A conducting material is made up of atoms that have easily shared orbiting electrons. As
a simple example, copper is a better conductor than aluminum because the copper atom's
electrons aren't as tightly coupled to its nucleus allowing its electrons to move around
more easily. An insulator has, for example, eight valence electrons tightly coupled to the
atom's nucleus. A significant electric field is required to break these electrons away from
their nucleus (and thus for current conduction). A semiconductor, like silicon, has four
valence electrons. Silicon's conductivity falls between an insulator and a conductor (and
thus the name "semiconductor"). As silicon atoms are brought together, they form both a
periodic crystal structure and bands of energy that restrict the allowable energies an
electron can occupy. At absolute zero temperature, (T=0 K), all of the valence electrons
in the semiconductor crystal reside in the valence energy band, Ev. As temperature