According to quantum mechanics, electrons bound to atoms occur
in specific electronic configurations. The highest energy
configuration (or energy band) that is normally occupied by
electrons for a given material is known as the valence band, and
the degree to which it is filled largely determines the
material's electrical conductivity. In a typical conductor
(metal), the valence band is about half filled with electrons,
which readily move from atom to atom, carrying a current. In a
good insulator, such as glass or rubber, the valence band is
filled, and these valence electrons have very little mobility.
Like insulators, semiconductors generally have their valence
bands filled, but, unlike insulators, very little energy is
required to excite an electron from the valence band to the next
allowed energy bandknown as the conduction band, because any
electron excited to this higher energy level is relatively free.
For example, the bandgap for silicon is 1.12 eV (electron
volts), and that of gallium arsenide is 1.4