Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or by various performance metrics, such as spectral response. Semiconductor-based photodetectors typically photo detector have 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 (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