If  a semiconductor is exposed to the radiations, its conductivity increases.
       The energy supplied by the incident light is used to ionize the covalent bonds in the semiconductor. Due to this the covalent bonds break and electron-hole pairs are generated, on top of those which are thermally generated. Due to the increase in electron-hole pairs the resistance of the material decreases and its conductivity increases. Hence such devices are called photoconductors or photoresistors.
       The principle of operation of photoconductor is shown in the Fig. 1.
Fig. 1 Principle of photoexcitation on semiconductors
       In intrinsic semiconductor, an electron moves from valence band to conduction band producing an electron-hole pair when excited by light. This is called intrinsic excitation. In extrinsic semiconductor; due to incident light, a donor electron may excite into conduction band or a valence electron may excite into an acceptor level. This is called impurity excitation. The photoconductivity is mainly due to intrinsic excitation as it is predominant of the two.
       Practically if the light intensity is changed by 100 foot-candle, the resistance of a photoconductor can change by several kiloohms.
1.1 Spectral Response
       For causing an intrinsic excitation, the minimum energy of photon must be equal to forbidden energy gap EG of the semiconductor material.
       The wavelength λc of the photon having energy equal to EG is given by,
       If the wavelength of the radiated light λ is greater than λc then energy of photon is less than and this can not cause intrinsic excitation.
Note : Thus the wavelength λc is called critical or cut-off wavelength or long-wavelength threshold of that material.
       For silicon having EG = 1.1 eV, the critical wavelength is,
                    λc = 1.24/1.1 = 1.1272 1.13 μm                   ............. For Si
       For germanium having = 0.72 eV, the critical wavelength is,
                    λc = 1.24/0.72 = 1.7222 = 1.73 μm               ............. For Ge
       These are the values obtained at the room temperature. The graph of spectral sensitivity of the incident light against the wavelength is called spectral response.
        The Fig. 2 shows the spectral response of Ge and Si materials.
Fig. 2  Relative spectral response of Si and Ge

       The graph shows that a given intensity of light of one wavelength will not produce the same number of charge carriers as produced by an equal intensity of light of another wavelength. Thus as the wavelength changes, the spectral response also changes.
Note : The spectral response or photoelectric yield depends on the frequency of the incident light.
       The range of wavelengths of visible light is shown in the Fig. 2, which is 0.38 to 0.76 m.
1.2 Application of Photoconductive Cells
       The important applications of photoconductive Cells are,
1. In light meters to measure a fixed amount of illumination.
2. In recording a modulating light intensity as on a sound track.
3. As an ON-OFF light relay used in digital or control circuits.
1.3 Commercial Photoconductive Cells
       The most commonly used photoconductive cell is made up of cadmium sulphide (Cds) alongwith a small amount of silver, antimony or indium impurities.
       The cds cell is most responsive to light as in dark its resistance of 2 MΩ  decreases to less than 10  Ω when excited by a strong light.
1.4 Why CdS Photoconductor cells are widely used ?
       The various features of CdS photoconductor cell are,
1. It has excellent sensitivity in the visible spectrum of light.
2. Their power dissipation capacity is high up to 300 mW.
3. When stimulated by light, the resistance decreases to very low value which can cause large output current.
4. Due to this, there is no need of an intermediate amplifier circuit.
       The CdS photoconductor cell can operate relay directly without any amplifier and hence widely used in various practical applications such as smoke detectors, burglar alarm, outdoor lighting control etc. The Fig. 3 shows the spectral response of CdS photoconductor cell. The shaded portion indicates that the response is very much sensitive for light having wavelengths between 0.63 to 0.8 m.
Fig. 3 Spectral response of CdS cell

1.5 Other Photoconductive Devices
       The following Photoconductive Devices are also in use as their responses are different for different wavelengths of light.
1. The cadmium selenide (CdSe) is commonly used as its spectral response is sensitive throughout the visible range.
2. The lead sulphide (PbS) cell shows a peak on spectral response curve at 2.9 m which indicates infrared region light. Hence such cell are used for infrared detection or infrared absorption measurements.

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