We report on highly-scaled Al 0.25 Ga 0.75 N channel high electron mobility transistors. Regrown ohmic contacts covering the sidewall of the compressively strained Al 0.25 Ga 0.75 N channel exhibited a low contact resistance of R c= 0.23 Ω· mm. Scaled devices with a T-shaped gate showed record high speed for any AlGaN-based transistors, f T/f max= 67/166 GHz, while simultaneously achieving high average breakdown field exceeding 2 MV cm− 1. The load-pull measurements performed at 10 GHz revealed a 20% peak power added efficiency with an output power density of 2 W mm− 1, which is mainly limited by the RF dispersion.