AlGaAs/GaAs tunneling emitter bipolar transistors (TEBT’s), grown by molecular beam epitaxy, have been fabricated. Device structures with two different tunneling barrier Al mole fractions and each for two different barrier thicknesses were characterized at room temperature. A differential current gain of 410 was achieved using a single 50 Å AlAs tunneling barrier. Devices with either thinner barriers (20 Å) or Al0.38Ga0.62As barriers had lower gains. Al0.24Ga0.76As/GaAs heterojunction bipolar transistors and GaAs homojunction bipolar transistors without tunneling barriers were also fabricated and characterized, for comparison. The performance of the homojunction devices was improved in all cases by inserting the tunneling barrier. The variations of the base and collector currents were measured for all devices and, for the TEBT’s they showed a functional dependence on the interfacial barrier Al mole fraction and thickness, which was attributed to carrier tunneling through the barrier. Furthermore, the above characteristics exhibit a clear electron-to-hole preferential tunneling.

Double-heterojunction NpN GaAs/InxGa1−xAs/GaAs bipolar transistor layers have been grown by molecular-beam epitaxy, and large-area devices have been processed and characterized. The indium mole fraction in the strained base layer, and thus the band offsets, has been varied with significant differences in current gains. From the gain versus indium-composition relation a valence-band offset of ΔEv =9.7 meV/% In is derived. We found that the highest base-In content yields the highest-gain devices despite the presence of interface misfit dislocations and dark-line defects.