EXAMPLE 3.18 Predicting extrinsic semiconducting properties Which of the oxides WO, Mgo, and Cdo are likely to show p or n-type extrinsic semiconductivity? Answer The type of semiconductivity depends on the defect levels that are likely to be introduced which Is, in turn, determined by whether the metal present can be easily oxidized or reduced. If the metal can easily be oxidized (which may be the case if it has a low oxidation number), then n-type semiconductivity is expected. On the other hand, if the metal can easily be reduced (which may be the case if it has a high oxidation number), then ptype semiconductivity is expected. Thus, Wo,, with tungsten present in the high oxidation state WVI), is readily reduced and accepts electrons from the O2 ions, which escape as elemental oxygen. The excess electrons enter a band formed from the W d orbitals, resulting in n-type semiconductivity Similarly, Cdo, like ZnO, readily loses oxygen and is predicted to be an n-type semiconductor. In contrast Mg+ ions are neither easily oxidized nor reduced, therefore MgO does not lose or gain even small quantities of oxygen and is an insulator Selftest 3.18 Predict p- or n-type extrinsic semiconductivity for V,O, and Coo.
