In this paper a theoretical study of diffusion of positrons to the surface of semiconductors with an internal electric field was reported. For the semi-infinite medium

by using a radiative boundary condition and δ-function method

the fraction of positrons diffusing back to the surface was calculated. The results are of universal significance. For the film with a thickness d

the fractions of positrons diffusing to the surfaces at x=0 and x=d under an absorbing boundary condition were also calculated with the same method. As d approaches infinity

in both cases of implantation profiles

solutions of the positron diffusion equation for the semi-infinite medium can be obtained. The results can be used to the analysis of experimental data for the overlayer/substrate system which contains one film or leveral films and a semi-infinite medium.