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UBC Theses and Dissertations

The effect of base grading on the gain and high frequency performance of AlGaAs/GaAs heterojunction bipolar transistors Ho, Simon Chak Man


A comprehensive, one-dimensional, analytical model of the graded-base AlGaAs/GaAs heterojunction bipolar transistor is presented, and used to examine the influence of base grading on the current gain and the high frequency performance of a device with a conventional pyramidal structure. Grading is achieved by varying the Al mole fraction x linearly across the base to a value of zero at the base-collector boundary. Recombination in the space-charge and neutral regions of the device is modeled by considering Shockley-Read-Hall, Auger and radiative processes. Owing to the different dependencies on base grading of the currents associated with these recombination mechanisms, the base current is minimized, and hence the gain reaches a maximum value, at a moderate level of base grading (x = 0.1 at the base-emitter boundary). The maximum improvement in gain, with respect to the ungraded base case, is about four-fold. It is shown that the reduction in base transit time due to increased base grading leads to a 60 % improvement in f[sub τ], in the most pronounced case of base grading studied (x = 0.3 at the base-emitter boundary). The implications this has for improving f[sub max] via increases in base width and base doping density are also examined. Finally, comparisons between predictions of the model and experimental data from fabricated devices reported in the literature are made.

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