Abstract: An ensemble Monte Carlo simulation has been developed to simulate the motion of electrons in a submicron GaN diode with a Al_xGa_1-xN heterojunction cathode. It is shown that the hot electron injection through the heterojunction cathode is effective to increase the mean electron velocity of carriers. The analysis has also shown that the mean drift velocity for electrons in the channel is about 2x10⁵ m sec^-1 at bias 4 V. Mean drift velocity in channel decrease with temperature and reach to saturated value about 1.5x10⁵ m sec^-1.