Barrier and recombination effects in the base-emitter junction of heterostructure bipolar transistors

Abstract

We show that high current densities with minimal space charge region recombination current can be obtained by a suitable choice of alloy grading parameters and doping of the junction. It is also shown that compositionally graded, but uniformly doped emitter junctions require a minimum doping level in the emitter in order to suppress an excess barrier that results from alloy grading in the presence of high current injection. The complexity of recombination phenomenon at a heterojunction due to its dependence on doping, alloy grading, and bias is demonstrated and related to the injection behavior. We conclude that use of a minimum doping with parabolic alloy grading is an adequate compromise between high injection behavior and recombination behavior for practical devices.