Photoconductivity Conceptual Explanations and Applications
Sarathy Kannan G
What is Photoconductivity? • Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation.
How does photoconductivity occurs? • When light is absorbed by a material such as a semiconductor, the number of electron hole pairs increases resulting in the increase in the number of charge carriers and raises its electrical conductivity.
When Photoconductivity occurs? • To cause excitation, the photon(hv) that strikes the semiconductor must have enough energy to raise electrons across the band gap, or to excite the impurities within the band gap(Eg) • If Eg be the minimum band gap, then the longest wavelength which may cause this effect is λ=hc/Eg
• In a homogeneous material, the increase in conductivity when exposed to electromagnetic waves is Δσ = e(Δn.μn + Δp.μp) • In a non homogeneous material the increase in conductivity is Δσ = e(n.Δμn +p.Δμp)
Photosensitivity • 'Photosensitivity' is the amount to which an object reacts upon receiving photons, especially visible light. S = σph / σd where σph = σ - σd total
The Spectral Response • The variation of photoconductivity with photon energy is known as spectral response. • The maximum value of photocurrent corresponds to band gap energy and spectral response. • The energy ranges from 3.7 eV for ZnS to 0.2 eV for cooled PbSe.
Spectral response for photo conducting materials.
Speed of response • It is the rate of the change in photoconductivity with change in photo excitation intensity. • For materials with exponential decay, the photocurrent reaches the dark current very quickly. • For materials with non exponential decay ,the decay of photocurrent takes a longer time to reach dark current.
Photoconductive Materials The desired characteristics are • High spectral sensitivity in the wavelength region of interest • Higher quantum efficiency • Higher photoconductive gain • Higher speed of response and • lesser noise
Cadmium sulphide (CdS) and Cadmium Selenide (CdSe)
• These are highly sensitive in the visible region of radiation. • They have high photoconductive gains (103 to 104) . • poor response time (about 50 ms). The response gets reduced at higher illumination levels indicating the presence of traps.
Lead Sulphide (PbS) • It has spectral response from 1 to 3.4 μm and hence very much suitable for fabricating nearinfrared detectors. • It has maximum sensitivity in the region of 2 μm • Has a typical response time of about 200 μs.
Indium Antimony (InSb) • These detectors have wavelength response extending out to 7 μm. • Exhibits response times of around 50 ns. • Can be operated at room temperature but has a improved noise performance at low temperatures.
Mercury Cadmium Telluride (HgxCd1-xTe) • This is an alloy composed of the semimetal HgTe and the semi-conductor CdTe. • Semi-metals have overlapping valence and conduction bands. • Depending on the composition of alloy, a semiconductor can be formed with a band gap varying between zero and 1.6eV. • Correspondingly the detector sensitivities lie in the range 5 to 14 μm. • Photoconductive gains of up to 500 are possible
Material
Symbol
Detection Range (μm)
Lead sulphide
Pbs
0.6-3.0
Indium Antimony
InSb
1.0-7.0
Mercury-doped Germanium
Ge:Hg
2.0-13
Cadmium mercury telluride
CdHgTe
3.0-15
Copper-doped germanium
Ge:Cu
2.0-2.5
Cadmium sulphide
CdS
0.4-0.8
Cadmium selenide
CdSe
0.5-0.9
Photo Diode • A photo diode is a reverse biased P-N junction diode which is designed to respond to photon absorption.
Principle • A reverse biased P-N junction diode has a reverse saturation current which is mainly due to flow of the minority carriers. If light is allowed to fall on such a reverse biased P-N junction diode , addition electron –hole pairs are generated in both P and N region . It produced a very large change in minority carriers conc. And hence increases the reverse current through the diode.
2000 Lm/m2 Diode
1500 Lm/m2
Current (mA)
1000 Lm/m2 Dark current
Reverse Voltage V
Uses • Photodiode can turn its current ON and OFF in nanosecond therefore , it is used where light is required to be switched OFF and ON at a very fast rate • A photodiode is used in light detection in light operated switches, reaching of computer punched cards and type etc. • In optical communication system. • Used in instrumentation, control automation and communication
PhotoVoltaic Effect • The "photovoltaic effect" is the basic physical process through which a PV cell converts sunlight into electricity. Sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum. When photons strike a PV cell, they may be reflected or absorbed, or they may pass right through. Only the absorbed photons generate electricity.
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Photo-Voltaic cell(Solar Cell) Becquarel in 1839 discovered that when a
pair of electrodes is immersed in an electrolyte and light is allowed to incident on one of them , a potential difference is created between electrodes . This phenomenon is called photovoltaic effect. Device based on this effect are known as Photo-Voltaic cell . This photovoltaic cell are the devices in which light energy is use to create a potential difference so developed is directly proportional to the frequency and intensity of incident light.
Semiconductor
Metal
Light
+ R -
Solar Cell
Uses of Photo-Voltaic cell • 1 – Operation of relays • 2 -- Photographic exposure • 3 -- Direct reading illumination metro.
Photoconductive Cell(or Photoresistor) • Principle Photoconductive Cell are based on the principle that the electrical resistant of semiconductor like bad sulphide, selenium etc., decreases when they are exposed to radiation . The decrease in resistance( or increase in conductivity) of semiconductors on exposing to light may be explained as follows
Sketch of Photoconductive device
Geometry of photoconductive cell.
• The four materials normally employed in photoconductive devices are: Cadmium Sulphide (CdS), Cadmium Selenide (CdSe), lead sulphide (PbS) and Thallium Sulphide (TlSIn a typical construction of photoconductive device, thin film is deposited on an insulating substrate. • The electrodes are formed by evaporating metal such as gold through a mask to give comb -like pattern as shown[above fig]. • The geometry results in a relatively large area of sensitive surface and a small inter electrode spacing. This helps the device to provide high sensitivity.
• When the device under forward bias is illuminated with light electron–hole pairs are generated. The electronhole pairs generated move in opposite directions. This results in a photocurrent.
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The photoconductive cell has very high resistance in dark called dark resistance. When illuminated the resistance falls. The spectral response of CdS cell is similar to that of the human eye. The illumination characteristics of the cell is shown in figure.
Photoconductor in circuit
Spectral response of CdS cell
Use of photoconductive cell • • • • • • • •
To measure the intensity of illumination To work as ON- OFF switch. In street lighting control. In camera exposure setting. In counting application. In aircraft and missile tracking system In burglar alarm. As voltage regulator
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