Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1994-05-27
1996-03-19
Westin, Edward P.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
257292, H01L 27146
Patent
active
055005225
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a photoresponsive device based on Gallium Arsenide (GaAs) Integrated Circuit (IC) MESFET technology. In particular it relates to a Gallium Arsenide MESFET imager. Elements of the invention may also be implemented utilizing GaAs JFET technology.
BACKGROUND OF THE INVENTION
Gallium Arsenide (GaAs) has been recognized as being very useful as the semiconductor base for very high speed VLSI circuits. The use of GaAs in integrated circuits has a number of advantages over conventional silicon technology. These include: High substrate bulk resistivity providing isolation and minimizes parasitic capacitance; Increased speed; Increased temperature tolerance; reduced power dissipation; and Improved radiation hardness.
GaAs is also useful as a photodetector and it has been known to produce photodetectors from GaAs which are analogous to existing silicon photodiodes.
Solid state imagers utilizing silicon technology are well known. Two kinds of imagers exist at the present time, these am charge coupled devices (CCD) and XY arrays. XY array is a generic term which includes charge injection devices (CID), mosaic arrays, transistor arrays and others. The charge coupled device is a somewhat esoteric and expensive imager, but offers good resolution. This technology is currently expanding the video camera market. The second type of imager is the XY array which consists of a matrix of transistors that can be addressed either by decoders or shift registers. It is simpler and cheaper but has lower resolution. It finds application in the industrial market where cosmetic image quality is not so important.
Although some attempts have been made to produce CCDs utilizing GaAs technology these are at an early stage. Due to the complicated nature of CCD devices and the cost of producing such a device, this technology is still formative. Also CCDs are more sensitive to defects than XY arrays and due to the higher defect density in GaAs, XY arrays offer some advantage.
Attempts have not been made to develop an XY imager based upon Gallium Arsenide. There are a number of reasons for this based upon perceived limitations of GaAs technology. Firstly, research has concentrated on the high speed processing aspects of GaAs MESFET IC technology whereas an XY imager is a low frequency device that, at first sight, would seem to be inappropriate for utilizing high speed technology. Secondly, it would seem that the lower doping of the GaAs substrate compared to silicon would result in a longer carrier diffusion path leading to increased crosstalk between imager pixels. Experiments performed by the inventors has proved that this is not the case. Thirdly, it was considered that the small voltage levels in GaAs would lead to a poor signal-to-noise ratio. However, this problem is not fundamental and the inventors have found that it can be overcome by an appropriate approach to circuit design. Furthermore, the inventors have recognized the advantage that GaAs ICe have a lower wiring capacitance than equivalent silicon devices, this leads to a lower kTC noise at the output and therefore a better than expected signal-to-noise ratio.
It is an intended object of this invention to provide a photoresponsive device based on standard Gallium Arsenide IC technology so that the imager can be integrated on the same substrate as the high speed digital processing elements. It is a further intended object to at least provide the public with a useful alternative to existing silicon devices.
An imaging device utilizing a GaAs digital IC process has the advantage that both the photoresponsive elements and the processing circuitry can be formed on the same substrate using very large scale integration (VLSi) techniques. This offers the opportunity of utilizing the high speed characteristics of GaAs for signal processing and the optical characteristics for imaging. This leads to cheaper and more compact imaging systems.
The inventors have also discovered that the optical gain of a GaAs MESFET sharply increases in the reg
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W. C. Koscielniak, R. M. Kolbas, M. A. Littlejohn and B. W. Licznershi, "Photocurrent enhancement in a GaAs metal-semiconductor-metal-semiconductor-metal photodetector due to ultrasmall Au islands", pp. 987-989.
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Abbott Derek
Eshraghian Kamran
Lee John R.
Luminis Pty. Limited
Westin Edward P.
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