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UBC Theses and Dissertations

Modeling of double heterojunction bipolar transistors Ang, Oon Sim


A one-dimensional analytical model in the Ebers-Moll formulation of a graded base double heterojunction bipolar transistor (DHBT) is developed and used to examine the effects of base grading, the emitter-base barrier and the base-collector barrier on the d.c. current gain, offset voltage and the high frequency performance of a N — Al[formula omitted]Ga₁[formula omitted]As/p — Al[formula omitted]Ga₁[formula omitted]As/N — Al[formula omitted]Ga₁[formula omitted]As DHBTs. Recombination processes considered in the space charge regions and the neutral regions are: Shockley-Read-Hall, radiative and Auger. The trade-off between base-grading, which reduces the base current, and the neutral base recombination, which is brought about by varying the aluminium the junctions, results in an optimum aluminium mole fraction profile regarding the d.c. current gain. For high frequency performance, a similar trade-off to that of the d.c. situation exists. In this case, the important manifestation of the increased collector-base barrier height is an increase in the base transit time. The aluminium mole fraction profile which optimises the unity gain cut-off frequency, f[formula omitted], and the unity power gain cut-off frequency, f[formula omitted], is established. DHBTs which are symmetrical, both in aluminium mole fraction and doping concentration profiles, are shown to have low common-emitter offset voltages, V[formula omitted],[formula omitted]. Base-grading reduces V[formula omitted],[formula omitted] in devices in which the difference between the emitter and collector aluminium mole fraction is < 0.1; otherwise, V[formula omitted],[formula omitted] increases as base-grading increases. The model is also used to examine the performance of a N-Al[formula omitted]Ga₁[formula omitted]As/p-In[formula omitted]Ga₁[formula omitted]As/N-Al[formula omitted]Ga₁[formula omitted]As DHBT. It is shown that radiative and Auger recombination limit the d.c. current gain in this device.

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