Pt/AlGaN/AlN/GaN Schottky diodes are fabricated and characterized for hydrogen sensing. The Pt Schottky contact and the Ti/Al/Ni/Au ohmic contact are formed by evaporation. Both the forward and reverse currents of the device increase greatly when exposed to hydrogen gas. A shift of 0.3V at 300K is obtained at a fixed forward current after switching from N₂ to 10%H₂+N₂. The sensor responses under different concentrations from 50ppm H₂ to 10%H₂+N₂ at 373K are investigated. Time dependences of the device forward current at 0.5V forward bias in N₂ and air atmosphere at 300 and 373K are compared. Oxygen in air accelerates the desorption of the hydrogen and the recovery of the sensor. Finally, the decrease of the Schottky barrier height and sensitivity of the sensor are calculated.