Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. A photo detector has a p–n junction that converts light photons into current. 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:
Photoemission or photoelectric effect: Photons cause electrons to transition from the conduction band of a material to free electrons in a vacuum or gas.
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 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. A photodetector or array is typically covered by an illumination window, sometimes having an anti-reflective coating.
Properties
There are a number of performance metrics, also called figures of merit, by which photodetectors are characterized and compared
Spectral response: The response of a photodetector as a function of photon frequency.
Quantum efficiency: The number of carriers 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.
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.
Photomultiplier tubes containing a photocathode which emits electrons when illuminated, the electrons are then amplified by a chain of dynodes.
Phototubes containing a photocathode which emits electrons when illuminated, such that the tube conducts a current proportional to the light intensity.
Microchannel plate detectors use a porous glass substrate as a mechanism for multiplying electrons. They can be used in combination with a photocathode like the photomultiplier described above, with the porous glass substrate acting as a dynode stage
Semiconductor
Active-pixel sensors are image sensors. Usually made in a complementary metal-oxide-semiconductor process, and also known as CMOS image sensors, APSs are commonly used in cell phone cameras, web cameras, and some DSLRs.
Cadmium zinc telluride radiation detectors can operate in direct-conversion mode at room temperature, unlike some other materials which require liquid nitrogen cooling. Their relative advantages include 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.
HgCdTeinfrared 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 and transformed into an electric signal.
Photoresistors or Light Dependent Resistors which change resistance according to light intensity. Normally the resistance of LDRs decreases with increasing intensity of light falling on it.
Photodiodes which can operate in photovoltaic mode or photoconductive mode. Photodiodes are often combined with low-noise analog electronics to convert the photocurrent into a voltage that can be digitized.
Phototransistors, which act like amplifying photodiodes.
Pinned photodiodes, a photodetector structure with low lag, low noise, high quantum efficiency, and low dark current, widely used in most CCD and CMOS image sensors.
Quantum dot photoconductors or photodiodes, which can handle wavelengths in the visible and infrared spectral regions.
Semiconductor detectors are employed in gamma and X-ray spectrometry and as particle detectors.
Photovoltaic cells or solar cells which produce a voltage and supply an electric current when sunlight or certain kinds of light shines on them.
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.
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
Photoreceptor cells in the retina detect light through, for instance, a rhodopsin photon-induced chemical cascade.
Chemical detectors, such as photographic plates, in which a silver halide molecule is split into an atom of metallic silver and a halogen atom. The photographic developer causes adjacent molecules to split similarly.
A graphene/n-type silicon heterojunction has been demonstrated to exhibit strong rectifying behavior and high photoresponsivity. Graphene is coupled with silicon quantum dots 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.
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.
