Conductors are those substances in which there is an overlap between valence band and conduction band (i.e almost no energy difference between valence and conduction band) so that electrons can easily move from valence band to conduction band and conduct electricity. For ex- Metals
In case of Semiconductors, there is small energy difference between valence band and conduction band(≈1eV, in Ge it is 0.78eV & in Si, it is 1.21eV) so that on applying electric field, that amount of energy is easily acquired by electrons in order to move from valence band to conduction band. Eg- n-type & p-type semiconductors
Insulators are those substances in which there is large energy difference with respect to electrons present in valence band and conduction band & which cannot be achieved by electrons pressent in valence band from any source.
The energy difference is called forbidden energy gap(For eg- Diamond, 6.6eV)