The effect of base grading on the gain and high-frequency performance of AlGaAs/GaAs heterojunction bipolar transistors

A comprehensive one-dimensional analytical model of the graded-base Al_xGa_1-xAs/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 Schockley-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 fourfold. It is shown that the reduction in base transit time due to increased base grading leads to a 30% improvement in f_T in the most pronounced case of base grading studied (x=0.3 at the base-emitter boundary). The implications this has for improving f _max via increases in base width and base doping density are briefly examined