Current gain and transit-time effects in HBTs with graded emitter and base regions

Two-dimensional simulations of the combined effects of emitter and base grading on the current gain and cutoff frequency f/sub t/ of heterojunction bipolar transistor (HBT) devices are presented. At low bias, the highest current gain was found to be obtained with an abrupt emitter and reduced by base grading, with f/sub t/ proportional to the collector current. At high bias, current gain was found to be enhanced by emitter grading, while base grading was found to reduce current gain if without emitter grading. Anticipated grading effects of lower band spikes and base transit time are found to be greatly modified by the changes of carrier density, lifetime, diffusion potential, and series resistance with bandgap.<
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