A direct comparison of Ti/Al/Ir/Au Ohmic contacts on AlGaN/GaN high electron mobility transistors with the more conventional Ti/Al/Ni/Au metallization is reported. The Ir-based contacts produce lower specific contact resistance (4.6 x 10(-5) Omega cm(2) compared to 2 x 10(-4) Omega Cm 2) after annealing at 850degreesC for 30 s, lower interdevice isolation currents, higher peak transconductance (133 mS/mm compared to 121 mS/mm), and higher device breakdown voltage (31 compared to 23 V) than the Ni-based contacts fabricated on the same wafer. The results are based on mapping of a large number of high electron mobility transistors (similar to250) on a 2-in.-diam wafer and show strong evidence that a relatively simple change in Ohmic metallurgy provides benefits in device dc and rf performance. (C) 2004 American Vacuum Society.