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Photodetector (5213 views - Electrical Engineering)

Photosensors or photodetectors are sensors of light or other electromagnetic energy. A photo detector has a p–n junction that converts light photons into current. The junction is covered by an illumination window, usually having an anti-reflective coating. The absorbed photons make electron-hole pairs in the depletion region. Photodiodes and photo transistors are a few examples of photo detectors. Solar cells convert some of the light energy absorbed into electrical energy.
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Photodetector

Photodetector

Photosensors or photodetectors are sensors of light or other electromagnetic energy.[1] A photo detector has a p–n junction that converts light photons into current. The junction is covered by an illumination window, usually having an anti-reflective coating. The absorbed photons make electron-hole pairs in the depletion region. Photodiodes and photo transistors are a few examples of photo detectors. Solar cells convert some of the light energy absorbed into electrical energy.

Types

Photodetectors may be classified by their mechanism for detection:[2][unreliable source?][3][4]

  • Photoemission or photoelectric effect: Photons cause electrons to transition from the conduction band of a material to free electrons in a vacuum or gas.
  • Photoelectric[dubious ]: Photons cause electrons to transition from the valence band to the conduction band of a semiconductor.
  • Photovoltaic: Photons cause a voltage to develop across a depletion region of a photovoltaic cell.
  • Thermal: Photons cause electrons to transition to mid-gap states then decay back to lower bands, inducing phonon generation and thus heat.
  • Polarization: Photons induce changes in polarization states of suitable materials, which may lead to change in index of refraction or other polarization effects.
  • Photochemical: Photons induce a chemical change in a material.
  • Weak interaction effects: photons induce secondary effects such as in photon drag[5][6] detectors or gas pressure changes in Golay cells.

Photodetectors may be used in different configurations. Single sensors may detect overall light levels. A 1-D array of photodetectors, as in a spectrophotometer or a Line scanner, may be used to measure the distribution of light along a line. A 2-D array of photodetectors may be used as an image sensor to form images from the pattern of light before it.

Properties

There are a number of performance metrics, also called figures of merit, by which photodetectors are characterized and compared[2][3]

  • Spectral response: The response of a photodetector as a function of photon frequency.
  • Quantum efficiency: The number of carriers (electrons or holes) generated per photon.
  • Responsivity: The output current divided by total light power falling upon the photodetector.
  • Noise-equivalent power: The amount of light power needed to generate a signal comparable in size to the noise of the device.
  • Detectivity: The square root of the detector area divided by the noise equivalent power.
  • Gain: The output current of a photodetector divided by the current directly produced by the photons incident on the detectors, i.e., the built-in current gain.
  • Dark current: The current flowing through a photodetector even in the absence of light.
  • Response time: The time needed for a photodetector to go from 10% to 90% of final output.
  • Noise spectrum: The intrinsic noise voltage or current as a function of frequency. This can be represented in the form of a noise spectral density.
  • Nonlinearity: The RF-output is limited by the nonlinearity of the photodetector[7]

Devices

Grouped by mechanism, photodetectors include the following devices:

Photoemission or photoelectric

Semiconductor

Photovoltaic

Thermal

  • Bolometers measure the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. A microbolometer is a specific type of bolometer used as a detector in a thermal camera.
  • Cryogenic detectors are sufficiently sensitive to measure the energy of single x-ray, visible and infrared photons.[12]
  • Pyroelectric detectors detect photons through the heat they generate and the subsequent voltage generated in pyroelectric materials.
  • Thermopiles detect electromagnetic radiation through heat, then genetating a voltage in thermocouples.
  • Golay cells detect photons by the heat they generate in a gas-filled chamber, causing the gas to expand and deform a flexible membrane whose deflection is measured.

Photochemical

Polarization

Graphene/silicon photodetectors

A graphene/n-type silicon heterojunction has been demonstrated to exhibit strong rectifying behavior and high photoresponsivity. Graphene is coupled with silicon quantum dots (Si QDs) on top of bulk Si to form a hybrid photodetector. Si QDs cause an increase of the built-in potential of the graphene/Si Schottky junction while reducing the optical reflection of the photodetector. Both the electrical and optical contributions of Si QDs enable a superior performance of the photodetector.[14]

Frequency range

In 2014 a technique for extending semiconductor-based photodetector's frequency range to longer, lower-energy wavelengths. Adding a light source to the device effectively "primed" the detector so that in the presence of long wavelengths, it fired on wavelengths that otherwise lacked the energy to do so.[15]

See also



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