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 cm(2)/V s showed ohmic behavior with a specific contact resistance on the order of 0.1 Omega cm(2). In contrast, Ni contacts deposited on Si-doped GaN with a mobility of over 100 cm(2)/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.