EFFECTS OF MOBILE DONORS ON POTENTIAL DISTRIBUTION IN GRAIN CONTACTS OF SINTERED CERAMIC SEMICONDUCTORS

Sintered ceramic semiconductors can be used to monitor the surrounding gas atmosphere. It is based on the conductivity response of the semic onductor to the surface reactions on the grains of porous material. So me nonstoichiometry defect donors, like oxygen vacancies in pi-type se miconductors, may be mobile at relatively low temperatures, where semi conductor gas sensors operate. In such case, the donor concentration m ay considerably decrease in the space charge layer at the semiconducto r surface, which may be reflected in the transport properties of elect rons through the neck contacts between grains. We model here various n eck contacts between spherical grains and compute the electrical poten tial at the conductive region from the Poisson-Boltzmann equation in c ases of mobile single and double donors. The solutions are evaluated n umerically with a finite difference method. According to the Schottky- defect model for oxygen vacancies, the surface concentration of donors is kept a fixed parameter in the calculations. The chosen material pa rameters are those of tin dioxide, which is the key material of semico nductor gas sensors. It is found that mobile donors may strongly modul ate the distribution of electrical potential around the neck contacts, and thus, modify the transducing properties of the microstructure of ceramic semiconductor gas sensors. (C) 1996 American Institute of Phys ics.