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Principle of operation A photodiode is a p-n junction or p-i-n structure. When light of sufficient photon energy strikes the diode, it excites an electron thereby creating a mobile electron and a positively charged electron hole. If the absorption occurs in the junction's depletion region, these carriers are swept from the junction by the built-in field of the depletion region, producing a photocurrent. Photodiodes can be used under either zero bias (photovoltaic mode) or reverse bias (photoconductive mode). 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. Avalanche photodiodes have a similar structure, but they are operated with much higher reverse bias. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device. Materials The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite an electron across the material's bandgap will produce significant photocurrents. Materials commonly used to produce photodiodes: Because of their greater bandgap, silicon-based photodiodes generate less noise than germanium-based photodiodes, but germanium photodiodes must be used for wavelengths longer than approximately 1 µm. Features Critical performance parameters of a photodiode include: responsivity: The ratio of generated photocurrent to incident light power, typically expressed in A/W when used in photoconductive mode. The responsivity may also be expressed as a quantum efficiency, or the ratio of the number of photogenerated carriers to incident photons and thus a unitless quantity. dark current: The current through the photodiode in the absence of any input optical signal, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. Dark current must be accounted for by calibration if a photodiode is used to make an accurate optical power measurement, and it is also a source of noise when a photodiode is used in an optical communication system. noise-equivalent power: (NEP) The minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 hertz bandwidth. The related characteristic detectivity (D) is the inverse of NEP, 1/NEP; and the specific detectivity () is the detectivity normalized to the area (A) of the photodetector, . The NEP is roughly the minimum detectable input power of a photodiode. When a photodiode is used in an optical communication system, these parameters contribute to the sensitivity of the optical receiver, which is the minimum input power required for the receiver to achieve a specified bit error ratio. Applications P-N photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices, and photomultiplier tubes. Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for remote controls in VCRs and televisions. In other consumer items such as camera light meters, clock radios (the ones that dim the display when its dark) and street lights, photoconductors are often used rather than photodiodes, although in principle either could be used. 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). They are also used in Blood gas monitors. PIN diodes are much faster and more sensitive than ordinary p-n junction diodes, and hence are often used for optical communications and in lighting regulation. P-N photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified charge-coupled devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night-vision equipment and laser range finding. Comparison with photomultipliers Advantages compared to photomultipliers: Disadvantages compared to photomultipliers: See also | ||||||||||
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