Photoconductivity
(also photoconductive effect), the increase in the electrical conductivity of a semiconductor when the semiconductor isexposed to electromagnetic radiation. Photoconductivity was first observed in selenium by W. Smith (Great Britain) in 1873.
Photoconductivity is usually caused by an increase in the concentration of charge carriers upon exposure to light; this effectis called primary photoconductivity. The photoconductive effect is the result of several processes whereby photons causeelectrons to be ejected from the valence band and injected into the conduction band (Figure 1). The number of conductionelectrons and holes increases simultaneously, and the effect is called intrinsic photoconductivity. When electrons from afilled band are injected into vacant impurity levels, the number of holes increases; this effect is referred to as p-typeextrinsic photoconductivity. If electrons are ejected from impurity levels and injected into the conduction band, the effect isknown as n-type extrinsic photoconductivity. The combined excitation of intrinsic and extrinsic photoconductivity is alsopossible. Such combined excitation is called exciton-induced photoconductivity and occurs when the excitation of intrinsicphotoconductivity leads, as a result of the ensuing processes of carrier trapping, to the occupation of impurity centers and,consequently, to the occurrence of extrinsic photoconductivity. Primary photoconductivity can result only from
Figure 1
excitation by sufficiently short-wavelength radiation, in which the photon energy exceeds either the width of the forbiddenband (in the case of intrinsic and exciton-induced photoconductivity) or the distance between the valence or conductionband and an impurity center (in the case of extrinsic photoconductivity).
All nonmetallic solids exhibit photoconductivity to some extent. The photoconductivity of semiconductors, such as Ge, Si,Se, CdS, CdSe, InSb, GaAs, and PbS, is the best studied and most widely used in technology. The magnitude of theprimary photoconductive effect is proportional to the quantum efficiency –n, which is the ratio of the number of carriersproduced and the total number of absorbed photons, and to the lifetime of excess photocarriers, that is, of excess chargecarriers generated by the light. When visible light is used for illumination, η, is usually less than unity because of competingprocesses that result in the absorption of light but are not associated with the production of photocarriers, that is, associatedwith the excitation or production of excitons, impurity atoms, or lattice vibrations. When a substance is exposed toultraviolet or harder radiation, η > 1, since the photon energy is high enough not only to eject an electron from a filled bandbut also to provide the electron with sufficient kinetic energy for impact ionization.
Figure 2. A typical relative spectral responsecharacteristic for intrinsic photoconductivity.The sharp decrease at longer wavelengths iscaused by an absorption edge, that is, by thecutoff of intrinsic absorption that occurs whenthe photon energy becomes less than the widthof the forbidden band. The smooth decrease atshorter wavelengths is caused by surfaceabsorption of light.
The free-carrier lifetime, that is, the mean time a carrier is free, is determined by recombination processes. During directrecombination, a photoelectron migrates directly from the conduction band to the valence band. In the case of recombinationat impurities called recombination centers, an electron is first trapped by such a center and then enters the valence band.Depending on the material’s structure, purity, and temperature, the free-carrier lifetime may range from a few fractions of asecond to 10–8sec.
The dependence of the photoconductive effect on radiation frequency is determined by the absorption spectrum of a givensemiconductor. As the absorption coefficient increases, photoconductivity first reaches a maximum and then declines(Figure 2). The decrease in photoconductivity is explained by the fact that when the absorption coefficient is large, all thelight is absorbed in the surface layer of the semiconductor, where the free-carrier recombination rate is very high.Recombination in the surface layer is called surface recombination.
Other types of photoconductivity are possible which are not associated with a change in the free-carrier concentration. Forexample, when long-wavelength electromagnetic radiation, which does not cause interband migration and does not ionizeimpurity centers, is absorbed by free carriers, the energy of the carriers is increased. Such an increase leads to a change incarrier mobility and, consequently, to an increase in electrical conductivity. Such secondary photoconductivity decreases athigh frequencies and is not frequency dependent at low frequencies. The change in mobility upon exposure to radiation maybe caused not only by an increase in carrier energy but also by the effect of the radiation on electron scattering in thecrystal lattice.
The study of photoconductivity is one of the most effective ways to investigate the properties of solids. Thephotoconductive effect is used to produce photoconductive cells and radiation detectors with a low time constant that aresensitive over a very broad wavelength range, namely, from γ-rays to microwaves.
REFERENCES
Ryvkin, S. M. Fotoelektricheskie iavleniia v poluprovodnikakh. Moscow, 1963.Stil’bans, L. S. Fizika poluprovodnikov. Moscow, 1967.
See also reference under SEMICONDUCTOR.
E. M. EPSHTEIN
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