In extrinsic semiconductors

In extrinsic semiconductors, formation of donor or acceptor level takes place in forbidden energy gap. Thus, some of the electrons in valance band acquire enough energy to jump to donor level or acceptor level present in forbidden energy gap. Thus, to obtain large number of electrons in conduction band some energy to be supplied to the electrons in valance band. For extrinsic semiconductors, it is nearly equal to 1 eV.