With the advent of bulk single crystal AlN substrates, AlxGa1‐xN epilayers can be grown pseudomorphically, under a compressive strain, with Al composition greater than 70%. Temperature dependent Hall effect measurements showed that Si dopant in AlxGa1‐xN has an activation energy ranging from 10 to 25 meV for x < 0.8, while the activation energy increases rapidly for x ≥ 0.85. Planar Ni/Au Schottky diodes fabricated on AlxGa1‐xN exhibit good diode rectification. Based on current‐voltage (I‐V) and capacitance‐voltage (C‐V) measurement, the effective Schottky barrier height was observed to monotonously increase with the increase of Al%. For a film with an n‐type carrier concentration of 1x1018 cm‐3 and Al content of 81%, the effective barrier height reached 1.8 eV with an ideality factor of 1.46. However, C‐V consistently showed much higher barrier height than I‐V, suggesting surface oxides play an important role in Ni/Au Schottky diode on AlGaN. The reverse breakdown voltage was also measured in a vertical structure without edge termination. The obtained high breakdown voltage exceeds the theoretical limit of 4H‐SiC at a similar carrier concentration, suggesting great potentials for high power switching application of AlGaN on AlN. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)