actually forbidden energy gap is the energy barrier that exists between the valence band and conduction band of a material (not necessarily a metal) .energy bands are formed in case of solids dur to the overlapping of the energy levels in atoms which lie very close to one another .valence band is one which has got the outermost electrons and it cannot b empty.on teh other hand the conduction band is one to which electrons from the valence band move after they sare supplied with the energy greaterr thatn or equal to that of the forbidden energy gap..

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