Theoretical analysis of heterojunction phototransistors

1 February 1972

journal article
Published by Institute of Electrical and Electronics Engineers (IEEE) in IEEE Transactions on Electron Devices

Vol. 19 (2) , 152-159
https://doi.org/10.1109/T-ED.1972.17391

Abstract

The optical gain and spectral response of heterojunction phototransistors with wide-gap emitters have been examined theoretically in idealized cases. The optical gain is found to be closely related to the current gain in the common-emitter configuration β of a heterojunction transistor. Because of Kroemer's factor, the injection efficiency of the emitter junction is very high, resulting in a high β or a large optical gain. However, β or the optical gain is limited by the base transport efficiency when the injection efficiency is extremely high. From the analyses, the optical gains and spectral responses of an n.GaAs-p. Ge-n.Ge heterojunction phototransistor and n-p-n homojunction phototransistors of GaAs and of Ge are numerically computed. It becomes evident that the heterojunction phototransistor has a higher optical gain and a wider spectral response (\sim5 \times 10^{5}at wavelengths ranging from 0.9 to 1.5 µm) than either of the homojunction phototransistors.

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