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Depletion-mode high-electron mobility transistors (HEMTs) based on a quaternary barrier In0.13Al0.83Ga0.04 N/AlN/GaN heterostructure on SiC substrate were fabricated. The 66-nm-long gate device shows a dc drain current density of 2.1 A/mm, a peak extrinsic transconductance of 548 mS/mm, and a record current gain cutoff frequency fT of 220 GHz for quaternary barrier GaN-based HEMTs, which is also among the highest fT for all GaN-based HEMTs. The large Lg ·fT product of 14.5 GHz ·μm with a gate-length-to-barrier-thickness aspect ratio of 5.8 indicates a high effective electron velocity of 0.9 ×107 cm/s, attributed to a high electron Hall mobility (1790 cm2/V ·s at an ns of 1.8 ×1013-2)-the highest reported in GaN-channel HEMTs with In-containing barriers. An intrinsic electron velocity of 1.7 ×107 cm/s, extracted from conventional Moll delay-time analysis, is comparable to that reported in the state-of-art AlGaN/GaN HEMTs.