AlGaN-channel high electron mobility transistors (HEMTs) are among a class of ultra wide-bandgap transistors that are promising candidates for RF and power applications. Long-channel Al x Ga 1-x N HEMTs with x = 0.7 in the channel have been built and evaluated across the −50 • C to +200 • C temperature range. These devices achieved room temperature drain current as high as 46 mA/mm and were absent of gate leakage until the gate diode forward bias turn-on at ∼2.8 V, with a modest −2.2 V threshold voltage. A very large I on /I off current ratio, of 8 × 10 9 was demonstrated. A near ideal subthreshold slope that is just 35% higher than the theoretical limit across the temperature range was characterized. The ohmic contact characteristics were rectifying from −50 • C to +50 • C and became nearly linear at temperatures above 100 • C. An activation energy of 0.55 eV dictates the temperature dependence of off-state leakage. AlGaN-channel high electron mobility transistors (HEMTs) are among a class of ultra wide-bandgap transistors (UWBG) that are promising candidates for power and RF applications.1-9 Their promise derives from the large critical electric field, which scales as a power law with the bandgap of the material, 10 e.g. E C ∼ E G 2.5 (the exact dependence is a topic of active research) and provides favorable power and RF figures of merit. Enhancement-mode HEMTs are required for power applications, while depletion-mode HEMTs are suitable for RF applications. Power electronics applications utilize GaN/AlGaN HEMTs with dielectric insulators as a means of achieving a large gate voltage swing for high current density and low on-resistance. The dielectric insulators also suppress gate leakage, but at the expense of substantial interface charge density and the potential for hot electrons inducing unwanted trapped charges in the dielectric. These latter effects make gates employing dielectric insulators unsuitable for RF applications with GaN/AlGaN HEMTs.Al y Ga 1-y N/Al x Ga 1-x N HEMTs with high Al in the channel, x = 0.7 and larger, have promising figures of merit and are candidates for next generation power and RF applications. 1,7,11,12 With a bandgap of >5.8 eV, the Al 0.85 Ga 0.15 N barrier is practically an insulator, but since it is combined with a modest conduction band offset to the Al x Ga 1-x N channel it has both insulator and Schottky-like properties. As a crystallographic insulator-like material, it has potential for a very good interface with nearly lattice matched Al x Ga 1-x N channel adjacent to the 2-dimensional electron gas (2DEG). Even as a Schottky barrier it can have a large turn-on voltage to enhance the voltage swing and current drive capability. Although promising, Al x Ga 1-x N channel HEMTs lack maturity and do not yet match the high current density of AlGaN/GaN HEMTs due both to the difficulty of achieving good ohmic contacts and the lack of aggressive dimensional scaling to compensate for limitations of the low-field electron mobility, which is limited by alloy scattering.In this work ...