Heterostructure bipolar transistor with enhanced forward diffusion of minority carriers

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.