Valence band

The term part of the valence band diagram of the electrical conductivity, in particular the semiconductor, is explained. The valence band is generally the highest occupied energy band electrons at absolute zero (temperature ) or are the bands which contribute electrons ( valence electrons ) to the chemical bond.

Explanations

As mentioned above the valence band is the highest occupied energy band at absolute zero (temperature). In semiconductors and insulators, this band is fully occupied and separated by the so-called band gap from the next higher energy band ( conduction band ). When ladders can - depending on the electron configuration of the element - the valence band either identical to the conduction band (for example, when sodium), or it may be with the next higher band (quasi the conduction band ) overlap. As a result, the valence band is only partially occupied with metals.

For the case of a monovalent metal, each atom contributes one valence electron in the crystal composite of binding at ( basic configuration 3s1 ). The valence electrons, as a cause of chemical bonding, the valence band belong to the solid state as a whole. The result in the case of sodium ( monovalent metal ) the 3s band, the valence band of sodium; to the emergence of the tapes see band model. Since sodium contributes only a valence of the corresponding energy level and thus the corresponding energy band, the band is 3s only half occupied (see Pauli principle).

This looks different for divalent metals such as magnesium. Magnesium has two valence electrons ( 3s2 ground configuration ), it would be expected, therefore, that its valence band fully occupied and therefore is an insulator. By energetically overlap with the next higher energy band (also called valence II, in the case of magnesium, the 3p - band ), electrons from the valence band I. The Second transgressed, so that both are only partially occupied; while the electrons are not simply pro rata basis, but depending on the density of states distributed ( see also band structure ). Analog is true in the case of aluminum ( 3s2 3p1 ground configuration ) in which the 3 -s- band fully occupied and the 3-p- band would be half full. The superposition of the energy bands but both bands are only partially occupied as in magnesium.

Semiconductors and insulators, the superposition of the valence band and described the next higher ( unoccupied ) band does not exist. For example, silicon has four valence electrons ( 3s2 3p2 ground configuration ). Similar to sodium, magnesium and aluminum are also here superimpose the two valence bands (3s and 3p band). However, since no overlap with the next higher band is present - to illustrate the power scheme can also be used by carbon - is the valence band ( here are often both simple valence bands combined ) fully occupied. The energy gap between the valence band and the conduction band is called a band gap of a quantum-mechanically forbidden zone for electrons. Since no free energy levels in the valence band exist is silicon at absolute zero (T = 0 K) is an insulator, since an outer (small ) electric field can not carry valence electrons in the free conduction band. Since it is possible with increasing temperature or light, electrons that can move in the conduction band is referred to as a silicon semiconductor.

Importance to the electrical lead

Ground state and the external electric field

Fully occupied bands can contribute to the conductivity does not contribute, because the application of an external electric field electron energy take from this field, they are raised to higher free energy terms in the band and it comes to band bending. Thus, electrons can move in the solid state free energy states are necessary. In a fully occupied band, the electrons can not accept higher energy level in the same band by the supplied energy of the electric field. Since a change in location of the entire electron brings no net transport of electric charge with them, a material with a fully occupied valence band is an insulator.

External power supply

However, a semiconductor thermal or photonic energy amount supplied is within the range of the band gap, the valence electrons are excited into the conduction band. These electrons in the conduction band can absorb the energy of an electric field and to make the material (along with the resulting electron holes, i.e., " holes" in the valence band ) conductive. This greatly increases with the temperature effect is called intrinsic, in the case of excitation by photons as photoconductivity. In contrast, the extrinsic, which can be produced by the introduction of impurity ( dopant ) in the semiconductor.

Semiconductors and insulators differ only in the width of the band gap. In this latter so large ( Eg > 3 eV ) is that electrons can not overcome by thermal excitation at finite temperatures.