Macroscopic theory of surface acoustic wave gas microsensors

Recently it has been experimentally demonstrated (see J.F. Vetelino et al., 1987) that a surface acoustic wave gas microsensor can detect concentrations of certain gases in the parts per million to parts per billion range. Here, a theoretical model is presented which is capable of predicting the change in sensor response as a function of gas concentration. The geometry of interest consists of a metal oxide film on a piezoelectric substrate. It is assumed that on exposure to a gas, the most significant change in the film is the creation of free charge carriers which cause the film to change from a dielectric to a semiconductor. The free carriers then interact with the electric field associated with the acoustic wave traveling at the film-piezoelectric interface, causing a corresponding change in the sensor response. Results are presented for a tungsten trioxide film on a lithium niobate substrate which has been exposed to hydrogen sulfide gas. The theoretical results are compared to experimental data.<>