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The photovoltaic effect in p-n junctions can be used to measure the diffusion lengths of minority carriers in a semiconductor. The short-circuit current in an irradiated p-n junction is Ik=egL, where e is the electron charge, g the generation rate (number of electron-hole pairs per unit volume and unit time generated by the radiation), and L a quantity which equals the diffusion length if the position of the p-n junction is suitably chosen. If g is known, the diffusion length can be calculated from the short-circuit current. The generation rate can easily be calculated if the junction is irradiated by Â¿ rays from a Co60 source. The method will be described more closely in this paper. Diffusion lengths were measured in Si, GaAs, and InP containing a p-n junction. In GaAs diffusion lengths up to 8 Â¿ were measured, and up to 130 Â¿ in InP.