Gallium nitride/gallium oxide GaN/Ga2O3 nanowire metal-oxide-semiconductor field-effect-transistors are shown to operate at an average electron velocity of ∼1.24 × 107 cm/s and threshold-voltage roll-off of −0.2 V as the transistor gate length Lg reduced from 500 to 50 nm. Improvement of saturation current to 120 μA and unity current/power-gain cut-off frequency to 150/180 GHz is observed on Lg = 50 nm devices. Our study reveals the advantages of using (i) polarization-induced positive charges and high-k dielectric at the {10}GaN/{002}Ga2O3 interface to provide carrier confinement and to shield the drain field, and (ii) polarization-induced negative charges at the (0001)GaN/sapphire interface to form a back-barrier to suppress leakage and improve the short-channel transport properties.