A novel gate-structure FET-type hydrogen sensor (Pt/Ti-FET) that has an adhesion layer (Ti) between the catalytic metal (Pt) and the gate insulator was fabricated. The dependence of the response characteristics of the sensitivity and selectivity on the Ti thickness was investigated. The sensitivity showed dependence on Ti thickness, and higher sensitivity was obtained with a decrement in Ti thickness. In addition, the Pt/Ti-FET showed good selectivity against other gases (O₂, C₂H₆) compared with the Pt-FET. After annealing at 260°C for 30 min under 5 Pa in a vacuum, the Pt/Ti-FET showed different response characteristics from an as-deposited Pt/Ti-FET. The annealed Pt/Ti-FETs with a thin Ti adhesion layer (below 10 nm) showed improved hydrogen sensitivity. In particular, the Ti 5-nm sample of the Pt/Ti-FET sensor showed a higher hydrogen response when exposed to 0.8% hydrogen gas (526.2 mV) compared to that of the Pt-FET sensor (415.7 mV). However, improvement in hydrogen sensitivity was not obtained in thick Ti samples. Additionally, the annealed Pt/Ti-FET showed better selectivity against O₂ than as-deposited Pt/Ti-FET.