High-gain resonant InGaAlAs/InGaAs heterojunction bipolar phototransistors grown by molecular beam epitaxy

Results obtained with two approaches that significantly enhance the gain and sensitivity of InP-based heterojunction phototransistors (HPTs) grown by molecular beam epitaxy (MBE) are described. The use of quaternary InGaAlAs (300 K, E/sub g/ approximately 1.08 eV) as the high-bandgap emitter layer improves the quality of the emitter-base interface and leads to high current gains, even at very low current levels. For heterojunction bipolar transistors (HBTs) with a circular (50- mu m diameter) emitter, DC current gains of more than 9000 were obtained at a current density of 1000 A/cm/sup 2/. In the second approach to enhancing the gain and sensitivity of HPTs while keeping the absorbing layer thickness the same, a resonant structure in which the active layers are part of a Fabry-Perot cavity with the end reflectors being gold (which also serves as the emitter contact) and an InAlAs/InGaAlAs quarter-wave stack was designed.