High-gain InGaAlAs/InGaAs heterojunction bipolar transistors andphototransistors

The characteristics of InGaAlAs/InGaAs heterojunction bipolar transistors (HBTs) grown by molecular beam epitaxy are described. A current gain of 15600 at a current density of ~10⁴ A/cm² and an emitter-base heterojunction ideality factor of 1.02 were measured. Appropriately designed InGaAlAs/InGaAs HBTs, when operated as phototransistors, also had high gains. A current gain of 1000 for a collector current of only 10 μA was obtained for phototransistors. Such high gains are due to low recombination currents as a consequence of the good crystalline quality of the InGaAlAs bulk and InGaAlAs/InGaAs interface