Holes, semiconductors

Chapman et al. (182-184) extended the work of Bevan et al. (19) by establishing a temperature difference of about 20° between the two platinum electrodes, and using the Seebeck effect (22,185) they determined the thermoelectric potential (g). From the sign of it was ascertained that chromia is a p-t3rpe (oxygen excess or positive hole) semiconductor in an oxygen atmosphere, and an -type (metal excess or electron) semiconductor in a hydrogen atmosphere. 

Although this picture is simple, it reveals an important feature. It is found that the vacancies left in the valence band when electrons are promoted to the conduction band also contribute to the conduction process. To a good approximation, these vacancies can be equated to positive electrons, and move in the opposite direction to the electrons in an applied field. They are called positive holes, or just holes. Semiconductors are characterised by an increase in conductivity with temperature because the number of mobile charge carriers, electrons and holes, will increase as the temperature increases. 

Singer, P., The Interconnect Challenge Filling Smnall, High Aspect Ration Contact Holes, Semiconductor International, pp. 57-64 (Aug. 1994)... 

Irradiation of a semiconductor with light of quantum energy greater than the band gap can lead to electron-hole separation. This can affect adsorption and lead to photocatalyzed or photoassisted reactions [187]. See Section XVIII-9F for some specifics. 

In an extrinsic semiconductor, tlie conductivity is dominated by tlie e (or h ) in tlie CB (or VB) provided by shallow donors (or acceptors). If tlie dominant charge carriers are negative (electrons), tlie material is called n type. If tlie conduction is dominated by holes (positive charge carriers), tlie material is called p type. 

In a defect-free, undoped, semiconductor, tliere are no energy states witliin tire gap. At 7"= 0 K, all of tire VB states are occupied by electrons and all of the CB states are empty, resulting in zero conductivity. The tliennal excitation of electrons across tire gap becomes possible at T > 0 and a net electron concentration in tire CB is established. The electrons excited into tire CB leave empty states in tire VB. These holes behave like positively charged electrons. Botli tire electrons in the CB and holes in tire VB participate in tire electrical conductivity. 

The application of a small external electric field A to a semiconductor results in a net average velocity component of the carriers (electrons or holes) called the drift velocity, v. The coefficient of proportionality between E and is known as the carrier mobility p. At higher fields, where the drift velocity becomes comparable to the thennal... 

In n type semiconductors, electrons are tire majority carriers. Holes will also be present tlirough accidental incoriioration of acceptor impurities or, more importantly, tlirough tlie intentional creation of electron-hole pairs. Holes in n type and electrons in p type semiconductors are minority carriers. 

There are many ways of increasing tlie equilibrium carrier population of a semiconductor. Most often tliis is done by generating electron-hole pairs as, for instance, in tlie process of absorjition of a photon witli h E. Under reasonable levels of illumination and doping, tlie generation of electron-hole pairs affects primarily the minority carrier density. However, tlie excess population of minority carriers is not stable it gradually disappears tlirough a variety of recombination processes in which an electron in tlie CB fills a hole in a VB. The excess energy E is released as a photon or phonons. The foniier case corresponds to a radiative recombination process, tlie latter to a non-radiative one. The radiative processes only rarely involve direct recombination across tlie gap. Usually, tliis type of process is assisted by shallow defects (impurities). Non-radiative recombination involves a defect-related deep level at which a carrier is trapped first, and a second transition is needed to complete tlie process. 

Light is generated in semiconductors in the process of radiative recombination. In a direct semiconductor, minority carrier population created by injection in a forward biased p-n junction can recombine radiatively, generating photons with energy about equal to E. The recombination process is spontaneous, individual electron-hole recombination events are random and not related to each other. This process is the basis of LEDs [36]. 

Brus L E 1984 Electron-electron and electron-hole Interactions In small semiconductor crystallites the size dependence of the lowest excited electronic state J. Chem. Phys. 80 4403-9... 

Photovoltaic Devices. For many inorganic semiconductors, absorption of light can be used to create free electrons and holes. In an organic semiconducting soHd, however, absorption of a photon leads to the formation of a bound electron—hole pair. Separation of this pair in an electric field can... 

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