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 Ndiscontinuous 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 r is shorter by a factor of iV 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 1 a 1 of the base transport factor a = 1 a 1 exp( --iwr) decreases so slowly with increasing frequency w that it becomes feasible to obtain an active behavior of the transistor above its own conventional cutoff frequencies.