In this study, the mechanism for ohmic contact of Ti/Al bilayer formation on as-grown, etched and postetch annealed GaN surfaces were investigated. A nonalloyed Ti/Al ohmic contact to etched GaN surface, with postetch annealing prior to metal deposition, was obtained. The specific contact resistance of 2.3x10(-4) Omega cm(2) was obtained. The nonalloyed ohmic contact may be attributed to the postetch annealing which generates nitrogen vacancies that result in a heavily n-type surface thereby forming a tunneling junction. On the other hand, the nonalloyed Ti/Al contact on as-grown and as-etched GaN surfaces exhibits non-ohmic behavior. After alloying at 500 degrees C for 5 min, Ti/Al contacts on as-grown, as-etched and postetched annealing GaN surfaces have specific contact resistances around 9.8x10(-5,) 1x10(-4), and 7.2x10(-5) Omega cm(2), respectively. Nonalloyed Ti/Al ohmic contacts would be especially useful for fabricating high breakdown, recessed-gate field effect transistors on GaN since the moderate postannealing condition converts only the near surface layer to heavily n type.