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A photodiode consists of an active P-N junction that generates a response when light falls on it, triggering the photoelectric effect or photoemission. This process produces free electrons in the intrinsic region of the diode, leading to a reverse current that is proportional to the amount of illumination. The linear dependence of the current on the light intensity makes photodiodes useful as photodetectors in various applications, including light-activated switches. Some photodiodes are based on silicon, which makes them sensitive to light throughout the visible spectrum and into the near-infrared region. For longer wavelengths, InGaAs (Indium Gallium Arsenide) photodiodes are available, though they tend to be more expensive. Germanium photodiodes can serve as a cheaper alternative to InGaAs diodes but have a slower response time.