The theory and practical behavior of P+−&pgr;−N+semiconductor photodiodes are given. It is shown that under operating conditions response may be anticipated for light modulated at frequencies in excess of 1010cps. The signal‐to‐noise ratio of such a diode is discussed both for simple detection of amplitude‐modulated light and for the case of optical heterodyne detection. In the latter case a favorable comparison is found with a photomultiplier tube detector. A description of the structure and fabrication of a germanium device is given, and results are presented which exhibit the long wavelength response, high quantum efficiency, and high frequency sensitivity expected. Measurements indicate no reduction in response at frequencies as high as 2000 Mc.