Types of photo detector and their application

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Photodetector

Photodetectors are electromagnetic radiation sensors converting the electromagnetic radiation into readable output through various mechanisms.

Types of photodetectors

Light incident on a material can cause various changes in them, depending on the phenomenon of conversion of light to any measurable signal, photodetectors can be classified as

Phenomenon Mechanism
photoelectric effect Photons cause electrons to shift from the conduction band of a material to valence band producing free electrons in a vacuum or gas.
Thermal Photons cause electrons to transition to mid-gap states then decay back to lower bands, making phonon which is heat.
Polarization Photons induce changes in polarization states of suitable materials, that can change the 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. detectors or gas pressure changes in Golay cells.

Configurations of Photodetectors

Arrangement of the sensors in a photodector, there are two types of configuration.

  • 1-D array - to measure the distribution of light along a line – Eg: spectrophotometer

  • 2-D array- form images from the pattern of light – Eg: image sensor

Properties of Photodetectors

The figure of merit for the selection of photodetectors are as follows

Parameters Description
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 on 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

Photodetector devices

Based on each phenomenon, there are various devices to choose from to be used in different application

Photoemission or photoelectric

Device Description
Gaseous ionization detectors Detect photons and particles with sufficient energy to ionize gas atoms or molecules. Electrons and ions generated by ionization cause a current flow which can be measured. Experimental particle physics
Photomultiplier A setup with multiple tubes that encloses a photocathode which emits electrons when illuminated, the electrons are then amplified by a chain of dynodes. A dynode is a vacuum tube that serves as an electron multiplier through secondary emission.
Phototubes A tube with photocathode which emits electrons when illuminated, such that the tube conducts a current proportional to the light intensity.
Microchannel plate detectors Silicon-based photomultipliers used for detection of single particles (electrons, ions and neutrons) and low intensity impinging radiation (ultraviolet radiation and X-rays).

Semiconductor

Device Description
Active-pixel sensors Image sensors usually made using cmos fabrication process, commonly used in cell phone cameras, web cameras, and some dslrs.
Cadmium zinc telluride radiation detectors Operating in direct-conversion (or photoconductive) mode at room temperature, unlike some other materials (particularly germanium) which require liquid nitrogen cooling. High sensitivity for x-rays and gamma-rays, due to the high atomic numbers of Cd and Te, and better energy resolution than scintillator detectors.
Charge-coupled devices Used to record images in astronomy, digital photography, and digital cinematography. Used instead of photographic plates a predecessor to cryogenic detectors.
HgCdTe infrared detectors. Detection occurs when an infrared photon of sufficient energy kicks an electron from the valence band to the conduction band. Such an electron is collected by a suitable external readout integrated circuits (ROIC) and transformed into an electric signal.
LEDs Reverse-biased to act as photodiodes.
Photoresistors/ Light Dependent Resistors (LDR) Change resistance according to light intensity. Normally the resistance of LDRs decreases with increasing intensity of incident light
Photodiodes Operate in photovoltaic mode or photoconductive mode,they are often combined with low-noise analog electronics to convert the photocurrent into a voltage that can be digitized.
Phototransistors Amplifying photodiodes.
Quantum dot photoconductors or photodiodes Detecting wavelengths in the visible and infrared spectral regions.
Semiconductor detectors Used in gamma and X-ray spectrometry and as particle detectors
Silicon drift detectors X-ray radiation detectors used in x-ray spectrometry (EDS) and electron microscopy

Photovoltaic

Device Description
Photovoltaic cells or solar cells A combination of layers of semiconducting material that can produce a voltage across it and supply an electric current when light is incident on it

Thermal

Device Description
Bolometers Measure the power of incident electromagnetic radiation by heating of a material that has temperature-dependent electrical resistance. A microbolometer is a specific type of bolometer used as a detector in a thermal camera.
Cryogenic detectors Sensitive to measure the energy of single x-ray, visible and infrared photons. Used in space applications
Pyroelectric detectors Detect photons through the heat they generate and the subsequent voltage generated in pyroelectric materials.
Thermopiles Detect electromagnetic radiation through heat, then generating 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

Device Description
Photoreceptor cells Used in bionics for the retina that detect light through biological function.
Chemical detectors - photographic plates A chemical reaction that results in splitting silver halide molecule into an atom of metallic silver and a halogen atom. The photographic developer causes adjacent molecules to split similarly.

Polarization

Device Description
photorefractive effect Used in holographic data storage.
Polarization-sensitive photodetectors Use optically anisotropic materials to detect photons of a desired linear polarization

Application of photodetectors

Photodetectors have application in various fields from industries to medical and residential to commercial uses.

  1. Sensing applications
  2. Communication couplers for electrical isolation between circuits
  3. As recording element.