The model of electron transport based on phonon-assisted tunnelling of electrons, was used as a basis for working out the theory of temperature dependence of thick film resistors (TFR). Tunnelling between localized impurity states in glass was assumed. The theory takes into account the effect of the activation energy, as well as that of the thermal expansion of the substrate. The contribution of the metallic type of conductivity to TFR resistivity is interpreted on the basis of the concept of overlapping wave functions of impurity d-states due to Ru admixtures in the glass. If the distances between impurities are smaller than the critical distance, a band of delocalized states arises which allows the metallic type of charge transport to take part in the process.
