Current gain increase in AlGaAs/GaAs heterojunction bipolar transistors using AlGaAs layer overgrowth

Reduction of the surface recombination current components in Npn AlGaAs/GaAs heterojunction bipolar transistors has been achieved by overgrowing the emitter-mesas with an AlGaAs layer approximately 0.5 μm thick. It was observed that the n=2 recombination current was reduced by ∼90%, to about 10% of the original value, with a corresponding 13-fold increase in current gain for 270×20 μm2 devices. In addition, devices with the same emitter-base area, but significantly different perimeters, were observed to have similar current gains and n=2 recombination current values. This was in contrast to devices fabricated without an overgrowth layer. These results suggest that the overgrowth layer causes a very significant reduction in the perimeter recombination current.