Characterization and reduction of edge leakage current in AlGaAs/GaAs heterojunction bipolar transistors for high frequency applications

N-p-n AlGaAs/GaAs heterojunction bipolar transistors of various emitter-mesa diameters have been fabricated to investigate the dependence on structural design considerations of the edge leakage current, mainly caused by surface recombination. It is shown that the inclusion of a thinned-emitter edge shoulder structure has a significant effect on the mesa edge leakage current of circular devices. MOVPE and MBE grown heterojunction bipolar transistors, with different base doping levels, were also assessed and the possibility of using a heavily carbon-doped base with doping levels varying from 1×10¹⁸ cm^-3 to 5×10¹⁹ cm, was examined. It was found that MBE-grown wafers show good transistor action, but poor current gain, whereas MOVPE-grown wafers have both good transistor performance and high current gain. However, a five-fold decrease in base doping level of MBE-grown devices produces an increase in current gain