Abstract
It is convenient to classify the impurity states in semiconductors into three groups: low, intermediate, and high. Even at high concentrations anomalous (i.e., nonmetallic) properties are observed. A simple "inhomogeneity" model is proposed in which the impurity states are regarded as spatial mixtures of metallic and nonmetallic regions. The model provides an explanation of the anomalous properties, at least qualitatively.