We have studied the microscopic electronic characteristics of Ni-Au layers on Mg-doped GaN, using electron beam induced current to measure the carrier separation associated with built-in electric fields. Scanning electron microscopy reveals that the metal layer has a large density of Ni islands within a Au matrix. An enhancement in the charge collection current is observed at these islands, in comparison with the matrix. These observations are explained by a local reduction in the Schottky barrier at the Ni islands. © 2005 American Institute of Physics.