ABSTRACT
Materials are normally classied into three categories, depending on energy of conduction band (CB) and valence band (VB) (Figure 3.1). The conduction band is the lowest unoccupied molecular orbital (LUMO), while the valence band is the highest occupied molecular orbital (HOMO). The conducting materials have a bandgap less than 1.0 eV or, sometimes, there may even be almost an overlap between these two bands, so that there is smooth conduction of electricity. The insulators (nonconducting material) have a wide bandgap that is more than 5.0 eV; therefore, they do not conduct electricity. There are some materials that have a bandgap of the medium order that is 1.5-3.0 eV. These are basically nonconducting in nature, but in the presence of some energy source, they become conducting. Therefore, such materials are called semiconductors.
