Electrons and holes in semi-counductor

Metals have free electrons available in conduction band for conduction of electric current. As the valence band overlaps the conduction band and no forbidden energy gap exists in the conductor, small electric field is sufficient to move an electron into higher state without creation of hole. The flow of electrons constitute electric current.

Hence only electrons are charge carriers in metals like aluminium, copper, sliver etc.

In semiconductors, the forbidden energy gap is small. At room temperature, the electrons from valence band jump into conduction band creating a hole in valence band. When the semiconductor is connected to terminals of the battery, electrons move towards positive terminal and holes move towards negative terminal.

In the applied electric field, the electron occupies the hole by moving towards positive terminal and a new hole is created in the place of electron moved. This hole acts as a positive charge carrier, while electron acts as a negative charge carrier. Both these electron and hole currents constitute the total current.

                 I = I_electrons + I_hole