On the Low‐Temperature Response of Semiconductor Gas Sensors

The present paper compares three different kinds of semiconductor gas sensing materials: metal oxides (MOX), hydrogen‐terminated diamond (HD), and hydrogenated amorphous silicon (a‐Si:H). Whereas in MOX materials oxygen is the chemically reactive surface species, HD and a‐Si:H are covalently bonded semiconductors with hydrogenterminated surfaces. We demonstrate that these dissimilar semiconductor materials exhibit the same kind of low‐temperature gas response. This low‐temperature response‐mechanism is mediated by a thin layer of adsorbed water with the semiconductor materials themselves acting as pH sensors. In this adsorbate‐limited state the gas sensitivity is limited to molecular species that can easily dissolve in H 2 O and subsequently undergo electrolytic dissociation. At higher temperatures, where a closed layer of adsorbed water can no longer exist, the gas response is limited by direct molecule‐semiconductor interactions. In this latter mode of operation, MOX gas sensors respond to adsorbed gases according to their different oxidising or reducing properties. Hydrogenated amorphous silicon (a‐Si:H), on the other hand, exhibits a significantly different cross sensitivity profile, revealing that gas‐surface interactions may largely be restricted to analyte molecules with lone‐pair and electron‐deficient three‐centre orbitals.