Metal Oxides and Sulfides as Extrinsic Semiconductors

Many metal oxides and sulfides exhibit semiconductor properties by virtue of their nonstoichiometry. In Section 5.3, we distinguished positive (p-type) and negative (n-type) semiconductors. 

The black phase persists on cooling. It contains 2Ni + in place of 2Ni +, plus a vacancy in the Ni sublattice, for every additional 0 . The Ni + ions, being smaller and more highly charged than Ni , act as charge carriers by skipping through vacant sites under an applied external electrical potential. 

It is possible to make nonstoichiometric solids that have ionic conductivities as high as 0.1-1000 S m (essentially the same as for liquid electrolytes) yet negligible electronic conductances. Such solid electrolytes are needed for high energy density electrical cells, fuel cells, and advanced batteries (Chapter 15), in which mass transport of ions between electrodes is necessary but internal leakage of electrons intended for the external circuit 

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