The thermal stability of the Ti/Al, Ti/Al/Ni/Au, Cr/Al and Cr/Al/Ni/Au ohmic contact metallizations on n-GaN were investigated. Bilayers of Ti/Al and Cr/Al, with and without Ni/Au overlayers, were deposited on n-GaN using an e-beam deposition method and ohmic contacts formed by a vacuum annealing technique. For both the Ti/Al and Ti/Al/Ni/Au minimum specific contact resistivities of similar to1 x 10(-5) Ohm cm(2) were achieved after vacuum annealing at 1100 degreesC for 2 min. Minimum specific resistivities of similarly formed Cr/Al and Cr/Al/Ni/Au ohmic contacts were 3.8 x 10(-5) and 2.3 x 10(-5) Ohm cm(2) respectively, obtained at 700 and 950 degreesC respectively. The above mentioned contacts were exposed to long term aging at temperatures of 300, 400 and 500 degreesC, for periods up to 100 h and their electrical and morphological characteristics were monitored at the various stages of the aging process. The Ti/Al and Cr/Al/Ni/Au proved to be the most stable of the four metallizations in terms of both electrical performance and surface morphology. The Cr/Al had shown stable morphological stability, but displayed the poorest electrical performance characteristics as a result of the aging process. (C) 2002 Elsevier Science Ltd. All rights reserved.