Corrections to the Hecht collection efficiency in photoconductive detectors under large signals: non-uniform electric field due to drifting and trapped unipolar carriers

The Hecht collection efficiency eta(0), and its modified expressions for exponential absorption, have been widely used in time-of-flight type transient photoconductivity experiments as well as in the assessment of the sensitivity of integrating-type radiation detectors. However, the equations apply under small signals in which the internal field remains uniform (unperturbed). We have used Monte Carlo simulation and the numerical solution of the continuity, trapping rate and Poisson equations to calculate the collection efficiency eta(r) (CE) for various levels of charge injection and deep trapping. The carriers are injected instantaneously very near the radiation receiving electrode and then drift under space charge perturbed conditions. The CE deviation from the ideal Hecht value has been quantified in terms of the injection ratio r and the normalized trapping time tau with respect to the transit time under small signals. The results can be represented by a scaled, compressed exponential with coefficients that depend on tau. A plot is provided for these coefficients. The CE drops significantly below the Hecht value as r increases and the deviation is more pronounced for smaller tau values. The errors in extracting tau from the application of the Hecht equation has been also calculated and mapped as a function of different r and tau values.