The thermal effect of GaN Schottky diode on the I-V characteristics was simulated with the application of the forward bias voltage. The simulated diode had the cylindrical structure with the Schottky and ohmic contacts at the same top surface. The Schottky contact of circular shape with 340 mum diameter was surrounded by the ring-type ohmic contact of the inner and the outer diameters of 440 and 740 mum, respectively. The diode was doped with four different concentrations: 1 x 10(15), 5 x 10(15), 5 x 10(16) and 1 x 10(17)/cm(3). The simulated. currents were obtained by increasing the forward bias up to 30V in these four diodes of different doping concentrations. At low level of forward bias, the thermal effect,, assisted the ionization of doped atoms and thus increased the currents flow. The current and the thermal effect increased with the doping concentration. The maximum temperature increase of the diode with 1 x 10(17)/cm(3) doping concentration was as high as 140K above room temperature. The induced heat did not seem to be high enough to ionize all the dopants but produced the thermally generated electron-hole pairs. With the increase of forward bias, especially at high doping concentration, the thermal effect on mobility slowed down the current increase. The various behaviors in I-V curves were resulted from the different thermal effect with different doping concentration levels. At low doping, the thermal effect was small but the inclusion of the thermal effect showed the increase of current through assisting the ionization process. At high doping, there seemed to be the decrease in mobility with the forward bias accompanied with enhanced ionization process. So it was concluded that the consideration of thermal effect in evaluating the precise I-V curve is very important irrespective of doping levels. (C) 2004 Published by Elsevier B.V.