RECENT DEVELOPMENTS IN SEMICONDUCTING THIN-FILM GAS SENSORS

A critical review of thin-film semiconducting materials that are used as gas sensors is presented in this paper. These materials can be approximately divided into two main groups: the first includes sensors that detect oxygen with variations of bulk or surface conductance; the second comprises all the materials that detect oxidizing and reducing gases in air at constant oxygen partial pressure by means of surface-conductance variations. The oxygen-detection mechanisms of surface- and bulk-conductance sensors are described. The long-term stability and the selectivity to a particular species are the main properties of a reliable gas sensor; these properties are also discussed in detail in relation to the second group of sensors. The properties of tin oxide, the most investigated material for gas sensing, are reviewed; some new ternary semiconducting compounds are also presented, together with their growth techniques. An almost novel method for growing thin films, the RGTO (rheotaxial growth and thermal oxidation) technique will be presented, since it is capable of preparing mixed oxide thin films with high surface area and nanosized crystallites. It is well known that the main possibility for sensor development lies in device arrays; therefore, the utilization of different materials for gas detection and the measurement techniques of signals supplied by sensor arrays will be outlined.