handbook of modern sensors

14.4 Photoresistors 421
(A)
(B)
Fig. 14.14. Structure of a photoresistor (A) and a plastic-coated photoresistor having a serpentine
shape (B).
which are semiconductors whose resistances change upon light entering the surface.
For its operation, a photoresistor requires a power source because it does not generate
photocurrent; a photoeffect is manifested in the change in the material’s electrical
resistance. Figure 14.14A shows a schematic diagram of a photoresistive cell. An
electrode is set at each end of the photoconductor. In darkness, the resistance of the
material is high. Hence, the applied voltage V results in a small dark current which
is attributed to temperature effect. When light is incident on the surface, the current
i p flows.
The reason for the current increase is the following. Directly beneath the conduction
band of the crystal is a donor level and there is an acceptor level above the
valence band. In darkness, the electrons and holes in each level are almost crammed
in place in the crystal, resulting in the high resistance of the semiconductor.
When light illuminates the photoconductive crystal, photons are absorbed, which
results in the added-up energy in the valence band electrons. This moves them into the
conduction band, creating free holes in the valence band, increasing the conductivity
of the material. Because near the valence band there is a separate acceptor level that can
capture free electrons not as easily as free holes, the recombination probability of the
electrons and holes is reduced and the number of free electrons in the conduction band
is high. Because CdS has a band gap of 2.41 eV, the absorption-edge wavelength is
λ = c/v ≈ 515 nm, which is in the visible spectral range. Hence, the CdS detects light
shorter than 515-nm wavelengths. Other photoconductors have different absorptionedge
wavelengths. For instance, CdS is most sensitive at the shorter-wavelength range,
whereas Si and Ge are most efficient in the near infrared.
The conductance of a semiconductor is given by
σ = ef (µ n τ n + µ p τ p ), (14.17)
where µ n and µ p are the free-electron and hole movements (cm/V s), τ n and τ p are
the free-electron and hole lives (s), e is the charge of an electron, and f is the number