There is a particular interest in the development of wide band gap semiconductor gas sensor because of their potential for high temperature operation and the ability to integrate them with power or microwave electrodes or with UV solar-blind detector and emitters fabricated in the same materials. AlGaN based devices are attractive for gas sensing in automotive exhausts and flow-gas, because of strong spontaneous polarization of AlGaN (free carrier concentration profiles inside this material that is very sensitive to any manipulation of surface change). In this report, we characterized the Ni/AlGaN/Sapphire Schottky barriers as hydrogen gas sensor at temperature range of 25 degrees C to 500 degrees C. A change in forward current was obtained in response to a change in ambient from pure N-2 to 2% H-2/98% N-2, higher than the change in forward current obtained in Ni/GaN or Ni/Si Schottky diodes measured under the same conditions. The sensor response time was independent on the rate of mass transport of gas into the test chamber, while at high temperature, dissociation of gas controlled by the diffusion of atomic hydrogen through the metal/AlGaN surface, increased the sensor response time.