We fabricated 120-nm-long-T-shaped-Ti/Pt/Au-gate AlGaN/GaN high electron mobility transistors (HEMTs) on sapphire substrates and annealed them thermally to improve their Schottky contacts (i.e., to reduce interface traps, increase the Schottky barrier height, and get better adhesion), thereby reducing gate leakage current, achieving better gate controllability, and obtaining better dc and rf characteristics compared to the HEMTs with as-deposited Schottky gate metals. Rapid thermal annealing (RTA) was carried out successively at 500, 600, and 650 °C. The drain-source current Ids decreased with RTA temperature up to 600 °C and turned to increase at 650 °C. The maximum transconductance gm_max, on the other hand, increased up to 650 °C, indicating that RTA improved the dc characteristics of the HEMTs. We confirmed the improvement of Schottky contacts by measuring gate leakage current Igs and calculating the Schottky barrier height ΦB. After RTA at 600 °C, the Igs at a gate-source voltage Vgs of -10 V was as much as three orders of magnitude lower than it was in the as-deposited condition and the ΦB was 0.27 eV greater than it was in the as-deposit- - ed condition (1.16 eV versus 0.89 eV). After the RTA at 650 °C, the ΦB was slightly less than it was after the RTA at 600 °C. RTA at 600 and 650 °C also improved the rf characteristics, increasing the cutoff frequency fT from 80 to 84 GHz (5% increase) and increasing the maximum oscillation frequency fmax from 102 to 122 GHz (20% increase).