Theory of Photoconductivity in Semiconductor Films

Abstract

Photoconductive films of the lead salt family are composed of a system of crystallites separated by intercrystalline barriers. The crystallites are lead salts while the intercrystalline barriers are an oxide of lead or of the lead salt. Space charge regions are present at the surface of the crystallites. The lifetime of hole-electron pairs is determined in part by surface states while the resistivity is strongly affected by intercrystalline barriers.We analyze a model which incorporates the above characteristics of a photoconductive film, except for the space charge effects. It is assumed that the primary photoeffect is absorption of light and production of hole-electron pairs in the crystallites. The change in conductivity results from a change in majority carrier density in the crystallites, and from reduction of intercrystalline barrier potentials.Equations are developed for the response to radiation, for the noise, and for the limit of sensitivity of the detector. These expressions contain familiar semiconductor parameters, and a new parameter which characterizes the relative importance of the change in carrier density as compared to the change in barrier potential. No attempt is made to calculate the parameters, but measurements necessary for their evaluation are briefly discussed. This permits a prediction of numerical values for responsivity, noise, and sensitivity which can be compared with experiment.