Ti/Al/Ir/Au Ohmic contacts on AlGaN/GaN high electron mobility transistors (HEMTs) show promising electrical performance, with lower specific contact resistance than obtained with the more conventional Ti/Al/Ni/Au metallization. HEMTs with both types of metallization have been measured up to 550 degreesC. We find that the dc performance of devices with Ir-based contacts is significantly better at each temperature up to this maximum value. with higher transconductance (g(m)) saturated drain-source current (I-DSS), and more stable threshold voltage (Vth). These contacts look very promising for HEMT power amplifier applications involving high temperature operation. (C) 2004 American Vacuum Society.