Linking atomic orbitals to bands

In the discussion below we will need of the following terms  

Homopolar semiconductors All atoms in the unit cell in these materials are the same so there is no charge transfer from one atom to another. Examples of such materials are Si and Ge. In these two examples, there are two atoms per primitive unit cell but both are the same kind of atom. 

Heteropolar semiconductors There are two or more different kinds of atoms organized regularly in the unit cell. These transfer electrons from one atom to the other and thus have different polarities. Examples of such materials are GaAs and AIN. In such materials the atoms are well organized into a compound (as distinct from a random alloy) with a specific lattice structure. 

Note A Si-Ge alloy has two kinds of atoms but they mix randomly and specific sites for one or the other cannot be distinguished. Therefore, such a mixture is an alloy of homopolar materials, not a heteropolar compound. 

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