800 $^\circ$C Air-Ambient Operation of Extreme-Bandgap Al$_{x}$Ga$_{1-x}$N (x>0.6) Digital and Random-Alloy Channel HEMTs
This work demonstrates the high-temperature (room to 800 $^\circ$C) operation of thin Al$_{0.62}$Ga$_{0.38}$N digital alloy (DA) short-period superlattice (SPSL), and random alloy (RA) Al$_{0.64}$Ga$_{0.36}$N channel MOSHEMTs on sapphire. The RA devices employ a doped $n^{+}$-85%-Al barrier and a highly doped $n^{++}$-Al$_{0.86\rightarrow0.20}$Ga$_{0.14\rightarrow0.80}$N reverse-graded layer (RGL) for surface ohmic contact formation, whereas the DA devices employ a binary AlN barrier and selectively regrown $n^{++}$-GaN source/drain contacts. At 800 $^\circ$C, the DA channel devices exhibit better thermal stability: their saturated drain current decreases from 460 to 172 mA/mm (62.6%) compared with 338 to 98 mA/mm (71.0%) for the RA channel, and a lower subthreshold swing (459 for DA versus 550 mV/dec for RA). All tested DA devices also exhibit superior threshold-voltage stability ($<\pm 10\%$ versus $>\pm 19\%$ for RA) and maintain an ON/OFF current ratio >$10^4$ at 800 $^\circ$C, compared with ~$10^3$ for RA, thus establishing the DA channel HEMTs as a promising new platform for high-temperature electronics.