The Energy Band Model
Throughout the book, the energy band model, or band diagram, is used to
describe the behavior of electrons and holes to externally applied forces
like light, heat, and/or voltage; therefore, learning how to interpret
them and sketch them is basic for this course. Here we are interested in
the microscopic things, like individual or groups of electrons and their
actions, so we have to use our imagination quite a bit here.
The band diagram models the allowed states in the semiconductor known
as energy bands. The two bands of allowed states are called the conduction
band and the valence band. The two are separated by an intervening forbidden
gap, which we call the band gap. To review what the allowed states are
take a look at the Bohr Model of the Atom.