Selective injection AlGaN/GaN heterojunction bipolar transistors with patterned regrown base contacts

We demonstrate graded AlGaN/GaN heterojunction bipolar transistors (HBTs) with selective injection of minority carriers across a p-GaN base and patterned regrown base contacts. The selective injection design regulates minority carrier transport under emitter–base forward bias through a thin base region, while thick and highly doped p+ GaN regrown layers patterned alongside the thin base regions are utilized to lower the base contact resistance. With SiO2 employed as a spacer between the emitter and the p+ regrown layers, the device with an interdigitated emitter/base-contact stripe design displayed a maximum collector current density (IC) of 101 kA/cm2 and a maximum current gain (β) of 70 at IC ∼1 kA/cm2 and ∼11 for IC > 50 kA/cm2. The reported results demonstrate the potential of the selective injection approach to break the long-existing HBT design trade-off between base resistance and current gain for next-generation radio frequency and millimeter wave applications.