Bandgaps

The bandgap in a nonmetallic solid is significant for electrical and optical properties. A solid with a little bandgap is a semiconductor with a conductivity that (different from the case with a metal) increases as temperature is raised. The bandgap also decides the minimum photon energy needed to excite an electron from the VB to the CB, and therefore the

threshold for optical absorption through a solid. In a covalent solid the bandgap is related to the energy splitting among bonding and antibonding orbitals and so to the strength of bonding. the bandgap is depicted by the energy needed to transfer an electron back from the anion to cation, in an ionic solid , that is related to the lattice energy. Bandgaps for binary compounds and elements follow some systematic trends.

1. In a series of isoelectronic solids like CuBr-ZnSe-GaAs-Ge the bandgap decreases with decreasing electronegativity variation between the two elements. This tendency reflects the decreasing energy difference among 'cation' and 'anion' orbitals.
2. In series like C-Si-Ge or LiF-NaF-KF the bandgap decreases as the group is descended and ions or atoms become larger. This trend reflects the decline in bond or lattice energies with larger atoms or ions.

A comparison among compounds of pre-transition metals (example Ca) and corresponding post-transition metals (example Cd) provides a good instance of the affect of the electronegativity differences. Bandgaps are smaller in compounds of the less electropositive post-transition metals. The colors of CdSe and CdS (used like yellow and red pigments) come from strong absorption of blue light, like the bandgaps correspond to photon energies in the visible spectrum. Analogous calcium compounds are not in colored like the larger bandgaps correspond to UV radiation.