Response mechanisms of hydrogen sensor based on a metal-insulator-SiC (MISiC) Schottky-barrier diode are analyzed. A physical model is established for the hydrogen sensor by combining thermionic emission with quantummechanical tunneling of charge carriers, and considering hydrogen-induced barrier-height modulation. Simulated results are in good agreement with experimental data. Relation between device performance and insulator thickness is investigated using the proposed model, and the optimal range of insulator thickness can be determined by taking into account the tradeoff between device sensitivity, reliability and resolution for high-temperature applications.