CHEMICAL SENSORS WITH CATALYTIC METAL GATES - SWITCHING BEHAVIOR AND KINETIC PHASE-TRANSITIONS

Rapid transitions in the response of platinum-based chemical sensors o ccurring at given hydrogen-oxygen concentration ratios are explained b y kinetic phase transitions or switching phenomena on the catalytic me tal surface. Below the transition point the response of platinum-insul ator silicon carbide devices is small and above the transition it is l arge. It is found that the critical ratio depends on the operation tem perature and the properties of the device. Three different cases are i dentified, namely, injection-, diffusion-, and reaction-rate-determine d transitions. At sufficiently large temperatures the transition is in jection limited and occurs at the stoichiometric ratio of hydrogen and oxygen in the gas mixture. The implications of the experimental obser vations on the applications of chemical sensors with catalytic sensing layers are discussed.