Double-heterojunction GaAlInAs/GaInAs bipolar transistor grown by molecular beam epitaxy

The first double-heterojunction In53Ga28Al19As/In53Ga47As bipolar transistors grown by molecular beam epitaxy on semi-insulating substrates were fabricated and characterized. We present a comparison between three types of structures: ones with two abrupt heterojunctions; ones with a thin small bandgap n-type layer (spacer) at the collector junction; and ones with two spacers. The use of the small bandgap spacer permits an increase in the collector efficiency while decreasing the recombination current in the emitter-base junction. The best devices of the third type (two spacers) exhibit a current gain up to 200 and an offset voltage of only 70 mV.