[thegoodwolf]

What is the reason that certain materials have different differences in energy between the conduction band and the valence band?  I understand that for metals it is the relative attraction between the valence electrons and the nucleus, and since the attraction is quite low there is essentially no energy difference between the valence band and the conduction band (this allows delocalized electrons).  But for semi conductors, why would the gap for one semiconductor be greater/less than another?