Two-dimensional analysis of surface recombination at the extrinsic base surface under various conditions in [formula omitted] heterojunction bipolar transistors

Abstract

In order to investigate surface effects, under improved surface conditions at the extrinsic base surface, on the current characteristics in AlGaAs GaAs HBTs, two-dimensional numerical simulations have been performed. The surface conditions at the extrinsic base surface have been realistically assumed to be determined by the magnitude of the surface recombination velocity, which is evaluated by the surface density and carrier capture cross section of the introduced surface states. From the simulated results, it is found that surface recombination at the extrinsic base surface occurs in a potential well which results from Fermi-level pinning. When the surface recombination velocity is low, the potential well either forms or does not, based on the surface density of the introduced deep states. However, current gain degrades significantly when the potential well is formed at the extrinsic base surface. Concurrently, the effects of ionized surface states on current gain degradation are investigated.