Theoretical comparison of base bulk recombination current and surface recombination current of a mesa AlGaAs/GaAs heterojunction bipolar transistor

Abstract

An analytical solution for the minority carrier concentration to the 2-D diffusion equation in the base of a mesa-structure bipolar transistor is derived. The solution is especially applicable for AlGaAs/GaAs heterojunction bipolar transistors for which the extrinsic base surface has a high surface recombination velocity if the surface is not passivated. The different components of the base current are determined from the derived 2-D minority carrier concentration in the base. The major components to the base current include: surface recombination at the extrinsic base surface, at the base ohmic contact and bulk recombination in the base. The relative importance of each component is discussed as a function of the extrinsic base length, emitter length and base doping. The effects of quasielectric field in the base and the passivation on the extrinsic base region are also evaluated. The results indicate that as base doping is increased, the current gain is limited by bulk recombination, independent of the details of the device geometry.