Experimental evaluation of separate contributions to ideality factor for the base surface recombination current in heterojunction bipolar transistors

Abstract

In this paper it is shown experimentally, for the first time, that the ideality factor n for the extrinsic base surface recombination current in AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is unity and is bias independent. The devices with various emitter radii have been fabricated and the n = 1 and n = 2 current components of the emitter-base junction current were calculated. It was observed experimentally that the n = 1 current component was proportional to the total area of the emitter-base junction plus the exposed extrinsic base surface, both at lower biases such as emitter-base voltage VBE=0.60V (when the n = 2 current component dominates), and at higher biases such as VBE=1.0V (when the n = 1 current component dominates). The ideality factor value for the diffusion current is 1. Therefore, these results suggest that the ideality factor of the extrinsic base surface recombination current in HBTs is unity and is independent of the emitter-base bias.