How can non-stoichiometric FeO act as a p-type semiconductor?

Consider iron oxide (FeO), which is made up of Fe2+ and O2- ions. On heating

FeO, some Fe2+ ions are oxidized to Fe3+ ions, thereby giving compounds with compositions ranging from  to . These oxides are actually a combination of FeO and Fe2O3.

For example, a crystal of compound  actually contains.

Since Fe3+ ion is short by one electron with respect to Fe2+ ion,  Fe3+ in the

FeO crysal lattice can be considered as a positive hole.

Consequently, under the influence of electric field, the positive holes can move from Fe3+ ions to Fe2+ ions, thereby producing electrical conductivity. Therefore, such non-stoichiometric crystals act as p-type semiconductors.