AlGaN/GaN heterojunction bipolar transistors by ammonia molecular beam epitaxy

We demonstrate AlGaN/GaN heterojunction bipolar transistors (HBTs) by ammonia molecular beam epitaxy (NH3 MBE). The several benefits offered by NH3 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 – 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, JC > 1 kA/cm2 was obtained from devices on both wafers. In CE operation, a current gain (β) of 1.5–3 is observed in devices on both wafers. High base resistance is found to limit the current gain in CE operation, by limiting the base-emitter bias voltage in the active region of the device. Maximum β values observed from Gummel plots at a fixed base-collector (BC) bias of zero volts, are β ∼ 15 at JC = 0.88 kA/cm2 for Wafer A and β ∼13.5 at JC = 3 kA/cm2 for Wafer B. In devices on both wafers, variation is observed in the plots of β versus IC (collector current) for devices with the same emitter dimensions and geometry but with different lateral spacing between emitter mesa and base contacts (WEB). Peak β obtained is also observed to vary with WEB. An explanation is provided for the observed behaviour. This work demonstrates the potential of NH3 MBE for the growth of GaN HBTs.