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Photodetectors

by Ron Kurtus (29 June 2006)

A photodetector is a device that responds to electromagnetic radiation and records its characteristics, usually as electrical signals. Methods used include the photoelectric effect, photovoltaic effect and resistance change. Photographic film is considered a chemical photodetector and is not usually discussed in electro-optics.

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Photoelectric effect

When electromagnetic radiation strikes certain materials (usually metallic), electrons are emitted. This is called the photoelectric effect and is used to detect various forms of light or electromagnetic radiation.

Although many metals exhibit photoelectric properties, a specialized coating on the metal surface is used to greatly increase the effect. The coating releases electrons more readily than the underlying metal. This coated metal is called a photocathode.

In order for the photoelectric effect to be useful, the number of measurable electrons should be increased. Photomultiplier tubes perform this task by amplifying the number of electrons released from the photocathode with a chain of dynodes. This makes such a device especially useful in detecting the low-energy photons in infrared radiation.

Photovoltaic effect

When illuminated by electromagnetic radiation, some materials generate sufficient electrons to provide direct current (DC) electricity in a circuit. This is called the photovoltaic effect.

Solar cell

The most common photodetectors using this method are solar cells and photovoltaic cells. Solar cells are sensitive to wavelengths and intensities radiated from the Sun. Photovoltaic cells work with lower levels of light to generate electricity from both the Sun and other sources of light, such as an incandescent bulb.

Photodiode

A photodiode also employs the photovoltaic effect to generate electricity. Photodiodes are semiconductor diodes that act as photodetectors for not only visible light but also ultraviolet radiation or X-rays if properly configured.

Phototransistor

A phototransistor consists of a bipolar transistor that includes a transparent case that allows light to reach the base-collector junction. Electrons that are generated in the base-collector junction are injected into the base, and this current is amplified by the transistor operation. A phototransistor has a slower response time than a photodiode but has a much higher sensitivity for light.

Pyroelectricity

Pyroelectricity is the ability of certain materials to generate an electrical potential when they are heated or cooled. As a result of this change in temperature, positive and negative charges migrate to opposite ends of the material, establishing electrical potential that can create a current.

Resistance change

The detection of electromagnetic radiation can be done by measuring the change in electrical resistance of a material. Thermal detection is when the electrical resistance a material increases with heating due to the light energy. Photoconductivity is when a material becomes more electrically conductive due to the absorption of electromagnetic radiation.

Thermal detection

The electrical resistance of most conductors increases with the increase in temperature. Since infrared is most effective in heating materials, most thermal detectors are used to detect radiation in that wavelength region. The greater the intensity of the light, the more the detector material is heated and the higher the resistance.

A bolometer is a thermal detector which changes its electrical resistance as a function of the radiant energy striking it. Other thermal detectors are pyroelectric detectors, Golay cells, thermocouples and thermistors.

Photoconductivity

Photoconductivity devices work just the opposite of thermal detectors in that their conductivity is increased and electrical resistance lowered when irradiated by higher frequency electromagnetic waves. The resistance lowers as the intensity of the light increases.

Photoresistors are made of a high-resistance semiconductor material. Light falling on the device gives bound electrons enough energy to jump into the conduction band. The resulting free electrons conduct electricity and lower the semiconductor resistance.

A photoresistors is also called a light-dependent resistor (LDR) or photoconductor.

Summary

Electro-optical photodetectors respond to electromagnetic radiation and record it as electrical signals. Methods used include the photoelectric effect, photovoltaic effect and resistance change, including photoconductivity. The photovoltaic effect includes the solar cell, photodiode, phototransistor and pyroelectricity devices.


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