A multi-step rapid thermal annealing process of Ti/Al/Ni/Au was investigated for ohmic contact of AlGaN/GaN high electron mobility transistor (HEMT). The samples were studied by Transmission Line Model (TLM), Scanning Electron Microscopy (SEM), Auger electron spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) measurements. By the multi-step annealing process, the specific contact resistance was decreased from 10(-5) Omega cm(2) level to 4-3 x 10(-6) Omega cm(2) and the surface morphology was improved. The AES measurements showed that the limitation indiffusion of Au and outdiffusion of Al were account for the surface morphology improvement and the surface Fermi level towards the conduction-band edge resulted in a lower specific contact resistance. (C) 2009 Elsevier Ltd. All rights reserved.