Gas sensitive thin metal-oxide-semiconductor (MOS) structures have been used to study the effect of different modifications of the gate metal. In order to increase the selectivity of the gas sensor photoresist was used as a gas permeable membrane. Furthermore thin polymer membranes containing molecular imprints against certain molecules were tested as the sensing layer.

MOS structures with thin discontinuous platinum as gate metal have previously been shown to be ammonia sensitive. After the metal gate was made hydrophobic by treatment with dichlorodimethyl silane, it was found that those structures when, put in anelectrolyte, served as gas permeable structures when their contact angle with water was sufficiently large. They could then be used as sensors for molecular species in the electrolyte. This was observed as a change of the electrical polarisation of the gate area, measured as a shift of the capacitance-voltage (C-V)-curve of a MOS capacitance or as a shift of the current-voltage (I-V)-curve of a MOS transistor along the voltage axis. In contrast to hydrophilic thin metal films, there was no change of the CV-curve when hydrophobic gates were exposed in electrolytes to different pH and/or ionic strengths. Charge migration out onto the semiconductor surface outside the metal gate was observed either as an increase of the inversion capacitance or as a photocapacitive current generated by a chopped light beam hitting the semiconductor surface at a distance from the contact. This charge migration is related to the wettability of the surface, and it occurs only if the surface has been in contact with ions. It is also connected to the differences between the ion sensitivity of hydrophobic and hydrophilic gates.