Oxygen plasma treatments have been performed prior to contact deposition on both n- and p-type GaN, and the effects of plasma pressure, rf power and treatment time on the contact characteristics are discussed. By exposing the surface of n-type GaN to an oxygen plasma prior to metal deposition, the as-deposited Ti/Al contacts change from rectifying to ohmic, and further improvements are observed after rapid thermal annealing (RTA). A specific contact resistivity better than 10(-7) Omega cm(2) was obtained using a plasma treatment of 20 s at 30 W and 0.2 mbar, followed by RTA at 500 degreesC in argon. The I-V characteristics of the Ti/Al contacts degraded when plasma treatments were performed for a longer time, at increased plasma pressure, or at higher rf power. However, unlike in the case for n-type GaN, oxygen plasma treatment prior to metal deposition deteriorated the electrical properties of the Ni/Au contacts to p-type GaN. X-ray photoelectron spectroscopy (XPS) was used in order to help elucidate the mechanism behind these effects. (C) 2004 Elsevier B.V. All rights reserved.