Then the region of intrinsic ionization should be at temperatures much higher than room temperature (note a slight increase in the slope of the lg(1/T) dependence for the FeO sample in Fig. in the high temperature region). In contrast, as the temperature decreases, the slope of the activation sections decreases, or the lg(1/T) dependencies reach saturation. In doped semiconductors, such behavior is usually associated with the transition from impurity conductivity to conductivity via the impurity band at decreasing temperature .