Extrinsic semiconductors

Key points: p-Type semiconductors are solids doped with atoms that remove electrons from the valence band; n-type semiconductors are solids doped with atoms that supply electrons to the conduction band. An extrinsic semiconductor is a substance that is a semiconductor on account of the presence of intentionally added impurities. The number of electron carriers can be increased if atoms with more electrons than the parent element can be introduced by the process called doping. Remarkably low levels of dopant concentration are needed—only about one atom per 109 of the host material—so it is essential to achieve very high purity of the parent element initially. If arsenic atoms ([Ar]4s²4p³) are introduced into a silicon crystal ([Ne]3s²3p²), one additional electron will be available for each dopant atom that is substituted. Note that the doping is substitutional in the sense that the dopant atom takes the place of an Si atom in the silicon structure. If the donor atoms, the As atoms, are far apart from each other, their electrons will be localized and the donor band will be very narrow (Fig. 3.73a). Moreover, the foreign atom levels will lie at higher energy than the valence electrons of the host structure and the filled dopant band is commonly near the empty conduction band. For T>0, some of its electrons will be thermally promoted into the empty conduction band. In other words, thermal excitation will lead to the transfer of an electron from an As atom into the empty orbitals on a neighbouring Si atom. From there it will be able to migrate through the structure in the band formed by Si–Si overlap. This process gives rise to n-type semi conductivity, the ‘n’ indicating that the charge carriers are negatively charged (that is, electrons). An alternative substitutional procedure is to dope the silicon with atoms of an element with fewer valence electrons on each atom, such as gallium ([Ar]4s²4p¹). A dopant atom of this kind effectively introduces holes into the solid. More formally, the dopant atoms form a very narrow, empty acceptor band that lies above the full Si band (Fig. 3.73b). At T 0 the acceptor band is empty but at higher temperatures it can accept thermally excited electrons from the Si valence band. By doing so, it introduces holes into the latter and hence allows the remaining electrons in the band to be mobile. Because the charge carriers are now effectively positive holes in the lower band, this type of semiconductivity is called p-type semiconductivity. Semiconductor materials are essential components of all modern electronic circuits and some devices based on them are described in Box 3.4.

Several d-metal oxides, including ZnO and Fe₂O₃, are n-type semiconductors. In their case, the property is due to small variations in stoichiometry and a small deficit of O atoms. The electrons that should be in localized O atomic orbitals (giving a very narrow oxide band, essentially localized individual O₂ ions) occupy a previously empty conduction band formed by the metal orbitals (Fig. 3.74). The electrical conductivity decreases after the solids have been heated in oxygen and cooled slowly back to room temperature because the deficit of O atoms is partly replaced and, as the atoms are added, electrons are withdrawn from the conduction band to form oxide ions. However, when measured at high temperatures the conductivity of ZnO increases as further oxygen is lost from the structure, so increasing the number of electrons in the conduction band. p-Type semiconduction is observed for some low oxidation number d-metal chalcogenides and halides, including Cu₂O, FeO, FeS, and CuI. In these compounds, the loss of electrons can occur through a process equivalent to the oxidation of some of the metal at oms, with the result that holes appear in the predominantly metal band. The conductivity increases when these compounds are heated in oxygen (or sulfur and halogen sources for FeS and CuI, respectively) because more holes are formed in the metal band as oxidation progresses. n-Type semiconductivity, however, tends to occur for oxides of metals in higher oxidation states, as the metal can be reduced to a lower oxidation state by occupation of a conduction band formed from the metal orbitals. Thus, typical n-type semiconductors include Fe₂O₃ , MnO₂, and CuO. By contrast, p-type semiconductivity occurs when the metal is in a low oxidation state, such as MnO and Cr₂ O₃.

Fig. 3.72 In an intrinsic semiconductor, the band gap is so small that the Fermi distribution results in the population of some orbitals in the upper band.

Fig. 3.73 The band structure in (a) an n-type semiconductor and (b) a p-type semiconductor.

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مواضيع ذات صلة

Liquid ammonia

Polyoxo compound formation

Amphoterism

Acidic and basic oxides

Periodic trends in aqua acid strength

Intrinsic semiconductors

Insulators

The Fermi level

Densities of states

Band formation by orbital overlap

The conductivities of inorganic solids

Nonstoichiometry