An interesting compound semiconductor Schottky diode hydrogen sensor based on a Pt-In0.5Al0.5P metal-semiconductor structure is fabricated and demonstrated. The hydrogen sensing characteristics, including hydrogen detection sensitivity and transient responses of the studied device under different hydrogen concentrations and temperatures, were measured and studied. The hydrogen detection sensitivity is related to a change in the contact potential at the Pt-semiconductor interface. Experimentally, the studied hydrogen sensor can be operated systematically under the applied bipolarity voltage biases. When the temperature is increased from 30 to 250 degrees C, the hydrogen relative sensitivity ratio (S-r), under the applied forward (reverse) bias of 0.3 V, is decreased from 181.2% (250.3%) to 28.2% (33.5%) upon exposing to the 9970 ppm H-2/air gas. Moreover, the hydrogen effect in the Schottky barrier height lowering is observed. The kinetic and thermodynamic properties of hydrogen adsorption were also studied. (c) 2007 The Electrochemical Society.