Electroreflectance studies of Pt/GaN Schottky diodes were performed in the temperature range between 5 and 300 K. The data were analysed using the electric field-dependent dielectric function of GaN and a multi-layer formalism. We observed a thermal activation of electric fields underneath the Schottky contact. The results are explained in terms of temperature-dependent ionised impurity concentration by a model with two donor levels. (C) 2003 Elsevier B.V. All rights reserved.