Characterization and reduction of edge leakage current in AlGaAs/GaAs heterojunction bipolar transistors

The dependence of the performance of circular n-p-n AlGaAs/GaAs heterojunction bipolar transistors on structural design considerations and different base doping levels is investigated. 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 of different emitter sizes. MOVPE and MBE grown heterojunction bipolar transistors, with different base dopants and doping levels, are assessed. The use of a heavily carbon-doped base with doping levels varying from 2×10 ¹⁸ cm^-3 to 5×10¹⁹ cm^-3, is also examined. MBE-grown wafers have shown good transistor characteristics, but poor gain. However, a five-fold decrease in base doping level of MBE-grown devices produces an increase in current gain of nearly the same order. MOVPE-grown wafers have both good transistor performance and high current gain with a 30% gain improvement for 100 μm compared to 200 μm emitter-sized devices