Carrier concentration distribution of overlap and non overlap photoconductive detectors was calculated and analyzed by solving the one dimensional ambipolar transport equation , according to the overlap structure proposed by M.A.Kinch. The results show that the overlap structure offers a region storing the minority carriers, which suppresses the sweepout and increases the average photo induced carrier concentration, thus improving the responsivily. The overlap structure was implemented by means of two kinds of technologies, and the measured data were given in this paper.