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The thermal annealing effects on Ti/Al/Ni/Au Ohmic contact to n-GaN are investigated by current–voltage (I–V) characteristics and transmission line method (TLM) measurements. The cladding layer of Ni/Au on Ti/Al plays two roles: preventing inter-diffusion of Ti, Al, Au and anti-oxidation of the contacting layer. The specific contact resistance (ρc) of Ti/Al/Ni/Au to n-GaN increases slightly at first with the increasing annealing temperature (Ta). When Ta increases above 500 °C, ρc decreases monotonously in the range of 400–900 °C. However, the morphology of the contact degrades gradually when Ta increases above 600 °C. The minimum of ρc is obtained as 9.65×10-7 Ω cm2 by two-step annealing method in this work. Finally, the roles of two-step annealing method in the formation mechanism of the Ohmic contact to n-GaN are also discussed. [Copyright &y& Elsevier]