We report MOCVD-grown Al 0.87 Ga 0.13 N/Al 0.64 Ga 0.36 N metal-oxide-semiconductor-heterojunction-field-effect-transistors on single crystal bulk AlN substrate. As compared to control devices on AlN template, thermal impedance for devices on single crystal AlN decreased to 1/3 from 31 to 10 K mm W− 1, comparable to SiC and copper heat-sinks. This represents a significant thermo-electric co-design advantage over other semiconductors. As a result, the peak drain saturation current increased from 410 to 610 mAmm− 1. A 3-terminal breakdown field of 3.7 MV cm− 1 was measured, which to date represents state-of-the-art performance for devices with similar Al x Ga 1− x N-channel composition. This translates to a measured Baliga figure of merit of 460 MWcm− 2.