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Photodetectors are semiconductor devices that are capable of detecting light through a quantum electronic process. There are a variety of photodetectors, such as infrared, ultraviolet, and those used for optical communications, which operate near the infrared wavelength region of 0.8-€“1.6 μm. In communications applications, optical detectors are also categorized by their semiconductor materials and the bandgap energies used to realize them (Table 8.1). A different way to categorize photodetectors is by their junction structure, PN, PIN, or avalanche photodetector (APD). In the PN photodiode (PD), electron-hole pairs are created in the depletion region of the NP junction proportional to the optical power. The electron and hole pairs are swept out by the electric field, creating a photocurrent. In the PIN photodetector, the electric field is concentrated in a thin intrinsic layer I sandwiched between N and P types. APD is similar to a PIN but has an additional layer (for instance, P+LPN+ where L is a lightly doped layer). Thus, this kind of diode experiences a photocurrent gain. APDs are used for high-sensitivity, long-haul systems, such as small form pluggable (SFP) transceivers or wireless free-space optical links (optical data telescopes).
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