An AlGaN/GaN wide band-gap semiconductor. with the Ta/Ti/Al/Ni/Au ohmic contact (7.5 x 10(-7) Omega cm(2)) was demonstrated by 700 degreesC annealing for 1 min. High-resolution electron microscopy and synchrotron-radiation x-ray diffraction showed that nitride phases were formed at the interface between the metal and the AlGaN layer. The thick formation of TaN/TiN interfacial layers appears to be responsible for the good ohmic contact behavior in Ta/Ti/Al/Ni/Au metal scheme. The surface morphology of Ta-based contacts is superior to that of the Ti/Al/Ni/Au metal scheme, The fabricated heterostructure field-effect transistor exhibited the saturation drain current density of 605 mA/mm and transconductance of 246 mS/mm. (C) 2005 American Vacuum Society.