Interface states modulated reverse leakage current through metal/AlxGa1-xN/GaN diodes has been studied. Reverse leakage current and breakdown voltage have been measured over wide temperature ranges. The investigation suggests that the piezoelectric polarization and the spontaneous polarization at the AlxGa1-xN/GaN heterostructure create polarization induced charges in AlxGa1-xN, and influence the two-dimensional electron gas at the GaN/AlxGa1-xN interface. Both of them dictate the barrier height of the strained AlxGa1-xN Schottky contact. High density of defect states at the metal/AlxGa1-xN interface leads to high reverse leakage current via Fowler-Nordheim emission and/or Frankel-Poole emission. The most notable finding is the reduction in the leakage current almost to zero at high temperatures due to strain relaxation, reduction in defect states, and barrier height enhancement at the metal/AlxGa1-xN interface. Such finding may have significant impact on the way we design high-power microwave devices.