A STUDY OF THE CONDUCTANCE AND CAPACITANCE OF PURE AND PD-DOPED SNO2 THICK-FILMS

Tin oxide based gas sensors are, at present, used in a variety of different applications and seem the most promising due to their inherent ability to undergo gas-induced conductivity changes even in the presence of low gas concentration. The gas response of thick-film tin dioxide depends crucially on electronic surface states involving absorbed oxygen, and on the rates of combustion reactions involving the gases to be detected. Despite their high sensitivity and short response time the devices generally exhibit low selectivity and limited long-term stability and are influenced by interfering gases, especially water vapour. The conductivity of high-porosity thick films, where the diameter of the grains is large compared with the Debye length, is described by the Schottky barrier model. The height of the energy barrier is extremely sensitive to the presence of additives, impurities, catalysts and water vapor, and the difference between the minimum and maximum of the energy barrier as a function of temperature has been proved suitable to be related to the sensitivity properties. Significant changes in the capacitance are also expected due to the large variation of charges in the surface states and in the space-charge layers during interaction with reducing gases. The aim of the present work is to study the behaviour of the space-charge and surface-state capacitance together with the conductance in the presence of reducing gas on screen-printed pure and Pd-doped samples. The corresponding depletion layers, that is, the Debye lengths of the electrons, are also discussed.