A theoretical model of the heterostructure-emitter bipolar transistor (HEBT) emphasizing the effects of heterointerface recombination and emitter set-back layer thickness is developed. Further, the bandgap narrowing effect is taken into account. In addition the role of the magnitude of valence band offset and heterointerface recombination velocity is discussed. It is found that set-back layer thickness strongly affects d.c. performance and frequency response. When emitter set-back layer thickness equals the base-emitter junction depletion layer width, the recombination effect in the set-back layer and the diffusion capacitance will be reduced because of minimized carrier storage in the emitter, resulting in an optimum HEBT. It is also found that the heterointerface recombination current can limit the HEBT current gain if the trapping density at the heterointerface is larger than 1 × 1012 cm-2. For an optimum design, the current gain of a HEBT can rise to 1000, which is comparable to the best experimental data reported in the literature. Quite good agreement between our model's predictions and reported experimental results is achieved. The presented model can be applied to InGaP/GaAs and to AlGaAs/GaAs structures.
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