Photoconductivity in Lead Selenide: Theory of the Dependence of Sensitivity on Film Thickness and Absorption Coefficients

Abstract

Recently developed theoretical expressions for the responsivity $R_s$ and noise of photoconductive films are examined. Assuming that the time constant is independent of film thickness we express $R_s$ and signal/noise ($\frac{S}{N}$) in terms of the absorption coefficient $\alpha$ and thickness $d$. Using a curve of $\alpha$ vs $\lambda$ for lead selenide obtained by Avery from crystal measurements, we calculate $R_s$ vs $\lambda$ for films of different thicknesses. These curves indicate that the photoconductive knee should lie near 5μ for PbSe films of all thicknesses. Experimentally, however, the position of the knee is observed to be a function of film thickness, being near 3.3μ in thin films and 5μ in thick films.