An analysis of space-charge-region recombination in HBT´s

The importance of including recombination in the base side of the emitter-base space-charge-region (SCR) in the computation of the current gain in AlGaAs/GaAs HBT´s is investigated. Recombination due to Shockley-Read-Hall, Auger and radiative processes is considered. The interaction of the base-side SCR recombination currents with the neutral-base current and the collector current, which occurs via their dependence on the quasi-Fermi level splitting (ΔE_fn) at the base-emitter junction, is not found to be a significant factor in the computation of ΔE_fn. However, it is confirmed that the quasi-Fermi level splitting, as calculated from a balancing of the thermionic/tunnel current with the neutral base and collector currents, must subsequently be included in the computation of the base-side SCR currents if the current gain is not to be severely underestimated. A discussion of why the ideality factor is ≈1 for the base-side SCR currents is given. Finally, simple analytical expressions for ΔE _fn and the SCR recombination currents are presented and should prove useful for HBT device- and circuit-simulation purposes