AlGaAs/GaAs heterojunction bipolar transistor with a two-dimensional electron gas emitter

We report the operation of the first two-dimensional electron gas (2-DEG) emitter heterojunction bipolar transistor. This device, which was grown by molecular beam epitaxy, incorporates an undoped GaAs spacer between the emitter and base of a standard AlGaAs/GaAs single-heterojunction bipolar transistor. The introduction of the spacer layer causes the formation of a 2-DEG at the AlGaAs/GaAs interface. This 2-DEG defines the emitter side of the junction and produces an emitter-base characteristic similar to that of the collector GaAs homojunction. Using a 300 Å GaAs spacer, offset voltages as low as 30 mV have been attained. These devices also exhibit current gains greater than 10 at emitter current densities of 3 A/cm2 and gains up to 400 in the high current density regime.