Semiconductors consist of atoms which are placed in an ordered form which is called a crystal. Crystals are identified based on their lattice structure. For instance the crystal structure of silicon is like that of diamond and refered to as the diamond lattice, shown in the figure below. Each atom in the diamond lattice has a covalent bond with four adjacent atoms which together form a tetrahedron. This lattice can also be formed from two face-centered-cubic lattices which are displaced along the body diagonal of the larger cube in the figure by one quarter of that body diagonal.