The effects of large signals on charge collection in photoconductive X-ray image detectors

A model for studying the effects of large signals on charge collection efficiency in amorphous selenium based X-ray image detectors is described by considering bimolecular recombination between drifting charge carriers and space charge effects. The continuity equations for both holes and electrons, and the Poisson´s equation across the photoconductor for a short step X-ray exposure are simultaneously solved by the finite difference method. The numerical results are compared with the Monte Carlo simulation results. It is found that the recombination plays practically no role on charge collection up to the total carrier generation rate q₀ of 10₁₉ EHPs/m²-s at the applied electric field of 10 V/mum. At large values of q₀ , the charge collection efficiency gradually decreases with increasing q₀ and approaches almost zero at q₀ larger than 10²⁴ EHPs/m²-s. The effect of recombination on charge collection increases with decreasing applied electric field and increasing the normalized absorption depth (absorption depth per unit thickness)