目 each band consists of a very large number of closely lying energy levels. The energy gap between the valence band and the conduction band is much smaller for a semi-conductor than that for an insulator,so that there is a real possibility for electrons to "jump the gap"into the empty band by thermal agitation  each band consists of a very large number of closely lying energy levels.  The energy gap between the valence band and the conduction band is much smaller for a semi-conductor than that for an insulator, so that there is a real possibility for electrons to “jump the gap” into the empty band by thermal agitation