Electron hole

As a defect electron, electron vacancy or hole in the (imaginary ) is called positive mobile charge carriers in semiconductors and provides the equivalent description of the absence of a (real) valence dar. The real charge transport continues to take place by electrons, since it is merely an alternative description and serves as a simplified mathematical treatment of processes in the semiconductor. In doped semiconductors is further makes the concept of the electron holes of the basic understanding of the forwarding mechanisms play an important role. The defect electron is a quasi-particle, its counterpart is the quasi-particle " electron crystal ".

General Description

Defect electrons are created generally by excitation of lattice atoms of a crystal. In pure semiconductor single crystals ( silicon, germanium, gallium arsenide, etc. ) are ( at absolute zero ) all valence electrons involved in the bonds, ie, all valence electrons are in the valence band and the conduction band is unoccupied. Therefore, there existed no defect electrons.

Therefore, some lattice atoms must be excited for the generation of electron holes. This can happen, for example, at higher temperatures ( thermal excitation ) or by absorption of a photon. In this case, the valence electrons are excited into the conduction band, leaving the corresponding interstitial an unoccupied Valenzelektronstelle ( a defect electron).

When a voltage is applied to the semiconductor, so contribute to both the floating electron in the conduction band and the defect electron in the valence band to the charge transport. This is called (in the case of pure semiconductors) of this intrinsic. However, in contrast to the conduction band electron to the defect electron can not move freely. It moves rather by a kind of " moving up " of valence electrons. In this case, a neighboring valence electron occupies the vacant site ( the defect electron) and leaves at its original turn a vacant site. This procedure can be interpreted as viewed from the outside, that a positively charged particle moving ( the defect electron) in the opposite direction (similar to a bubble in a liquid).

Defect electrons in the doped semiconductor

A further possibility of generating the defect electron excitation of impurity atoms in the semiconductor crystals. A semiconductor single crystal impurities produce energy levels within the band gap. For a suggestion, therefore, less energy is required than in a pure semiconductor crystal. For this reason, a significant increase in conductivity is observed at low temperatures; one speaks in this case of impurity conduction. Depending on the value of the foreign atom different impurities may arise. In semiconductor technology, such impurities are ( for the most silicon, boron or phosphorus ) into the semiconductor crystal ( doping) to change the conductivity of the starting material targeted.

The so-called p-type doping is particularly noteworthy for the generation of electron holes. In this case, a semiconductor doped with an impurity of lower valency. This impurity is one or more valence less than would be necessary to replace the semiconductor substituted atom; in the case of a tetravalent semiconductor such as silicon, for example, boron Energetically these vacancies are slightly above the valence band, which an electron from the valence band only little energy is required to change to the (fixed ) defect level. In this case, the associated lattice atom is ionized and in turn generates a positive hole in the valence band.

Application

The interesting physics of the semiconductor ( conductivity, optical transitions ) takes place in a minimum of the conduction band ( curvature positive = effective mass of electrons, positive) and in a maximum of the valence band ( curvature negative = effective mass of the electron negative) from ( while in a metal other configurations occur ). An electron has in the valence band ( in the vicinity of the maximum), a negative effective mass () and thus moves in external electric field to lower electric potentials, that is to negative pole (as opposed to electrons in the conduction band or in metals ). Is an electron ( charge ) from the valence band in momentum with an acceptor, or in the conduction band over ( thermal or optical excitation ) and then remains in the valence band back an unoccupied state. The fully occupied band, for every positive pulse as great a negative within the Fermi surface (in the simplest case, the Fermi sphere ). Thus, after excitation in the valence band an unoccupied state and a resulting impulse remains. In addition, a resulting positive charge is left in the previously neutral fully occupied band by removing an electron. This can be described equivalently as a hole with a positive charge, positive momentum and positive effective mass. Thus, the movement direction is equal to that of the electron in the valence band, ie in the external electric field to the negative pole.

The removed electron (English missing electron ) had exactly the same speed as the residual after excitation hole ( engl. hole):

An external electric field accelerates the missing electron, when it would sit in original condition, just like the hole:

Another important characteristic quantities of semiconductors are the charge-carrier mobility, and their effective mass. Both, however, are not automatically the same for electrons and holes and hang for example, material, doping, mechanical stress state, temperature, direction of motion and so on.

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