We report the experimental studies of hot electron energy relaxation and capture in a low electron mobility GaN/ GaAIN HEMT structure grown with MBE on sapphire substrate. Pulsed I- V characteristics indicate that at elevated electric fields, current pulse decays exponentially with time. The threshold electric field (F(th)) above which the current pulse exhibits the decay, increases with temperature (E(th)similar to200V/cm at T = 77K, E(th)similar to350V/cm at T = 300K). The magnitude of the decay decreases with increasing temperature and increases with applied field. Time constant associated with this decay decreases rapidly with increasing applied field. These observations indicate the presence of hot electron capture over potential barriers. Electron temperature as a function of applied electric field is obtained by comparing the measured electric field dependence of the mobility mu(E) at a low lattice temperature, with the lattice temperature dependence of the mobility at a low electric field. Electron energy loss rate is then determined from the electron temperature dependence of the power loss using the power balance equations. Our results show that the experimental power loss has an exponential factor, hw/k which is much smaller than the value expected from LO phonon emission. Furthermore, the magnitude of the experimental power loss is also much lower than the theoretical value. This observation is explained in terms of capture of hot electrons. (C) 2002 Elsevier Science B.V. All rights reserved.