Abstract
We demonstrate AlGaN/GaN heterojunction bipolar transistors (HBTs) by ammonia molecular beam epitaxy (NH~3~ MBE). The several benefits offered by NH~3~ MBE for the growth of GaN based vertical electronic devices are discussed. To obtain good ohmic contacts to the base layer, devices were fabricated with two different processes involving regrowths β regrown base contact (Wafer A) and regrown emitter (Wafer B). Devices on Wafers A and B had similar DC performance. Common emitter (CE) operation with a peak collector current density, J~C~β>β1βkA/cm^2^ was obtained from devices on both wafers. In CE operation, a current gain (Ξ²) of 1.5β3 is observed in devices on both wafers. High base resistance is found to limit the current gain in CE operation, by limiting the baseβemitter bias voltage in the active region of the device. Maximum Ξ² values observed from Gummel plots at a fixed baseβcollector (BC) bias of zero volts, are Ξ²ββΌβ15 at J~C~β=β0.88βkA/cm^2^ for Wafer A and Ξ² βΌ13.5 at J~C~β=β3βkA/cm^2^ for Wafer B. In devices on both wafers, variation is observed in the plots of Ξ² versus I~C~ (collector current) for devices with the same emitter dimensions and geometry but with different lateral spacing between emitter mesa and base contacts (W~EB~). Peak Ξ² obtained is also observed to vary with W~EB~. An explanation is provided for the observed behaviour. This work demonstrates the potential of NH~3~ MBE for the growth of GaN HBTs.