In this work, sub-micron-thick AlN/GaN transistors (HEMTs) grown on a silicon substrate for high-frequency power applications are reported. Using molecular beam epitaxy, an innovative ultrathin step-graded buffer with a total stack thickness of 450 nm enables one to combine an excellent electron confinement, as reflected by the low drain-induced barrier lowering, a low leakage current below 10 µA/mm and low trapping effects up to a drain bias V DS = 30 V while using sub-150 nm gate lengths. As a result, state-of-the-art GaN-on-silicon power performances at 40 GHz have been achieved, showing no degradation after multiple large signal measurements in deep class AB up to V DS = 30 V. Pulsed-mode large-signal characteristics reveal a combination of power-added efficiency (PAE) higher than 35% with a saturated output power density (P OUT) of 2.5 W/mm at V DS = 20 V with a gate-drain distance of 500 nm. To the best of our knowledge, this is the first demonstration of high RF performance achieved with sub-micron-thick GaN HEMTs grown on a silicon substrate.