Wafer-fused aluminum gallium arsenide/gallium arsenide/gallium nitride heterojunction bipolar transistors

(Al)GaN materials have established themselves in high speed and high power applications, owing to their large band gap energies and high anticipated electron saturation velocities. Unlike the rapid development of AlGaN/GaN high electron mobility transistors (HEMTs), that of AlGaN/GaN heterojunction bipolar transistors (HBTs) has been greatly impeded by the highly resistive p-GaN material. A proposed solution is to utilize AlGaAs/GaAs heterostructures as the emitter-base and keep GaN as the collector. Combining the unity emitter injection efficiency and the large base transport factor of Al-GaAs/GaAs, and the high breakdown field of GaN, an AlGaAs/GaAs/GaN npn HBT is expected to operate at high speed and high power densities. Unfortunately, the large lattice mismatch between GaAs and GaN makes it very difficult to epitaxially grow high quality GaAs on GaN. Instead, direct wafer fusion has been used to heterogeneously integrate AlGaAs/GaAs and GaN for HBT fabrication. Encouraging device performance has been achieved in wafer-fused AlGaAs/GaAs/GaN HBTs, demonstrating the feasibility of forming active device regions by direct wafer fusion.;By optimizing the wafer fusion and substrate removal process, smooth and continuous (Al)GaAs films have been obtained on GaN. The fused p +-GaAs/n-GaN heterojunctions have been characterized by electrical measurements. A large number of interface states and an electron energy barrier may exist at the fused GaAs/GaN interface resulting in adverse electron blocking effects at the base-collector junction. A broad range of fusion conditions have been explored and current gain as high as 20 has been achieved in an AlGaAs/GaAs/GaN HBT fused at 450°C for 2 h. The fused HBTs were found to exhibit larger breakdown voltages than the as-grown AlGaAs/GaAs/GaAs HBTs. The first wafer-fused RF HBT has been fabricated and characterized as well. Current gain cutoff frequency (fT) of 2.6 GHz and maximum oscillation frequency (fmax) of 1.0 GHz have been obtained in a self-aligned AlGaAs/GaAs/GaN HBT with an emitter area of 5 x 7 mum2. Finally, various mechanisms limiting the fused HBT performance have been examined and the fused GaAs/GaN interface is believed to be the dominant factor governing the fused device characteristics.