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We present a detailed investigation of the different factors that rule the temperature dependence of the AlGaN/AlN/GaN Hall-FET devices in the temperature range 300-500 degrees C. To understand the origin of the apparent increase in drive-in current density which affects devices above 300 degrees C, several series of experiments have been done. We checked: (i) the constitutive materials resistance to elevated temperature, (ii) the insulation of the buffer layer, (iii) the stability of the carrier density in the two-dimensional electron gas and, finally, (iv) the performance of devices after temperature cycling. We found that, both, the material properties and insulation of the buffer layers remains satisfactory up to 500 degrees C. The carrier density remains also very stable. However, we observe some material deterioration after temperature cycling which suggests that, in such non-passivated devices, the thermal drift could be due to partial deterioration of the surface layers and/or the gate Schottky contact. (C) 2008 Elsevier B.V. All rights reserved.