Typically, the activation energy in physics is defined as the minimum amount of energy which electrons of donor (acceptor) impurity must to receive in order to get into the conduction band.

However, in the case of the hopping conductivity, it can be defined as energy required to hope from one defect state to another. As the electron can jump only from the busy donor on free, a necessary condition of hopping conductivity is existence of empty seats in an impurity zone which at low temperatures can be provided only with compensation, i.e. introduction of the acceptor impurity which is taking away part of electrons from donors.

Best regards, Leonid

In metal oxides, the electronic charge carriers are the electrons and holes. The concentrations of the charge carriers are directly related to the defect structure of the oxide.

A donor is a defect with an electron close to the conduction band. It is thus easily ionized to give an electron in the conduction band, given rise to n-type conductivity Similarly, an acceptor would accept an electron from the valence band and is called p-type conductivity. Hence, an electronically conducting oxide is an n-type semiconductor if transport of electrons predominates and a p-semiconductor if holes triumph.

The probability to excite free carrier due to thermal activation can be expressed as exp(-Eact/kT), where Eact is the activation energy which depend on the Fermi energy,  kT is the product of the Boltzmann constant, k, and the temperature, T. The activation energy is Fermi-energy dependent.

• In the intrinsic semiconductor at zero Kelvin,   Eact =1/2Eg, where Eg is the band gap of the material. 
•  For the n-type, Eact =1/2(Ec-Ed), Ed is the donor level, Ec-conduction band energy.
•  For the p-type semiconductor, Eact=1/2(Ev-Ea), Ea is acceptor level energy, Ev is the valence band energy .

Thank you for your answers.

why are some metal oxides n-type and some metal oxides p-type?