We fabricated 120-nm-T-shaped-Mo/Pt/Au-gate AlGaN/GaN high electron mobility transistors (HEMTs) on sapphire substrates. The gate leakage current I-gs of the Mo/Pt/Au-gate HEMT at a gate-source voltage V-gs of -5 V was as much as five orders of magnitude lower than that of the Ni/Pt/Au-gate HEMT under the as-deposited condition. The off-state breakdown voltage, defined as the gate-source voltage when the gate-source current is -1 mA/mm, was about -60 V for the Mo/Pt/Au-gate HEMT. These dc performances are comparable to those of the Ni/Pt/Au-gate HEMTs in which the Schottky contacts were improved with rapid thermal annealing at 500 degrees C. The Mo/Pt/Au-gate HEMTs also exhibited good rf performance without RTA. The cutoff frequency f(T) was more than 50 GHz and the maximum oscillation frequency f(max) was about 100 GHz. (c) 2005 American Vacuum Society.