An enhancement of the conductivity of certain semiconductors due to the absorption of electromagnetic radiation. Photoconductivity can result from the action of radiation in the visible portion of the spectrum in some materials.

Radiation, of frequency ν, can be considered as a stream of photons of energy hν, where h is the Planck constant. If a semiconductor is exposed to radiation an electron in the valence band can be excited by the radiation. If the photon energy is sufficiently great the energy absorbed by the electron causes it to be excited across the forbidden band into the conduction band and a hole remains in the valence band. Thus when the photon energy, hν, exceeds the energy gap, E_g, there is a sudden marked change in the conductivity of the material due to the creation of excess charge carriers. For small intensities of illumination the increase in conductivity is approximately proportional to the intensity. If the photon energy exceeds the work function, Φ, of the material, the electron is liberated from the solid by the photoelectric effect. Photoconductivity resulting under the condition

is sometimes termed the internal photoelectric effect. Band-to-band transitions as described above result in intrinsic photoconductivity.

Measurement of the absorption spectra of semiconductors can yield valuable information on the magnitude of E_g. Direct-gap semiconductors have an absorption edge corresponding exactly to E_g (Fig. a). Indirect-gap semiconductors yield a value larger than E_g (Fig. b). Even if the photon energy is only equal to E_g so that a direct transition is not possible, indirect photoconductivity can occur in indirect-gap semiconductors provided that a phonon is simultaneously created or destroyed. If k is the wavevector and thus represents the momentum of the electron in the crystal lattice (see momentum space), transitions of energy E_g can only take place if there is a change in momentum so that Δk ≠0. This results in a small absorption edge in the spectrum corresponding to E_g. The momentum change required for the occurrence of indirect transitions has to be absorbed or released by the assisting phonon in order to conserve momentum.

(a) Energy diagram of direct-gap gallium arsenide semiconductor (b) Energy diagram of indirect-gap germanium semiconductor

Extrinsic photoconductivity is an effect observed in some semiconductors when the photon energy, hν, of the incident radiation is not sufficiently large to cause band-to-band transitions; instead the energy corresponds to the energy gap required to excite an electron from the valence band, energy E_v, into an acceptor level or from a donor level into the conduction band, energy E_c (Fig. c). E_a and E_d are the acceptor and donor energy levels. In this case electron-hole pairs are not created: an increase in the p- or n-type carriers results, respectively.

(c) Extrinsic photoconductivity transitions

Photoconductive materials have a short response time to the incident radiation because the excess carriers generated disappear very quickly due to recombination. They are very useful as switches and photodetectors and as photocells can produce an a.c. signal if the incoming radiation is suitably modulated, as by a mechanical chopping device.