MOS hydrogen sensors with ultrathin layers of palladium

MOS gas sensors were fabricated on n- and p-type silicon (5-7 Ω. cm) with thermal oxide layers ranging in thickness from 66 to 269 Å. A layer of palladium 30 Å thick was deposited through an evaporation mask. A layer of gold 350 Å thick was deposited over the palladium to provide a continuous electrical contact. Measurements were made of the capacitance at 1 MHz as a function of gasambient and time. The devices showed sensitivity to hydrogen at room temperature. Compared to devices fabricated with 300-Å layers of palladium, the thin film devices showed much faster response and recovery at room temperature and reduced magnitude of response. Devices fabricated on p-type silicon were noticably faster than those fabricated on n-silicon. First-order reaction kinetic fits showed general agreement with the data.