Ever had to explain to your teenage son why LEDs or solar cells work? If so, you might appreciate the discussion of bandgaps recently posted on M.I.T.'s Web site. For those of us who are a bit fuzzy on valence bands and conduction bands, the explanation offered is quite helpful and down-to-earth, and completely understandable by anyone who knows what electrons are. One selection:
"When electrons get excited (by getting heated, or by being hit with a particle of light, known as a photon), they can jump across the gap. If an electron in a crystal gets hit by a photon that has enough energy, it can get excited enough to jump from the valence band to the conduction band, where it is free to form part of an electric current. That’s what happens when light strikes a solar cell, producing a flow of electrons."
Here's the link: http://web.mit.edu/newsoffice/2010/explained-bandgap-0723