A type of diode used as a light detector in optical transmission systems. A PIN comprises three layers of semiconducting material in the forward current-carrying direction. The first layer is chemically doped, i.e., infused with impurities, to create a positive (p) electromagnetic region.The second layer is either undoped or lightly doped to retain its intrinsic (i) properties and, therefore, is neither strongly positive nor strongly negative. The third layer is doped to create a negative (n) electromagnetic region. So, the diode structure is Positive, Intrinsic, and Negative, or PIN. A PIN generates a single electron from each photon received and, therefore, does not provide a significant gain, or increase in signal strength. However, PINs are fairly rugged and inexpensive. PINs generally are matched with light-emitting diode (LED) and vertical cavity surface-emitting laser (VCSEL) light sources and multimode (MMF) glass optical fiber (GOF) or plastic optical fiber (POF) in relatively low speed, short haul fiber optic transmission systems (FOTS). See also diode, dopant, FOTS, GOF, laser diode, LED, light detector, MMF, POF, and VCSEL.