New Experimental Interpretation for the Emitter-size Effect of AIGaAs/GaAs heterojunction bipolar transistors.

Abstract

AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have an excellent frequency performance. One of the major limitations of these devices is current gain degradation with decreased emitter-base junction size, which is known as the emitter-size effect. Several experimental and theoretical studies have been reported on the emitter-size effect in AIGaAs/GaAs HBTs. These papers suggest that it is due to the extrinsic base surface recombination current. However, in thts paper. experimental evidence is reported which suggests that it is the perimeter recombination current which causes the emitter-size effect. Further analysis or previously published results, from rectangular geometry AIGaAs/GaAs HBTs fabricated with and without an overgrown A1GaAs layer approximately 0.5 um thick around the emitter-mesas, has been carried out. This analysis suggests that the perimeter recombination current rather than the extrinsic base surface recombination current causes the emitter-size effect. Devices with the overgrown layer and with the same emitter-base area. but significantly different perimeters, had similar current gains and n=2 recombination current values. In addition, for similar geometry devices without the overgrowth layer, the n=2 recombination current was roughly proportional to the device perimeter and due to the emitter-size effect current gains were decreased when the perimeter was increased. Since the reduction in the perimeter recombination current leads to a reduction of the emitter-size effect, it is suggested that the emitter-size effect is not due to the extrinsic base surface recombination current. Consequently, these results suggest that the emitter-size effect is due to the perimeter recornbination current.