An emitter guard-ring structure for GaAs high-gain heterojunction bipolar transistors

An emitter guard-ring structure is adopted for GaAs/AlGaAs heterojunction bipolar transistors (HBT's). The guard ring is added around the central emitter and has nearly the same dc bias as the base, so that the carrier injection as well as the surface recombination at the emitter periphery is blocked. As a result, the current gain at low operating currents is improved remarkably. It remains almost constant at 600 as the collector current is decreased from 20 mA down to 5 µA. This is the best result to date for a GaAs HBT at low currents. This technique also makes it possible to investigate the interface quality without interference from surface effects.