposite to that of those blocked by the blocking layer 15; a counter 
electrode 12 and a power source 18 are fundamental portions thereof. 
Reference numeral 17 denotes an incident light. However, in this 
construction, the blocking layer 13 or 15 may be omitted, if a 
satisfactory rectifying contact can be obtained between the 
photoconductive layer 14 and the electrode 12 or the electrode 16. 
To the photoelectric conversion device having the construction indicated in 
FIG. 1A an electric field necessary for realizing the avalanche 
multiplication in the amorphous semiconductor layer is applied by the 
power source 18. As the inventors of this invention have discovered, in a 
device according to this invention, in which the amorphous semiconductor 
layer (although it is not shown in the figure, it must constitute at least 
a part of the photoconductive layer 14) is made of a material belonging to 
the tetrahedral system containing hydrogen or halogens and forms a 
blocking type structure, it is possible to apply a high electric field 
over the whole amorphous semiconductor layer and further to keep dark 
current at a value, which is smaller than 1/100 of that in a crystalline 
semiconductor, inspite of its large area. 
In this state, when it is irradiated with light on the transparent 
electrode 16 side, incident light is absorbed within the amorphous 
semiconductor layer so as to generate electron-hole pairs, which travel in 
the directions determined by the polarity of the applied electric field, 
opposite to each other. Consequently, if the thickness of the amorphous 
semiconductor layer and the direction of the electric field are so set 
that charge carriers having a larger ionization ratio between 
photogenerated electrons and holes for the adopted amorphous semiconductor 
run through the amorphous semiconductor layer under the high electric 
field so that the charge multiplication is realized with a high 
efficiency, it is possible to obtain a device characteristic of operating 
stably with a high sensitivity with a photoconductive gain greater than 1 
even for the temperatures higher than 80.degree. C., maintaining the high 
speed photoresponse. For example, in the construction in FIG. 1A, it is 
mainly electrons that travel through the photoconductive layer 14 
(amorphous semiconductor layer). 
Furthermore, an amorphous semiconductor can be easily formed in a 
homogeneous and large thin layer and it is possible to deposit it on 
arbitrary substrate by a simple process. Consequently, from the view point 
that a uniform multiplication factor can be obtained, it can be understood 
that this invention is very useful. 
As the tetrahedral amorphous semiconductor material, which is preferable 
for realizing this invention, compounds composed mainly of Si can be 
cited. These compounds have a feature that their band gap can be varied by 
varying fabrication conditions or the composition ratio of Si and that 
they are excellent in the thermal stability property. 
Further, the charge multiplication phenomena are observed also in amorphous 
materials composed mainly of the compound of carbon and silicon (silicon 
carbide) containing hydrogen and/or halogens. The content of the added 
element of hydrogen or halogens is preferably 0.5-30 at. % and more 
preferably 5-20 at. %. This amorphous material is more excellent in the 
thermal stability with respect to Se and has in general a wider forbidden 
band and fewer thermally excited carriers with respect to amorphous Si. 
For these reasons, it has a smaller increase in dark current even at the 
high temperature operation. Further, with the material described above, 
since the forbidden band width can be easily varied by varying the 
composition ratio of carbon and silicon, it is possible to choose a 
material having the forbidden band width optimum for the utilization 
conditions and therefore the material is extremely useful. In this way, 
also with an amorphous semiconductor mainly composed of silicon carbide it 
is possible to resolve the problem at the high temperature operation, 
which is characteristic to a multiplication type imaging device using 
amorphous Se. 
The inventors of this invention have examined the avalanche multiplication 
in amorphous silicon carbide more in detail and found that amorphous 
silicon carbide with forbidden band width of 1.9 to 2.6 eV can be obtained 
for carbon composition ratio between 5 and 50%, and in such material, 
avalanche multiplication occurs efficiently, and that the thickness of the 
amorphous silicon carbide is preferably not less than 0.5 .mu.m in order 
to obtain a satisfactory multiplication factor. 
Further, a small amount of elements of V family such as P, As, etc. or 
elements of III family such as B, Al, etc. may be added to amorphous 
silicon carbide for obtaining an avalanche multiplication layer. But, in 
this case, it is desirable to keep the resistivity at the room temperature 
over 10.sup.10 .OMEGA.cm. 
Furthermore, the amorphous silicon carbide layer is not necessarily simply 
a homogeneous layer, but, in particular, in the case of a-SiC, a p-n 
junction may be formed therein, in order to have a structure, in which an 
avalanche multiplication can be produced efficiently in the junction 
portion, or the charge multiplication factor may be increased effectively 
by controlling the forbidden band width while varying the carbon 
concentration in the direction of the layer thickness. A device having a 
p-n junction in the amorphous silicon carbide layer is shown in FIG. 9. In 
this figure, 911 is a transparent substrate; 916 a transparent electrode, 
915 an electron blocking layer, 914a a p-type amorphous silicon carbide 
layer portion (doped with B), 914b an n-type amorphous silicon carbide 
layer portion (doped with P), 913 a hole blocking layer, an 912 and Al 
electrode. 
By the way, in the structures indicated in FIGS. 1A and 1B, in the case 
where the carrier blocking characteristic is intensified by disposing the 
blocking layers 13, 15, the layer described below is useful. 
That is, as the hole blocking layer, 113, amorphous silicon carbide or 
silicon nitride containing at least one of hydrogen and halogens, or n 
conductivity type amorphous silicon carbide or silicon nitride containing 
at least one of hydrogen and halogens and at least one of the element of V 
family such as P, As, etc., or oxide of at least one of Ce, Ge, Zn, Cd, 
Al, Si, Nb, Ta, Cr and W, or a combination of more than 2 of the layers 
described above is suitable. 
Further, as the electron blocking layer, 115, amorphous silicon carbide or 
silicon nitride containing at least one of hydrogen and halogens, or p 
conductivity type amorphous silicon carbide or silicon nitride containing 
at least one of hydrogen and halogens and at least one of the elements of 
III family such as B, Al, etc., or oxide of Ir, or at least one of 
calcogenides such as Sb.sub.2 S.sub.3, As.sub.2 S.sub.3, As.sub.2 
Se.sub.3, Se-As-Te, etc., or a combination of more than 2 of the layers 
described above is suitable. 
Here, for the silicon carbide used as the carrier blocking layer, 
contrarily to the silicon carbide used as the charge multiplication layer, 
the carbon content may be varied to a value greater than 50% depending on 
the sign of the carrier (i.e. electron or hole), whose injection should be 
blocked. 
Although a photoelectric conversion device using the charge multiplication 
effect in a tetrahedral amorphous semiconductor layer has been described 
above, for an amorphous semiconductor, contrarily to a crystal, since it 
is possible to overlay arbitrary different materials upon one another, the 
photoconductive layer may be constituted not by a single layer, but 
together with other thermally stable amorphous semiconductor layers 
overlaid thereon and having a similar charge multiplication. In addition, 
the whole photoconductive layer is not necessarily amorphous 
semiconductor, but it may have a construction, in which a crystalline 
semiconductor layer such as Si, etc. and an amorphous semiconductor layer 
are overlaid on each other. Further, it may be so constructed that it is 
deposited on a substrate including signal reading-out circuits, etc. What 
is essential to this invention is that there is disposed an amorphous 
semiconductor layer composed mainly of an element belonging to the 
tetrahedral system containing at least one of hydrogen and halogens as at 
least some of the layers constituting the photoconductive layer, and in 
which the charge multiplication is made to occur so as to enhance the 
sensitivity. Consequently, it is also possible that it is mainly the other 
layers in the photoconductive layer that have the function to absorb the 
incident light so as to produce photocarriers and that the amorphous 
semiconducting layer is used mainly for multiplying the carriers. In this 
case, there is a layer for generating photocarriers on the side, which is 
exposed to the incident light, in the photoconductive layer and the 
amorphous semiconductor layer may be made therebehind (in the direction of 
the propagation of the incident light). 
FIGS. 2 and 3 indicate effects obtained by realizing this invention. FIG. 2 
indicates the temperature dependence of the dark current and the 
photoconductive gain when two photosensitive elements (A) of this 
invention and (B) of the prior art are driven by applying such an electric 
field that an avalanche multiplication can occur. Photosensitive element 
(A) consists of the transparent electrode 112, the hole blocking layer 
113, the intrinsic amorphous silicon layer as photoconductive layer 114, 
the electron blocking layer 115 and Al as electrode 116 successively 
deposited on the transparent glass substrate 111, as indicated in FIG. 1B, 
and has an effective area of 1 cm.sup.2. Photosensitive element (B) has a 
p.sup.+ .pi. pn junction, and has an amorphous silicon layer disposed on a 
transparent electrode deposited on a transparent substrate and Al 
electrode deposited thereon, and has an effective area of 1 cm.sup.2. With 
the photosensitive element (B), the gain is insufficient in all 
temperature regions and the dark current increased significantly with 
increasing temperature, while both the gain and the dark current of the 
photosensitive element (A) have satisfactory behavior. 
FIG. 3 indicates the endurance of the element (A) described above and an 
element (C) driven continuously for 100 hours at 80.degree. C., the 
element (C) consisting of a transparent electrode, a hole blocking layer, 
an amorphous Se layer and an Au electrode successively deposited on a 
transparent substrate and having an effective area of 1 cm.sup.2. The 
endurance of the element (A) is remarkably improved with respect to that 
of the element (C). In the case where silicon carbide is used as the 
amorphous semiconductor, effects similar to those indicated in FIGS. 2 and 
3 can be obtained. 
Although, in the above, examples in which the avalanche effect in a 
thermally stable tetrahedral amorphous semiconductor is applied mainly to 
a photoelectric conversion device have been described, it is a matter of 
course that this invention can be applied as well to more general 
amplifying elements and switching elements apart from photoelectric 
conversion devices. 
Hereinbelow, this invention will be explained more in detail by using 
examples. 
EXAMPLE 1 
The example 1 will be explained referring to FIG. 1A. 
A transparent electrode 16 composed mainly of iridium oxide is formed on a 
transparent substrate 11. An a-Si:H having a layer thickness of 0.5-10 
.mu.m is formed thereon as a photoconductive layer 14 including amorphous 
semiconductor by the plasma CVD method by using SiH.sub.4 as source gas. 
Further, an a-SiC:H having a layer thickness of 10 nm, doped with P at 50 
ppm, is formed thereon as a hole blocking layer 13 by using SiH.sub.4 and 
C.sub.2 H.sub.6 as source gases and PH.sub.3 as doping gas. A 
photoelectric conversion device is obtained by depositing an Al as 
electrode 16 as a counter electrode further thereon. 
EXAMPLE 2 
FIGS. 4A and 4B indicate the construction of a one-dimensional image 
sensor, which is an embodiment of this invention. FIG. 4A is a plan view 
showing a part thereof and FIG. 4B is a cross-sectional view along the XX' 
line in FIG. 4A. 
A transparent conductive film composed mainly of iridium oxide is deposited 
on a transparent substrate 41. Then the conductive film is separated into 
a plurality of portions by the photoetching so as to form individual 
reading-out electrodes 46. An a-SiC:H having a layer thickness of 10 nm, 
doped with B at 50 ppm, is formed thereon as an electron blocking layer 45 
by means of a mask by using SiH.sub.4 and C.sub.2 H.sub.6 as source gases 
and B.sub.2 H.sub.6 as doping gas. An a-Si:H having a layer thickness of 
0.5-10 .mu.m is formed further thereon as a photoconductive layer 44 
containing amorphous semiconductor by means of the same mask by sputtering 
an Si target, using a mixed gas of Ar and H.sub.2. An a-SiC:H having a 
layer thickness of 10 nm, doped with P at 50 ppm, is formed further 
thereon as a hole blocking layer 43 by means of the same mask by using 
SiH.sub.4 and C.sub.2 H.sub.6 as source gases and PH.sub.3 as doping gas. 
Al is deposited further thereon as a common electrode 42 by means of a 
mask different from that described above. Thereafter, the reading-out 
electrodes 46 are connected to a scanning circuit disposed on the 
substrate by a method such as bonding, etc. so as to obtain a 
one-dimensional image-sensor. 
In the case where an electric field higher than 5.times.10.sup.7 V/m is 
applied to the photoelectric conversion device of the example 1 or 2 so 
that the transparent substrate 11, 41 side is negative with respect to the 
counter electrode 12, 42 and incident light 17 is projected to the 
transparent substrate 11, 41, a high sensitivity with a gain greater than 
1 can be realized without impairing the photoresponse. Further, even in 
the case where it is driven continuously for a long time at 80.degree. C., 
no degradations in the characteristics are caused. 
EXAMPLE 3 
FIG. 5 shows the construction of an image pick-up tube, which is another 
embodiment of this invention. A transparent electrode 52 composed mainly 
of In.sub.2 O.sub.3 is formed on a glass substrate 51. An a-Si:H having a 
layer thickness of 10 nm, doped with P at 50 ppm, is formed thereon as a 
hole blocking layer 53 by using PH.sub.4 as doping gas. Then an a-Si:H 
having a layer thickness of 0.5-10 .mu.m is deposited by the plasma CVD 
method by using SiH.sub.4 as source gas so as to obtain a photoconductive 
layer 54. Then Sb.sub.2 S.sub.3 is deposited to a layer thickness of 100 
nm as an electron blocking layer 55 in an Ar atmosphere of 13.3 Pa 
(10.sup.-1 Torr). A target portion 50 of an image pick-up tube can be 
obtained by those described above from 51 to 55. An image pick-up tube is 
obtained by mounting this target portion 50 in a glass tube 58 and 
evacuating the glass tube 58. 
EXAMPLE 4 
An example, in which this invention is applied to an image pick-up tube, 
similarly to Example 3, is shown. In this example also Ge is used as the 
element belonging to tetrahedral system. In FIG. 5, a transparent 
electrode 52 composed mainly of In.sub.2 O.sub.3 is formed on a glass 
substrate 51. An a-Si:H having a layer thickness of 10 nm, doped with P at 
50 ppm, is formed thereon as a hole blocking layer 53 by using PH.sub.4 as 
doping gas. Then an a-Ge:H having a layer thickness of 0.5-10 .mu.m is 
formed by the plasma CVD method by using GeH.sub.4 as source gas so as to 
obtain a photoconductive layer 54. Thereafter amorphous material composed 
of Se-As-Te is deposited to a layer thickness of 100 nm as an electron 
blocking layer 55 in an N.sub.2 atmosphere of 13.3 Pa (10.sup.-1 Torr). An 
image pick-up tube is obtained by using an image pick-up tube target 
portion 50 thus obtained in the same way as in Example 3. 
When an electric field higher than 8.times.10.sup.7 V/m is applied to the 
photoconductive layer in the image pick-up tube in Example 3 or 4 so that 
the transparent electrode 52 is positive, it is possible to realize a high 
sensitivity with a photoconductive gain greater than 1 without impairing 
the photoresponse. Further it is confirmed that its characteristics are 
thermally stable. In the figure reference numeral 19 indicates a load 
resistance. 
Now, several examples will be explained, in the case where the 
photoconductive layer composed of amorphous silicon carbide is formed as 
the amorphous semiconductor layer for charge multiplication. In the 
following examples amorphous silicon carbide (a-Si.sub.1-x C.sub.x :H) is 
formed by the plasma CVD method using SiH.sub.4, SiF.sub.4, etc. and 
CH.sub.4, C.sub.2 H.sub.6, C.sub.2 H.sub.4, etc. as source gases or by the 
reactive sputtering method of Si in a gaseous medium of H, Ar, CH.sub.4, 
etc. At this time, the concentration of carbon in the layer is controlled 
by regulating the flow rate of the source gas and the partial pressure of 
the gas of the atmosphere. Further, in the method described above, an n 
conductivity type a-SiC:H is obtained by adding a gaseous compound of an 
element of V family such as P, As, Sb, etc. and an p conductivity type 
a-SiC:H is obtained by adding a gaseous compound of an element of III 
family such as B, Al, etc. 
EXAMPLE 5 
Explanation will be made, referring to FIG. 1A. 
A transparent electrode 16 composed mainly of iridium oxide is formed on a 
transparent substrate 11. An a-Si.sub.70 C.sub.30 :H having a layer 
thickness of 0.5-10 .mu.m is formed thereon as a photoconductive layer 14 
including amorphous semiconductor. An a-Si.sub.50 C.sub.50 :H doped with P 
at 50 ppm is formed further thereon to a thickness of 10 nm so as to form 
a hole blocking layer 13. A photoelectric conversion device is obtained by 
depositing an Al electrode further thereon as a counter electrode 12. 
EXAMPLE 6 
FIG. 6 shows the schematical construction of a light sensitive element, 
which is an embodiment of this invention. This light sensitive element is 
formed by depositing successively a transparent electrode 62 composed 
mainly of indium oxide, an electron blocking layer 65, a photoconductive 
layer 64, a hole blocking layer 63 and an Au electrode 66 on a transparent 
substrate 61. The electron blocking layer 65 consists of 3 layers 651 made 
of a-Si.sub.60 C.sub.40 :H and 3 layers 652 made of a-Si.sub.70 C.sub.30 
:H doped with B at 10 ppm overlaid alternately on each other, each of the 
layers being 5 nm thick. The photoconductive layer 64 is a layer 2-8 .mu.m 
thick and made of a-Si.sub.80 C.sub.20 :H doped with P at 50 ppm. The hole 
blocking layer 63 consists of 4 layers 631 made of a-Si.sub.70 C.sub.30 :H 
doped with As at 50 ppm and 4 layers 632 made of a-Si.sub.50 C.sub.50 :H 
overlaid alternately on each other, each of the layers being 2.5 nm thick. 
As described above, it is possible to a obtain carrier blocking layer made 
of a-SiC:H, with effective wide band gap and effective p or n conductivity 
type, having good characteristics, and having an excellent carrier 
blocking ability if it consists of undoped a-SiC:H layers having a wide 
band gap and doped a-SiC:H layers having a narrow band gap. 
EXAMPLE 7 
Explanation will be made, referring to FIG. 8A. 
An electrode 82 composed mainly of Cr is formed on a semi-insulating 
semiconductor substrate 81. a-SiN:H is deposited thereon to a layer 
thickness of 10 nm as a hole blocking layer 83. Then a-Si.sub.80 C.sub.20 
:H is deposited thereon to a layer thickness of 0.5-10 .mu.m as a 
photoconductive layer 84. Thereafter a thin layer made of silicon oxide is 
deposited thereon to a layer thickness of 8 nm as an electron blocking 
layer 85. A photoelectric conversion device can be obtained by forming a 
transparent electrode 86 composed mainly of tin oxide further thereon. 
EXAMPLE 8 
FIG. 7A indicates a schematical construction of a light sensitive element, 
which is still another embodiment of this invention. The overall 
construction is identical to that indicated in FIG. 1B. This element 
consists of an electrode 72 composed mainly of Ta, a hole blocking layer 
73, an amorphous semiconductor layer 74, an electron blocking layer 75, 
and a transparent electrode 76 successively formed on an arbitrary 
substrate 71. The hole blocking layer 73 is made of CeO.sub.2 having a 
layer thickness of 10 nm and the electron blocking layer 75 is made of 
a-Si.sub.70 C.sub.30 :H doped with B at 100 ppm and having a layer 
thickness of 10 nm. The amorphous semiconductor layer 74 is a layer 2 
.mu.m thick made of a-SiC:H. At this time a first portion 741 which is 100 
nm thick is so formed that the C concentration in the layer decreases from 
35% to 10%. This operation being considered to be one period, the 
amorphous semiconductor layer 74 is completed by repeating similar 
operations (by about 20 periods). As a result, as indicated in FIG. 7B, 
the band gap varies from 2.3 eV to 2.0 eV with a period of 100 nm. In this 
case, at the discontinuous portions 77 of the band gap its value varies 
significantly. However, if the band gap is varied by varying the 
composition of the compound consisting of Si and carbon as in this 
example, almost all the difference of the band gap at the discontinuous 
portions 77 is attributable to displacement of the conduction band edge. 
Consequently, in the case where this element is used under the condition 
in which the transparent electrode is negatively biased, traveling 
electrons gain energy corresponding to the energy difference of the 
conduction band edge, when they pass through the discontinuous portions 77 
of the forbidden band width and thus it is possible to increase the 
effective electron multiplication factor with respect to that obtained in 
the case where the band gap is continuous. 
EXAMPLE 9 
Explanation will be made by referring to FIGS. 8A and 8B. 
An electrode 82 composed mainly of n.sup.+ conductivity type crystalline Si 
is formed on a semiinsulating semiconductor substrate 81. A hole blocking 
layer 83 made of a-Si.sub.70 C.sub.30 :H doped with P at 100 ppm is 
deposited thereon to a layer thickness of 5 nm. Then an amorphous 
semiconductor layer 84, 0.6-10 .mu.m thick and composed mainly of a-SiC:H 
and comprised of 841 and 842 is formed further thereon. At this time 
portion 842, where the band gap is small, is disposed within the amorphous 
semiconductor layer by controlling the C concentration in the layer. For 
example, the C concentration in the layer is so controlled that it is 30 
at. % in the first portion 841, 1.5 .mu.m thick; it is decreased from 30 
at. % to 0% in the succeeding portion 50 nm thick; it is constant at 0% in 
the succeeding portion 0.5 .mu.m thick; and finally it is increased 
continuously from 0% to 30 at. % in the succeeding portion 50 nm thick. As 
a result, the forbidden band width is so shaped that it is 2.2 eV at the 
portion 841 where the C concentration is 30 at. % and 1.8 eV at the 
portion 842 where the C concentration is 0%, as shown in FIG. 8B. 
Therefore, the forbidden band width has a shape, as principally conduction 
band edge is narrowed. After that, an electron blocking layer 85, 10 nm 
thick and made of a-Si.sub.70 C.sub.30 :H doped with B at 100 ppm is 
formed further thereon. A transparent electrode 86 composed mainly of tin 
oxide is formed further thereon so as to obtain a photoelectric conversion 
device. The efficiency of the photoelectric conversion is increased 
especially by light having long wavelengths for adopting this structure so 
that the portion 842 having a narrow band gap absorbs incident light 17 
with a high efficiency. In addition, in this way, since the charge 
generation layer and the charge multiplication layer are substantially 
separated, it is possible to suppress noise generation accompanied by the 
charge multiplication. 
When an electric field higher than 5.times.10.sup.7 V/m is applied to a 
photoelectric conversion device described in one of the Example 5 to 9, it 
is possible to realize a high sensitivity with a photoconductive gain 
greater than 1 without impairing the photoresponse. Further no variations 
in characteristics are caused, even if they are operated continuously for 
a long time at a temperature of 80.degree. C.