Dopant effects on the response of gas-sensitive resistors utilising semiconducting oxides

Conductivity changes induced in semiconducting oxides as a consequence of the presence of small concentrations (ppm level) of reactive gases in air are presumed to be caused by non-equilibrium changes in surface coverage of ionised oxygen species (surface acceptor states). Responses of porous pellets of different oxide materials to small concentrations of different reducing gases in the air can usually be classified as either p-type (resistance increases) or n-type (resistance decreases). However, some materials show n-type responses to some reducing gases and p-type to others, and some materials can show a change in sign of the response with increasing concentration of the reducing gas; such effects can be brought about by systematic substitution into the lattice of complex oxides. This paper illustrates these effects and develops a simple model, the principal features of which are that the bulk donor density is low enough, the average grain size small enough and the surface acceptor state density high enough that the grains are fully depleted of conduction electrons. It is then shown that the conductivity goes through a minimum with increasing surface density of acceptor states. Since it is the surface density of acceptor states which is varied as a consequence of interaction with the reducing gas, the observations are thereby accounted for.