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Depletion-mode quaternary barrier In0.13Al0.83 Ga0.04N high-electron-mobility transistors (HEMTs) with regrown ohmic contacts and T-gates on a SiC substrate have been fabricated. Devices with 40-nm-long footprints show a maximum output current density of 1.8 A/mm, an extrinsic dc transconductance of 770 mS/mm, and cutoff frequencies fT/fmax of 230/300 GHz at the same bias, which give a record-high value of √fT ·fmax = 263 GHz among all reported InAl(Ga)N barrier HEMTs. The device speed shows good scalability with gate length despite the onset of short-channel effects due to the lack of a back barrier. An effective electron velocity of 1.36 ×107 cm/s, which is comparable with that in the state-of-the-art deeply scaled AlN/GaN HEMTs, has been extracted from the gate-length dependence of fT for gate lengths from 100 to 40 nm.