do you know the main difference between insulators and semiconductors

 A conductor readily conduct electric current, they have excess free electrons, according to band energy theory;  the conductor, semiconductor and insulator are separated by sort of energy gaps. Namely the conduction band  , valence band  and forbidden gap.

 The conductor has only two energy gaps, the conduction band and the valence band, the two band are merged, electrons flow readily from the conduction band to the valence band .

 In semiconductors three regions exist, the conduction, valence and forbidden gap.  The forbidden  gap is narrowed , a little excitation can bridge the forbidden gap and electrons can easily move from the conduction band to the valence band whereas an insulator has all the three energy band,  but here the forbidden gap is very wide that no amount of thermal excitation can cause electrons to flow from the conduction band  to valence . Therefore the main difference between insulators and semiconductor is that the forbidden gap in  the semiconductor can be bridged while that of insulators cannot be bridged