gain and sensitivity of photodetector 1 current gain is defined as a ratio of the number of charge carriers crossing a particular cross section of photodetector 1 to the number of primary photons photon 6 absorbed by photodetector 1 simplification of this ratio using a standard technique allows the gain factor to be calculated as where r is the reflectance of reflector 16 n a is the photon absorption efficiency of n layer 10 and p layer 11 and n e is the photon emission efficiency of p layer 12 and n layer 13 for the photodetectors shown in fig1 and 4 both n e and n a are substantially equal to unity it is appreciated that by properly selecting materials which exhibit very high values for n e n a and r a gain factor of 100 or more is easily obtained photodetector devices of the type shown in fig1 have been fabricated using epitaxial growth techniques liquid phase epitaxy has been predominantly used but molecular beam epitaxy is also applicable these techniques yield devices which are about 100 microns square the thickness of photodetector 1 is substantially equal to the number of pn and pn junctions times approximately 5 microns plus the substrate thickness typical substrate thicknesses are on the order of 75 microns hence the thickness of photodetector 1 is slightly greater than 87 microns during epitaxial growth of the devices impurities are introduced into each layer the type of impurity and the concentration of the impurity affect the conductivity of each layer impurity types and impurity concentrations for the various layers of an exemplary embodiment of photodetector 1 are tabulated.