Heterostructure bipolar transistor with enhanced forward diffusion of minority carriers

Abstract

We consider the minority transport in a heterostructure bipolar transistor whose base band gap narrows down toward the collector in N discontinuous steps. Assuming that the potential energy drop at each step is sufficiently large to prevent the reverse flow of minority carriers, we show that the total base propagation delay τ is shorter by a factor of N compared to the diffusive delay in a flat base of the same width. Moreover, if the length of each step is sufficiently narrow, then for large N the magnitude ‖α‖ of the base transport factor α=‖α‖exp(−iωτ) decreases so slowly with increasing frequency ω that it becomes feasible to obtain an active behavior of the transistor above its own conventional cutoff frequencies.