Pd-gate MIS hydrogen sensors have serious drift problems and decreased sensitivity after long term operation. We have shown that the serious drift can be eliminated with Pd/Pt hydrogen sensing gate and Ag/Cu/Pd/Pt reference gate differential-type sensors. The decrease of sensitivity can be miniized by high temperature annealing in air. The decrease of sensitivity after 1000 hours of operation is less than 10%. It is considered that this decrease is due to a decrease of hydrogen adsorption sites on the Pd gates surface, which becomes oxidized during high temperature operation in air. This oxidation is observed with XPS and AES. The sensitivity abruptly decreases for operation at 100°C and below. This decrease is due to water adsorption on the Pd surface. H2O and OH peaks are observed in FTIR spectra for Pd powder in 1% hydrogen gas in synthetic air, and the solubility of Pd metal for hydrogen abruptly decreases because of water adsorption on the surface at 100 °C and below.
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