344 · CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 1: GENERAL APPROACHES properties of the base oxide. At somewhat higher concentration it can contribute to limiting the electro- conductivity of the metal oxide matrix. And at the final stage, at a certain combination of electrocon- ductivity and gas sensitivity for two metal oxide phases in the gas-sensing matrix, the second oxide phase can produce either full blockage of interaction of the base oxide with the surrounding atmosphere, or shunting of the matrix of the base oxide through a more conductive second metal oxide phase. All this will certainly lead to a significant change of both the electrophysical and gas-sensing properties of the metal oxide matrix. In this case, these properties will not be determined by the base oxide. After analyzing results connected with the study of gas-sensing properties of SnO 2 modified by Ni, Fe, Co, and Cu, it was concluded that at excess concentration of the second oxide phase, the wors- ening of gas-sensing characteristics may also be conditioned by formation of additional p-n heterojunc- tions in the gas-sensing matrix. This conclusion was made on the basis of the following considerations (Morrison 1986; Williams 1999): a. Metal oxides formed by transition metals from the end of Period IV in the lower oxidation state, i.e., with a deficiency of oxygen, exhibit usually p-type conductivity. b. The adsorption of oxygen with formation of acceptor-like species on p-type semiconductors is difficult due to the lack of conduction electrons. c. Charge transfer in transition metal oxides, where the number of d electrons exceeds 3, is not de- scribed by the zone model. In this case we have to take into account the Geitler-London model