Fig 6Why some solids conductelectron energies in a solidsilicon at 300 K
electron free to movehole left behindIn a solid, the electrons' allowed energies merge into bands. Electrons can only carry current if there are empty energies just above theirs to move into. A metal's top band is only partly filled, so its electrons always can. In an insulator the top filled band is separated from the next empty one by a large gap, so no electron can move. A semiconductor has a small gap, and heat lifts a few electrons across it, leaving holes behind. Raise the temperature and the number of free electrons in silicon climbs steeply, so semiconductors conduct better when hot, while metals conduct worse.