Solid state physics is the study of how solids behave, in particular, how the presence of other atoms effect properties of other nearby atoms and the applications of these properties. Band theory, an important aspect of solid state physics, explains how electrons behave in solids. Much like the energy levels of a single atom, the structure of a solid requires that electrons can only be in specific "bands" of energy. Each band can only hold a specific number of electrons and those bands with the least energy associated with them fill up first. The highest, unfilled band is called the conduction band. The highest filled band is called the valence band. The energy difference between the conduction and valence bands is called the energy gap. If the highest band is only partially filled, the material is a conductor. If the energy gap is large...