The low field mobility of carriers at an AlxGa1-xN/GaN hetero structure is investigated using an ensemble Monte Carlo method. The mobility values are obtained in the temperature range 77-1020 K using the energy levels, electron concentrations and wavefunctions obtained from the self-consistent solution of Poisson/Schrodinger equations at the hetero interface. The free electron concentration at the interface results solely from the spontaneous and piezoelectric polarization charges at the interface and the exchange-correlations effects are also considered in the potential energy calculations. Both the role of interface roughness scattering on mobility as a function of temperature and the dependence of carrier concentration at the AlGaN/GaN hetero interface on Al-content are investigated in detail. We found that interface roughness scattering is very effective especially at low temperatures on the drift mobility and it has no observable effect beyond a certain temperature. While increasing alloy composition increases the free electron concentration, it reduces the mobility of electrons because the steepness of the potential at the junction increases. The results of our simulations well-matched with the existing experimental data. (C) 2013 Elsevier Ltd. All rights reserved.