AlN/GaN heterostructures with 1700-cm 2 /V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (L G ) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. L G = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency f T of 165 GHz, a maximum frequency of oscillation f max of 171 GHz, and intrinsic average electron velocity v e of 1.5 ×10 7 cm/s. The 40-GHz load-pull measurements of L G = 140 nm devices showed 1-W/mm output power, with a 4.6-dB gain and 17% power-added efficiency. GaN substrates provide a way of achieving high mobility, high v e , and high RF performance in AlN/GaN transistors.