219. How do photoconductors work?
When the atoms and molecules in a solid join together, some of their electrons may become shared between them. These electrons can travel about the solid as waves. Because they travel as waves, they can only follow paths that bring them back perfectly in phase with how they started out, like steady ripples on a pond. As a result, they can only follow certain paths and can only have certain energies. For complex and fundamental reasons, only two electrons can adopt any particular path, so the electrons take turns filling up all of these paths or "levels" from the lowest energy ones up. The electrons fill up these levels until there are no more electrons seeking a path. The behavior of the solid depends on the nature of the levels remaining after all of the electrons have found a path. The last few levels filled with electrons are called "valence levels" and the first few empty levels are called "conduction levels". If there are no more empty levels at energies near the last one filled, the material will behave as an insulator. The conduction levels are far higher in energy than the valence levels. If there are empty levels at energies near the last one filled, the material will behave as a conductor. The conduction levels and valence levels are right nearby. A photoconductor is of the former type: there are no conduction energy levels near the last one filled valence level so it is an insulator. But it becomes a conductor when exposed to light because the light can move the valence level electrons into empty conduction levels at much higher energies.