Distinguish between metals, insulators and semiconductors on the basis of their energy bands.
The energy band diagram for metal is shown in fig. (i).
Insulators: In insulators, the valence band is completely filled, the conduction band is empty and energy gap is quite large (Eg> 3 eV). As a result of huge energy gap, no electron will jump from the valence band to the conduction band even if electric field is applied. The energy band diagram of insulator is shown in figure (ii). 
Thus, electrical conduction is not possible and hence these materials behave as insulators. eg. diamond ( Eg = 6 eV).
Semiconductors: In semiconductors, the valence band is completely filled and conduction band is empty but, energy gap between them is less than 3 eV. The energy band diagram of a semiconductor is shown in figure (c.) 
At 0 K, conduction band remains totally empty because the electrons in the valence band are not able to get excited to the higher level. However, at room temperature, some electrons in the valence band acquire thermal energy greater than energy band gap and, jump over to the conduction band where they are free to move under the influence of even a small electric field. As a result of it, the semiconductor acquires small conductivity at room temperature.
The resistance of semiconductor would not be as high as that of insulator.
