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The ultra-wide bandgap of Al-rich AlGaN is expected to support a significantly larger breakdown field compared to GaN, but the reported performance thus far has been limited by the use of foreign substrates. In this Letter, the material and electrical properties of Al0.85Ga0.15N/Al0.6Ga0.4N high electron mobility transistors (HEMT) grown on a 2-in. single crystal AlN substrate are investigated, and it is demonstrated that native AlN substrates unlock the potential for Al-rich AlGaN to sustain large fields in such devices. We further study how Ohmic contacts made directly to a Si-doped channel layer reduce the knee voltage and increase the output current density. High-quality AlGaN growth is confirmed via scanning transmission electron microscopy, which also reveals the absence of metal penetration at the Ohmic contact interface and is in contrast to established GaN HEMT technology. Two-terminal mesa breakdown characteristics with 1.3 μm separation possess a record-high breakdown field strength of ∼11.5 MV/cm for an undoped Al0.6Ga0.4N-channel layer. The breakdown voltages for three-terminal devices measured with gate-drain distances of 4 and 9 μm are 850 and 1500 V, respectively. [ABSTRACT FROM AUTHOR]

Despite being the most widely used dielectric for passivation of GaN-based lateral devices, amorphous silicon-nitride still faces many stability challenges, which arise from its complex bulk electronic and interface properties on the polar (Al)GaN surfaces. In this investigation, SiN has been applied as an ultra-thin interlayer (∼3–5 nm) in vertical contact structures on Ga-polar and N-polar GaN templates to study the metal–insulator–semiconductor- (MIS-) like system and better understand the interaction between the polar surface and its dielectric overlayer. We describe the role of amphoteric ≡Si centers in SiN in passivating and providing the polarization countercharge to Al/GaN of different polarities. The consequent requirements of the concentration profile of the amphoteric defects and the corresponding chemical profile of SiN is discussed. The importance of SiN surface termination and their influence on the interface potential on Al/GaN that determines device performance and reliability is also shown. Finally, a pathway to highly stable and reliable ohmic contacts to n-type Ga-polar GaN without instabilities associated with metal directly alloying with GaN as in the case of traditional contacts is proposed. [ABSTRACT FROM AUTHOR]