The effect of defects in n-type GaN epitaxial layers on the electrical characteristics of Ni contacts was studied. The defect density of GaN layers was characterized, and Ni dots deposited on four n-GaN wafers with different mobilities were examined by current–voltage (I–V) and capacitance–voltage (C–V) measurements. Ni contacts deposited on undoped GaN with a mobility of 6.7 cm2/V s showed ohmic behavior with a specific contact resistance on the order of 0.1 Ω cm2. In contrast, Ni contacts deposited on Si-doped GaN with a mobility of over 100 cm2/V s exhibited Schottky behavior with a Schottky barrier height of 0.75 eV from I–V and 1.10 eV from C–V. These results suggest the formation of small areas with low barrier height at the interface due to defects and dislocations. © 1999 American Vacuum Society.