The electrical characteristics of the indium tin oxide (ITO) contacts on n-GaN with various doping concentrations have been studied. Ohmic behavior was observed for ITO films on highly doped n-GaN (n = 1 x 10(19) cm(-3)) without thermal annealing and the measured specific contact resistance was 5.1 x 10(-4) Omega cm(2). This result could be attributed to the formation of a tunneling junction on the heavily n-type GaN surface. However, as the thermal annealing was performed to the ITO/n-GaN (n = 1 x 10(19) cm(-3)) Ohmic contact, it exhibited Schottky characteristics. This result might be due to the microscopic interfacial reaction among In, Sn, O and GaN and their alloys which extend into GaN films, thereby influencing the electrical properties of ITO/n-GaN contacts. (C) 1999 Elsevier Science Ltd. All rights reserved.