Gain degradation mechanisms in wafer fused AlGaAs∕GaAs∕GaN heterojunction bipolar transistors

Abstract

The authors have compared AlGaAs∕GaAs∕GaN heterojunction bipolar transistors (HBTs) formed by wafer fusion with AlGaAs∕GaAs∕GaAs as-grown HBTs subject to high temperature annealing conditions similar to those used in the wafer fusion process. The high temperature annealing alone is found to cause gain degradation by a factor of 2–6, a result of reduction in minority carrier lifetime in the base. Detailed analysis indicates that the fused HBTs also suffer from higher recombination in the emitter-base junction, exacerbated base degradation as well as effective potential barriers formed at the GaAs base/GaN collector junction.

Influences of ALD Al2O3 on the surface band-bending of c-plane, Ga-face GaN

“…However, limited by the well-known poor p-type doping in GaN, the most critical challenge, implementing practical GaN-based BJTs has been impossible to date [6]. In additional to this major challenge, the preliminary investigations of GaN-based BJTs revealed that the surface energy band-bending of GaN needs to be taken into crucial consideration for GaN-collector bipolar transistor applications [7,8].…”

The role of setback layers on the breakdown characteristics of AlGaAs/GaAs/GaN HBTs

“…It can be seen that the breakdown voltage of the GaAs homojunctions is ~ 20 V, in good agreement with the theoretical prediction, while that of the fused heterojunction with no setback is only ~ 90 V, much smaller than the calculated value. After inserting a 20 nm p-GaAs setback layer, an obvious soft breakdown appears before the hard breakdown at ~ 75 V. The smaller than expected breakdown voltages of the wafer-fused heterojunctions are attributed to the large reverse leakage currents [4]. The common emitter breakdown voltages V CEO of HBTs have also been measured using floating-base setup.…”

“…Since the breakdown characteristics of HBTs depend mainly on the collector material, HBTs fabricated on AlGaAs/GaAs/GaN double heterostructures are expected to be able to operate at both high-frequency and high power densities. Recently, we have reported AlGaAs/GaAs/GaN HBTs formed by direct wafer fusion, exhibiting dc current gain of ~ 5-10 [3,4]. Wafer fusion, instead of epitaxial growth, was employed to form these heterostructures because of the large lattice mismatch between GaAs and GaN.…”

4H-SiC/Si Heterojunction Bipolar Transistors Fabricated by Surface Activated Bonding

“…The interface sates degrade the current gain by combined effects of localized recombination and trapped charge that affect the band bending. 17 The current gain of HBT decreased as the interface states density increased. 18 CNL E CNL are assumed to have donor-like (acceptor-like) features.…”