High-gain and high-temperature applicable phototransistor...

Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation

Reexamination Certificate

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C257S076000, C257S077000

Reexamination Certificate

active

06225672

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Technology
The present invention relates to techniques of inserting each of the two low impurity layers (i-layer) into base-collector and base-emitter respectively for a monocrystal Silicon Carbide (SiC) phototransistor, particularly to the construction and fabrication process of the low impurity layer and the phototransistor.
2. Prior Art
As more and more specified electronic devices are requested to operate under severe conditions, the demand of high-gain solid state sensing elements workable at high temperature subsequently grows up time to time. Unfortunately, most part of the known high-gain photodetectors, such as avalanche photodiodes (APD), phototransistors (PT), etc are made by using narrow bandgap materials like monocrystal silicon, amorphous silicon, or group III-V compounds. When temperature is elevated, dark current of the mentioned elements will increase rapidly to thus deteriorate photosensitivity thereof, so that, the operating temperature is confined less than 100° C. Pallab Bhattacharya, Semicinductor Opto-electronic devices Englewood Cliffs, N.J.: Prentice Hall, 1944.
The wide bandgap semiconductor materials, such as Silicon Carbide (SiC), diamond, and gallium nitride (GaN), etc have been used to fabricate high temperature electronic elements lately Morkoc, S. Strite, G. B. Gao, M. E. Lin, B. S-verdlov, and M. Burns, “Largeband-gap SiC, III-V nitride, and II-VI ZnSe-based semiconductor device technologies,” J. Appl. phys., vol. 76, no. 3, pp. 1363-1398, wherein the relative techniques of SiC is the most matured and most compatible with silicon ICs in fabricating process. SiC do have several vantage features, such as wide bandgap, high electron mobility, and high thermal conductivity, etc L. Harris, Propertis of Silicon Carbide. London, United Kingdom: INSPEC, the institution of Electrical Engineers, 1995. However, for the time being, the developments of high temperature SiC phototransistors are curbed due to lack of high-gains. This weakness is supposed to be brought about from a drastic recombination of minority carrier in the base of a conventional n-p-n construction thus lower current gain of a transistor and reduce optical gain accordingly B. Casady and R. W. Johnson, “Status of Silicon Carbide (SiC) as a Wide-bandgap Seminconductor for High-Temperature Applications: A Review, ‘Solid-State Electronics, vol. 39, no. 10, pp. 1490-1422, 1996.
Aiming at the above-depicted defects, the present invention is to propose a newly developed construction and fabrication process for a high-gain monocrystal SiC phototransistor capable of operating at high temperature.
SUMMARY OF THE INVENTION
1. Objective of the Invention
The invention is intended to provide a high-gain phototransistor workable at high temperature. As the fabrication process is compatible with that of silicon semiconductor basically, it is advantageous for cutting down production cost and enhancing the relative techniques.
2. Description of Technology
In view of the fact that the present high-gain photodetectors made by silicon amorphous silicon or materials of group IV-V can only operate at temperature lower than 100° C., while Silicon Carbide (SiC) phototransistors of n-p-n type have a gain too poor to fit practical applications, hence, this invention has chosen a newly developed n-i-p-i-n structure for making monocrystal SiC phototransistor. This choice is also based on an exemplification made to point out that amorphous silicon n-i-p-i-n phototransistors have larger optical gain with lesser noise comparing with that of n-p-n type B.S. Wu, C. Y. Chang, Y. K. Fang, and R. H. Lee, “Amorphous Silicon Phototransistoron a Glass Substrate,” IEEE Trans. On Electron Devices, vol. ED-32, no. 11, pp. 2192-2196, 1985 and C. Y. Chang, J. W. Hong, and Y. K. Fang, “Amorphous Si/SiC phototransistors and avalanche photodiodes,” IEE Proceedings-J, vol. 138, no. 3, pp. 226-233, 1991.
The monocrystal SiC phototransistor, grown on a silicon substrate, is basically formed by a 5-layer structure sequentially comprising collector electrode (ITO)/collector (n-type monocrystal SiC)/i
1
low impurity layer (i-type monocrystal SiC)/base (p-type monocrystal SiC)/i
2
low impurity layer (i-type monocrystal SiC)/emitter (n-type monocrystal SiC) (refer to FIG.
1
). In addition, a silicon wafer is employed as the indispensable substrate, and a buffer layer added is to reduce effect of lattice mismatch between silicon and Silicon Carbide for obtaining better quality SiC film. It is clear that the present invention has two extra low impurity layers interfacing base-emitter as well as base-collector respectively than a conventional n-p-n phototransistor.
The element of the present invention is designed to thoroughly deplete the base and the two low impurity layers under zero bias, and a triangular potential barrier is formed between the collector and emitter (refer to FIG.
2
A). When bias V
CE
(V
CE
>0) and illumination is applied simultaneously, photo-induced electrons start moving toward collector, while holes move in the opposite direction. Some of the holes may accumulate in the barrier region and neutralize some negative space charges to lower down the barrier potential (as shown in
FIG. 2
b
). Consequently, a large optical gain may be thus obtained inasmuch as much more electrons can move over a lowered barrier to gather and form a relatively larger current. Because of the difference in conduction mechanism between n-i-p-i-n type and n-p-n phototransistor, though the minority carrier recombination may also occur in the former, the effect to optical gain is rather smaller comparing with result of lowered potential barrier. Therefore, a far larger optical gain may be obtained in a SiC n-i-p-i-n phototransistor than that of a SiC n-p-n type.
Furthermore, the lowered quantity of potential barrier in n-i-p-i-n type phototransistor B. S. Wu, C. Y. Chang, Y. K. Fang, and R. H. Lee, “Amorphous Silicon Phototransistoron a Glass Substract,” IEEE Trans. On Electron Device, vol. ED-32, no. 11, pp. 2192-2196, 1985 may be expressed as &Dgr;&phgr;
b
=(KT/q)ln(q&phgr;
L
J
d
) where &phgr;L is photo-carrier flux, J
d
is dark current.
From above formula, we understand that when temperature goes up, an increment in J
d
will inevitably cause a decrement in &Dgr;&phgr;
b
, however, a larger T will result in a greater &Dgr;&phgr;
b
, and this compensation function can reduce effect of temperature fluctuation to &Dgr;&phgr;
b
, and so to photocurrent. In other words, the optical gain of n-i-p-i-n type phototransistors is relatively temperature-independent. Consequently, by combination with high temperature characteristics of SiC, the SiC n-i-p-i-n type phototransistor can obtain a far greater optical gain than that of SiC n-p-n type.


REFERENCES:
patent: 4810662 (1989-03-01), Chang
patent: 5243216 (1993-09-01), Noguchi et al.
patent: 5311047 (1994-05-01), Chang
patent: 0 322 615 A1 (1989-07-01), None

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