Current voltage (I-V) and capacitance voltage (C-V) measurements have been performed versus temperature on GaN Schottky diodes. The results show an increase of the Schottky barrier height ϕb and a decrease of the ideality factor n both with the increase of the temperature. We show that this behavior originates in the existence of an interface state density distribution, which is determined via the analysis of the temperature dependence of the I-V measurements, and allows the tunneling of the carriers from the semiconductor to the metal. Those interface states are shown to be responsible for interface inhomogeneities which result in two Gaussian voltage dependent Schottky barrier distributions. We show also that, in the presence of this interface state distribution, C-V measurements, without the correction of the built in voltage by taking into account the effect of both the high values of the ideality factor and series resistance, lead to erroneous values of the Schottky barrier height ϕb.