NEW TRENDS AND FUTURE-PROSPECTS OF THICK-FILM AND THIN-FILM GAS SENSORS

There are a number of ways in which the interaction of gas molecules with a semiconducting oxide can be successfully exploited in order to provide information about the composition of the atmosphere. A wide range of gases is capable of reacting with surface oxygen ions to produce neutral products and to re-inject electrons into the oxide valence band. For n-type oxides this process produces a conductivity increase and for p-type oxides a conductivity decrease results. Surface conductance changes may also arise from a number of other mechanisms: (1) The products of surface oxidation reactions may produce a secondary effect. For example, the surface combustion of gas molecules that contain hydrogen generally produces water, which contributes a change in conductance. (2) Gases that do not undergo reactions with surface oxygen may cause resistance changes by a direct chemisorption process. (3) Charge may be transmitted between surface-adsorbed species and the conduction band of a semiconducting oxide via surface particles of precious metals such as platinum. The various processes mentioned above are all capable of giving rise to changes in the measured conductivity of semiconducting artifacts. In addition, it is possible to monitor changes in the atmospheric composition via gas reactions taking place at the surface of a solid electrolyte in contact with the semiconductor surface. In this case measurements are made of potential rather than resistance.