Browse Prior Art Database

Heterojunction Phototransistor

IP.com Disclosure Number: IPCOM000051882D
Original Publication Date: 1981-Mar-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 25K

Publishing Venue

IBM

Related People

Rogers, DL: AUTHOR

Abstract

High frequency response is improved in a heterojunction phototransistor by using a small area emitter and a high conductivity base.

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Heterojunction Phototransistor

High frequency response is improved in a heterojunction phototransistor by using a small area emitter and a high conductivity base.

Heterojunction phototransistors have found limited use in fiberoptic applications due to the need for simultaneous high speed response and low noise gain. With previous designs the emitter-base capacitance has been proportional to the active area, making it difficult to minimize emitter-base capacitance. With a large emitter-base capacitance it is necessary to operate the transistor at a relatively large bias current to achieve optimum frequency response. This bias current in turn increases the shot noise at the emitter and collector junctions.

The figure illustrates a heterojunction phototransistor constructed to minimize the emitter-base capacitance while at the same time retaining high speed performance. The device is constructed using steps similar to those ordinarily used to construct heterojunction phototransistors. Three epitaxial layers are grown on a suitable semiconductor substrate 10: a lightly doped collector layer 16, a heavily doped thin base layer 14, which may have a larger band gap than the collector, and a moderately or lightly doped emitter layer 12 with a larger band gap than the base. Additional layers could be grown to facilitate making contact to the three regions. The area of the emitter is reduced by using an etch that preferentially attacks the wide gap emitter material to create a small mesa onto which the emitt...