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 @M03796@ potential applied to the @R05054@ creates a region at the @I03082@ 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 @M03796@ in the photovoltaic @M03958@ where the photodiode is actually generating the @E01935@ supplied to the load. In a biased @M03958@, the photoconductive @M03958@, the reverse current is proportional to the @I03255@. A Schottky-barrier photodiode is constructed by deposition of a metal @F02366@ on a @S05591@ surface in such a way that no @I03082@ layer is present. The barrier thickness depends on the impurity @DT07203@ concentration in the @S05591@ 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 @M03796@ 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 @M03796@ potential near to breakdown potential so that further electron-hole pairs are formed leading to @S05472@ of the photocurrent. This operational @M03958@ for @P04629@ is the so-called Geiger @M03958@, similar to that of the gas filled @G02598@. Avalanche photodiodes can also be operated in the proportional @M03958@.