Photodetectors, based on photon absorption or thermal emission, are made from semiconductor materials that are very sensitive in the spectral range from ultraviolet to far infrared. Detector figures of merit such as responsivity, noise, noise equivalent power and detectivity, and ideal performance and cooling requirements are given. Modes of operation include charge‐coupled device, photovoltaic diode, photoconductor, and bolometer. Most semiconductor detectors operate as photon detectors where the minimum photon energy detected is defined by a discrete activation energy of the semiconductor. However, micromachined bolometers and superlattice infrared detectors are in development for thermal imaging and chemical spectroscopy. Materials and detector types are discussed. Detector material preparation technologies are presented for intrinsic detectors where the photon absorption coefficient is high, and for the low absorption coefficient detectors such as impurity‐doped Ge or Si. Fabrication technology making possible large photodetector focal planes that detect photons and perform signal processing are integrated structures either in monolithic form or in hybrid form. The monolithic arrays range up to 10 6 detectors. The hybrid focal planes, mostly for infrared detection, contain up to 10 5 . Photodetector arrays have been developed for imaging across the spectrum and are used in CCD cameras, chemical spectroscopy, missile seekers, and night vision equipment.