SCALING IN NPN AND PNP HETEROSTRUCTURE BIPOLAR-TRANSISTORS

Calculating the cutoff frequency f//T of bipolar transistors from the emitter-to-collector delay neglects the heavy influence of parasitic reactances on the frequency response of realistic transistors. A more complete equivalent circuit modelling reveals that the speed advantage of N pn against Pnp heterojunction bipolar transistors of common geometry, base width, and doping profile decreases as the transistor is scaled up in size. For power applications, the f//T of InP/GaInAs Pnp devices may even surpass that of N pn transistors.