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Herein, a 3.5 GHz (S‐band) power performance is reported on an AlGaN/GaN high‐electron‐mobility transistor (HEMT) with AlN buffer on SiC substrate. A 4 inch epi‐wafer is grown by high‐temperature metal–organic chemical vapor deposition. The fabricated devices with a 400 nm gate and direct current and radio‐frequency (RF) characteristics are examined. These device shows a transconductance of 233 mS mm−1 and a maximum drain current of 820 mA mm−1. The pulsed current–voltage (I–V) characteristic shows a low slump ratio with a 0.36% and 2.2% for Z1 and Z2, respectively. The power performance shows output power = 5.5 W mm−1 with 54.3% power added efficiency, and VDS was 50 V. The potential of an AlN buffer HEMT is demonstrated by the results for use in next‐generation high‐power RF devices.