Sometimes valence electrons are shared, becoming a bond between two atoms - covalent bonding. This is the bonding type in diamond-crystal lattice semiconductors such as silicon semiconductors. However, it is more interesting to analyze energy-related aspects rather than spatial aspects such as bonds. Therefore the concept of energy bands is coming in handy.

An almost continuous band of allowed energies of electrons comes about when atoms are brought in close proximity to each other, this is because of the interatomic forces and is foreseen in the Pauli exclusion principle. “Almost”, well, one energy level is split into N levels when N atoms are brought together, and these N levels can accommodate at most 2N electrons due to spin degeneracy.

Remember, N is huge! Now, since the separation between the energy levels within the band is much smaller than the thermal energy possessed by an electron at room temperature the band can be viewed as continuous. Ec is the lowest possible conduction band energy, while Ev is the highest possible valence band energy. The band gap energy, Eg, is furthermore defined as (Ec - Ev). Eg is the energy it takes to break a bond in the spatial view of the crystal. The band gap energies for some semiconductors at T = 300 K are: Eg = 1.42 eV in GaAs and 1.12 eV in Si. You do remember that 1 eV = 1.602?10-19 J, don’t you?