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 degrees C. The drain-source current I-ds decreased with RTA temperature up to 600 degrees C and turned to increase at 650 degrees C. The maximum transconductance g(m_max), on the other hand, increased up to 650 degrees C, indicating that RTA improved the dc characteristics of the HEMTs. We confirmed the improvement of Schottky contacts by measuring gate leakage current I-gs and calculating the Schottky barrier height phi(B). After RTA at 600 degrees C, the I-gs at a gate-source voltage V-gs of -10 V was as much as three orders of magnitude lower than it was in the as-deposited condition and the phi(B) was 0.27 eV greater than it was in the as-deposited condition (1.16 eV versus 0.89 eV). After the RTA at 650 degrees C, the phi(B) was slightly less than it was after the RTA at 600 degrees C. RTA at 600 and 650 degrees C also improved the rf characteristics, increasing the cutoff frequency f(T) from 80 to 84 GHz (5% increase) and increasing the maximum oscillation frequency f(max) from 102 to M GHz (20% increase). (c) 2005 American Vacuum Society.