Valence and conduction bands

Used in quantum mechanics, the term conduction band refers to an area of combined orbitals, or a band, for electrons in a molecule. From the perspective of molecular orbital theory, the conduction band can be said to be the LUMO (Lowest Unoccupied Molecular Orbital).

Unlike the valence band, the conduction band rarely contains electrons. In excited states, electrons will momentarily move into the conduction band before releasing their energy and falling back into the lower electronic orbitals. Understanding the behavior of electrons with respect to this band is useful for understanding the behavior of various substances. In quantum mechanics, the concept of the conduction band is addressed in band theory.

For a conductor, the valence band (i.e. the one totally occupied) and the conduction band overlap. In the case of semiconductors, the energy gap is low and therefore conduction can occur once provided a certain potential. In insulators, the potential difference is too high (the limiting value is usually placed at E > 1.6 eV).

In the fundamental state of a many-electron independent system, a potential is obtained by filling the single-electron band states up to the Fermi energy. The Fermi energy separates the filled levels (below) from the empty levels (above). Depending on the total number of electrons in the solid, the level corresponding to the Fermi energy can end up either within one (or a few) energy bands or within a band gap. In the first case, the electrons that are in the partially filled bands near the Fermi energy are ready to receive excitation energy (provided, for example, by an external electromagnetic field). In particular, the application of an electric field can accelerate electrons, which can therefore conduct electric current: the solid with this characteristic is a metal. On the contrary, all electrons in full bands must obey to Pauli exclusion principle; a solid where bands are either all full or all empty, with Fermi energy in a forbidden zone, is called an insulator. In solids, the most energetic band that is completely filled is called the valence band, while the first empty or partially empty energy band is called the conduction band.

In extrinsic conductors, the energy gap is artificially lowered through doping, which generates additional higher energy valence bands (n doping) or lower energy conduction bands (p doping).

Leave a Comment