A sensitive detector of ionizing radiation in which the output is a current pulse proportional to the energy falling in or near the depletion region of a reverse-biased semiconductor junction. The first types were made by evaporating a thin layer of gold on to a polished wafer of n-type germanium; however, gold–silicon devices can be operated at room temperature and these have superseded the germanium type, which have to be operated at the temperature of liquid nitrogen to reduce noise. When the gold–silicon junction is reverse-biased a depletion region, devoid of charge carriers (electrons and holes), forms in the silicon. Incoming ionizing radiation falling in this depletion region creates pairs of electrons and holes, which both have to be collected in order to give an output pulse proportional to the energy of the detected particle.

Junction detectors are used in medicine and biology as well as in space systems.