Emitter-base bias dependence of the collector current ideality factor in abrupt Pnp AlGaAs/GaAs heterojunction bipolar transistors

Starting with the 4x4 Luttinger-Kohn Hamiltonian and making use of the axial approximation, we calculate the emitter current as a function of the applied forward emitter-base bias for a typical Pnp AlGaAs/GaAs single heterojunction bipolar transistor (HBT). While including the effects of emitter series resistance and recombination in the quasi-neutral base and emitter-base space-charge region, we then calculate the collector current density versus emitter to base bias and find it to be in excellent agreement with the experimental results for a Al0.4Ga0.6As/GaAs Pnp HBT recently reported in the literature. For that structure, the collector current ideality factor is found to increase from 1.1 at low forward bias V-EB to 3.0 at large applied emitter-base forward bias approaching the built-in potential. Experimental values are equal to 1.2 and 2.25 at low and large V-EB, respectively. (C) 2000 American Institute of Physics. [S0021-8979(00)01503-6].