Two-dimensional analysis of the surface recombination effect on current gain for GaAlAs/GaAs HBTs

Current-gain degradation induced by surface states are investigated for GaAlAs/GaAs heterojunction bipolar transistors with a mesa-type extrinsic base using a two-dimensional numerical model. Surface Fermi level pinning and surface recombination effects are introduced into the model, which is based on W.E. Spicer et al.´s (1979) unified defect model. It is shown that surface recombination is significant only at the boundary region between the intrinsic base and extrinsic base. As a result, although current gain degrades with the existence of surface states, it does not depend on the spacing between the emitter and the base electrode edges. The degradation is shown from calculations to be suppressed for a very-thin-base or a graded-base structure. The suppression is especially pronounced in the graded-base structure because of the inherent potential barrier to electrons formed in the extrinsic base layer, which prevents electrons from reaching the surface region where the recombination centers exist