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Electrical transport phenomena have been investigated in (Al,Ga)N/GaN heterostructures (without any intentional doping) with an Al content ranging from 10 to 32 %. Conductivity and Hall Effect measurements have been performed as a function of temperature up to 700 K. To analyse the temperature dependence of the two-dimensional electron gaz concentration (2DEG) n, a fully self-consistent calculation (coupled Schrodinger and Poisson equations), was developed for the conduction band edge of the heterostructures. The observed 2DEG concentration changes vs. temperature have been compared with the theoretical predictions taking into account the electric fields in the structure and the surface states. A calculation based on the relaxation time approximation has been developed to describe the temperature dependence of the mobility. The electrical performance of GaN/AlxGa1-xN heterostructures with x=0.32 % indicate that they could be good candidates for Hall sensor working at high temperature (up to 600 K). At room temperature, the magnetic field sensitivity is K K-H0= 60 V/A/T. Its temperature sensitivity S, is lower than +500 ppm/K up to 600K. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.