A pseudomorphic heterojunction bipolar transistor

Examines the suitability of the GaAs/GaAs_ySb_1-y system for application to HBTs. It has been speculated that there is no offset in the conduction band for this system, in which case the spike and notch should therefore be absent. However, there is considerable lattice mismatch between GaAs and GaSb (approximately 7%) and, therefore, also between GaAs and GaAs_ySb_1-y alloys. However, a reasonably large ΔEv (=ΔEg) can be obtained while keeping the GaSb mole fraction low in the ternary GaAs _ySb_1-y alloy. This suggests that, in the pseudomorphic growth regime, high quality heterojunctions between GaAs and GaAs_ySb_1-y can be grown by a suitable technique, such as molecular beam epitaxy (MBE). This could be applied to HBTs using the wider band gap GaAs as the emitter and the GaAs_y Sb_1-y as the base. There are many advantages in using a GaAs-GaAsSb system instead of the commonly used AlGaAs-GaAs system. The compatibility of growing the GaAs and GaAs_ySb_1-y at the same temperature (500-600°C but typically 550°C) will solve growth related problems such as Be (p-type dopant) movement towards the growth front. Shallower donor levels in a GaAs emitter than those in an Al_xGa_1-xAs one should reduce any device temperature stability problem. Additionally a GaAs emitter instead of an Al_xGa_1-xAs one would lead to lower turn-on voltage and higher current density for a given bias. The authors have grown by MBE and fabricated N-p-N double-heterojunction bipolar transistors (DHBTs) in this system having abrupt and graded emitter-base and base-collector interfaces employing GaAs_0.9Sb_0.1 bases