Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1990-08-01
1992-05-19
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
357 30, H01J 4014
Patent
active
051151235
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an improvement in a photosensor. More particularly, it relates to an improvement in the contact type photoelectric transducer which comprises amorphous silicon as the main component.
BACKGROUND ART
The conventional contact image sensors may be classified into two types: photoconductor type and photodiode type. In the photoconductor type image sensors, their photoconductivity is utilized and a current proportional to the voltage applied can be obtained. However, they are disadvantageous in that the response speed is slow. Accordingly, the photodiode type image sensors, in which a layer of amporphous silicon carbide or the like is used as a blocking layer and in which the rectifying effect of the diode is utilized so that carriers photoelectrically generated alone can afford the output current, have become a focus of attention.
However, while the photodiode type is advantageous in that the bright/dark current ratio is greater as compared with the photoconductor type, the photodiode type still has a problem to be solved. Thus, the photocurrent intensity is limited to a level lower than 1 when expressed in terms of quantum efficiency. Accordingly, charge accumulation and amplification, for instance, are required, presenting difficulties from the circuit viewpoint.
It is an object of the invention to provide a photosensor in which the dark current is slight and the bright current is fairly strong.
DISCLOSURE OF THE INVENTION
The invention provides a photosensor which comprises a first electrode layer, a light-transmitting second electrode layer, and a laminate structure disposed between the two electrode layers and comprising a first photoconductive layer, a blocking layer and a second photoconductive layer for charge injection as a result of modification of the electrical properties of the first photoconductive layer and of the blocking layer, the main component of the first photoconductive layer being silicon, the blocking layer being made of a substance having a broader band gap as compared with hydrogenated amorphous silicon and composed mainly of silicon and carbon or mainly of silicon and nitrogen or mainly of silicon and oxygen, and the main component of the second photoconductive layer being silicon and the silicon of the second photoconductive layer being doped with an element(s) of group III of the periodic table or an element(s) of group V of the periodic table as an impurity(ies). In this photosensor, the bright current can be increased while the dark current remains suppressed.
In the above-mentioned photosensor, the magnitude of charge injection may be controlled by controlling the impurity level in the second photoconductive layer within the range of 1 ppm to 10,000 ppm.
In the above-mentioned photosensor, the magnitude of charge injection may be controlled by adjusting the thickness of the above-mentioned second photoconductive layer to a value not less than 20 angstroms but smaller than 5,000 angstroms.
In the above-mentioned photosensor, the magnitude of charge injection may be controlled by controlling the band gap of the above-mentioned blocking layer within the range of 1.7 eV to 6 eV.
In the above-mentioned photosensor, the magnitude of charge injection may be controlled by adjusting the thickness of the blocking layer to a value not less than 20 angstroms but smaller than 5,000 angstroms.
In the above-mentioned photosensor, an impurity(ies) of group III or group V of the periodic table may be added to the above-mentioned blocking layer.
In the above-mentioned photosensor, the magnitude of charge injection may be controlled by controlling the impurity level in the blocking layer within the range of 0 ppm to 10,000 ppm.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows, in longitudinal cross section, the photosensor of a first embodiment of the invention;
FIG. 2 shows, in longitudinal cross section, the photosensor of a second embodiment of the invention;
FIG. 3 shows, in longitudinal cross section, the photosensor of a third embodiment of th
REFERENCES:
patent: 4348611 (1982-09-01), Ruppel et al.
patent: 4728997 (1988-03-01), Szydlo et al.
Hayashi Katsuhiko
Nakayama Takehisa
Yamaguchi Minori
Kanegafuchi Chemical Industry Co. Ltd.
Nelms David C.
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