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Photodiode

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A photodiode

A photodiode is an electronic component and a type of photodetector. It is a p-n junction designed to be responsive to optical input. Photodiodes are provided with either a window or optical fibre connection, in order to let in the light to the sensitive part of the device. They may also be used without a window to detect vacuum UV or X-rays.

Photodiodes can be used in either zero bias or reverse bias. In zero bias, light falling on the diode causes a voltage to develop across the device, leading to a current in the forward bias direction. This is called the photovoltaic effect, and is the basis for solar cells — in fact, a solar cell is just a large number of big, cheap photodiodes.

Diodes usually have extremely high resistance when reverse biased. This resistance is reduced when light of an appropriate frequency shines on the junction. Hence, a reverse biased diode can be used as a detector by monitoring the current running through it. Circuits based on this effect are more sensitive to light than ones based on the photovoltaic effect.

A phototransistor is in essence nothing more than a bipolar transistor that is encased in a transparent case so that light can reach the base-collector junction. The phototransistor works like a photodiode, but with a much higher sensitivity for light, because the electrons that are generated by photons in base-collector junction are injected into the base, this current is then amplified by the transistor operation. A phototransistor has a slower response time than a photodiode however.

Features

Advantages compared to photomultipliers:

  1. Excellent linearity of output current as a function of incident light
  2. Spectral response from 190 nm to 1100 nm (silicon), longer wavelengths with other semiconductor materials
  3. Low noise
  4. Ruggedized to mechanical stress
  5. Low cost
  6. Compact and light weight
  7. Long lifetime
  8. High quantum efficiency, typically 80%
  9. No high voltage required

Disadvantages compared to photomultipliers:

  1. Small area
  2. No internal gain (except avalanche photodiodes, but their gain is typically 10–10 compared to up to 10 for the photomultiplier)
  3. Much lower overall sensitivity
  4. Photon counting only possible with specially designed, usually cooled photodiodes, with special electronic circuits
  5. Response time for many designs is slower

Applications

P-N photodiodes are used in applications similar to photoconductors.

Consumer items such as camera light meters, clock radios (the ones that dim the display when its dark) and street lights usually seem to use photoconductors rather than photodiodes, although in principle either could be used.

Receivers for remote controls in VCRs and televisions often use photodiodes.

Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a better, more linear response than photoconductors.

They are also widely used in various medical applications, such as detectors for Computed tomography (coupled with scintillators) or instruments to analyze samples (immunoassay).

P-N photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified CCDs (ICCDs) or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night-vision equipment and laser range finding.

PIN diodes are much faster and more sensitive than ordinary p-n junction diodes, and hence are often used for communications.

Materials

Materials commonly used to produce photodiodes:

Material Wavelength range (nm)
Silicon 190–1100
Germanium 800–1700
Indium gallium arsenide 800–2600
lead sulfide <1000-3500

See also

External links

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