A two-electrode, radiation-sensitive junction formed in a semiconductive material. A junction is formed by two successive regions of a semiconductive material having, respectively, an excess of electrons (n-type) or holes (p-type). A @[email protected] potential applied to the @[email protected] creates a region at the @[email protected] that is depleted of majority carriers. Each incident photon produces electron-hole pairs in the depletion region resulting in a measurable signal current. The photodiode can be operated either with zero @[email protected] in the photovoltaic @[email protected] where the photodiode is actually generating the @[email protected] supplied to the load. In a biased @[email protected], the photoconductive @[email protected], the reverse current is proportional to the @[email protected] A Schottky-barrier photodiode is constructed by deposition of a metal @[email protected] on a @[email protected] surface in such a way that no @[email protected] layer is present. The barrier thickness depends on the impurity @[email protected] concentration in the @[email protected] layer. The incident radiation generates electon-hole pairs within the depletion region of the barrier where they are collected efficiently and rapidly by the built-in field. A PIN (p-intrinsic-n) diode is a planar diffused diode consisting of a single crystal having an intrinsic (undoped or compensated) region sandwiched between p- and n-type regions. A @[email protected] potential applied across the detector depletes the intrinsic region of charge carriers, constituting the radiation-sensitive detector volume. The number of electron-hole pairs produced is dependent on the energy of the incident photons. An avalanche photodiode is a photodiode in which the photogenerated electron-hole pairs are accelerated by a @[email protected] potential near to breakdown potential so that further electron-hole pairs are formed leading to @[email protected] of the photocurrent. This operational @[email protected] for @[email protected] is the so-called Geiger @[email protected], similar to that of the gas filled @[email protected] Avalanche photodiodes can also be operated in the proportional @[email protected]