This material can be classified as either a conductor, an insulator, or a semi-conductor. These three distinct classes of material arise from a difference in the structure of the allowed electron energy levels. In particular, every material possesses both a valence and a conduction band for electrons, and the energy difference between these two bands will determine how easily an electric current will pass through the material.

As the name implies, the valence band contains the valence electrons of a substance. At absolute zero, all of the electrons in a substance would be contained in the valence band. However, if the substance is at a higher temperature, thermal energy can excite electrons out of the valence level and into an excited energy level. The conduction band is composed of the excited energy states of a substance, and it contains electrons that have been thermally or otherwise excited from the valence band. The electrons in the conduction band are able to freely move about the substance and conduct electricity if an external electric field is applied.

Due to the lattice spacing of the atoms and other relevant factors, there is an energy gap between the highest- energy electron valence level and the lowest-energy conduction level. The width of this gap is dependent on the temperature and the pressure of the material and determines whether a material will be a conductor, insulator or semi- conductor. For reference, an energy level diagram for each type of material is shown in figure.

In conductors, the valence band and the conduction band overlap. Consequently, there is no energy gap to cross in order to reach the conduction band, and any energy that is added to the electron is sufficient to propel it into the conduction band. There are many electrons that are free to move about a conductor, so it very easy for current to flow if an external electric field is applied.

In insulators, there is a distinct separation of the two bands and there is a large energy difference between them. This energy difference is so large that the thermal energy of an individual electron is not large enough to propel it from the valence band to the conduction band. Consequently, there are not many electrons in the conduction band, and it is difficult for current to flow when an external electric field is applied.

As in insulators, there are two distinct bands in semi- conductors. However, the energy gap between these two bands is neither as large nor as significant (typically around one electron-volt) as is the band gap in insulators. At normal temperatures, the thermal energy of the material is sufficient to propel some electrons from the valence band into the conduction band, allowing some electrons to be free to conduct current. The number of free charge carriers increases with supplied energy, so the conductivity of a semi-conductor can be manipulated by outside potentials