Doping of semiconductors

Controlled valancy semiconductors:

Some transition metal compounds can be conductors due to the presence of an element in more than one oxidation state. NiO is a very good example. On oxidation the compound goes black and becomes a relatively good conductor. Some of the Ni²⁺ ions have been oxidised to Ni³⁺ and some Ni²⁺ ions diffuse out to maintain charge balance leaving cation holes.

The reason for the conduction is the ability of electrons to transfer from Ni²⁺ to Ni³⁺ ions. This basically allows the Ni³⁺ ions to move and black NiO is therefore a p-type semiconductor. Slightly different to the p-type discussed earlier this type is known as a hopping semiconductor because the transfer process is thermally controlled and therefore highly dependent on temperature.

This makes controlling the conductivity a tricky process. Therefore controlled valancy semiconductors rely on control of the concentration of, in this case, Ni³⁺ ions by controlled addition of a dopant (such as lithium). Instead of having NiO, you now have Li⁺_xNi²⁺_1-2xNi³⁺_xO, hence, the concentration of Li⁺ ions controls the conductivity.