Ohmic contact formation by Ti/Al/Mo/Au, Mo/Al/Mo/Au, and V/Al/Mo/Au on AlGaN/GaN heterostructure field effect transistor layers have been studied and compared. Mo/Al/Mo/Au ohmic contacts exhibited the lowest contact resistance of 0.22 +/- 0.02 Omega mm over a range of anneal temperatures from 650 to 800degreesC. The minimum contact resistances of Ti/Al/Mo/Au and V/Al/Mo/Au ohmic contacts were measured to be 0.38 +/- 0.04 Omega mm at an anneal temperature of 800degreesC and 0.35 +/-0.07 Omega mm at an anneal temperature of 700degreesC, respectively. Long-term thermal stability measurements were performed at 500, 600, and 700degreesC. Of the three metallization schemes, Ti/Al/Mo/Au exhibited the best thermal stability (up to 8.5 h at 700degreesC). Intermetallic reactions were investigated using Auger electron spectroscopy and x-ray diffraction measurements. (C) 2004 American Vacuum Society.