Transport of minority- carriers in graded-composition semiconductors and its impact on graded-band-gap base transistors operated at high currents

A comprehensive model for minority-carrier transport due to a quasi-electric-field in graded-composition semiconductor materials is developed including effects due to nondegenerate high-level injection of excess electrons and holes. For low-level injection, equivalent to a one-sun photoexcitation of uniformly doped direct- or indirect-band-gap material the total quasifield is given by the standard gradient of band-gap energy. High-level injection asymptotically reduces quasifield components due to band-gap and effective-mass gradients to lower values, and essentially eliminates components due to equilibrium- and excess-carrier concentration gradients. The impact of these results on performance of p-n-p and n-p-n graded-band-gap base transistors operated at high currents is examined.