Scaling in Npn and Pnp heterostructure bipolar transistors

Calculating the cutoff frequency fT 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 Npn 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 fT of InP/GaInAs Pnp devices may even surpass that of Npn transistors.