SURFACE RECOMBINATION CURRENT OF ALGAAS/GAAS HETEROJUNCTION BIPOLAR-TRANSISTORS

Electron-hole recombination at the surface of a GaAs-related compound is more significant compared to that of Si. This leads to non-negligible emitter and base surface recombination currents in AlGaAs/GaAs heterojunction bipolar transistors (HBTs) due to considerable electron-hole recombination taking place at the surface of the emitter and base peripheries. Thus, unlike Si bipolar transistors in which surface recombination can be neglected, it is imperative to include surface recombination current when modeling the HBT base current. Based on the finite-difference approach and relevant device physics such as the Shockley-Read-Hall recombination statistics and Fermi level pinning, this paper develops a physics-based model for predicting emitter and base surface recombination currents of AlGaAs/GaAs HBTs. All components of the HBT base current calculated from the present model are compared, and their relative importance assessed. We show that base surface recombination current depends on exp(qV/2kT) whereas emitter surface recombination current follows an exp(qV/kT) dependence. Furthermore, it is suggested that the importance of base surface recombination current decreases as the emitter-base voltage increases, and vice versa for emitter surface recombination current. Emitter surface recombination current also becomes more significant as the ratio of emitter-base junction perimeter to area increases resulting from HBT dimension scaling. Our theoretical predictions compare favorably with numerical simulations and an empirical model available in the literature.