BAND AID

by Dr. James D. Livingston

Pessimists see the band as half empty
Optimists see it as half full
Either way, with full states next to empties
Electrons respond to field's pull

And with empty states next to full ones
Low-E photons electrons excite
So partly-filled bands allow metals
To conduct and be opaque to light

But if bands are all full or all empty
And the gaps between bands are quite large
Light goes through and there is no conduction
Just some bands full of immobile charge.

Ah, but if the gap is a bit smaller
Charge can jump it occasionally
And we now have a semiconductor
And an electronics industry

They've more glamour than good insulators
And more glamour than metals, it seems
This logic leads us to the moral:
It's best to avoid all extremes

Energy bands for Magnesium. (Click to enlarge.) Even though Mg is divalent, it is still a metal, because the 3s and 3p bands overlap, as shown here.
(Image taken from this site of chemistry images.)

Britney Spears guide to semiconductor physics
(Other than the pictures of Britney, this is actually a serious physics site!)