Abstract

In semiconductor X-ray photodetectors, efficient charge transport and collection are critical for their performance. This thesis presents an experimental and theoretical investigation into charge carrier mobility, trapping, and collection efficiency. Using time-of-flight (TOF) and interrupted-field time-of-flight (IFTOF) transient photoconductivity techniques, amorphous selenium (a-Se) photoconductive films of various thicknesses were characterized to measure carrier mobility and deep trapping lifetimes. A high-voltage transient photoconductivity setup was implemented to mitigate displacement current and switching transients, enabling precise capture of photocarrier transit dynamics. Measurements across different alloy-stabilized a-Se films revealed well-defined trap lifetimes and mobilities, shedding light on how alloy doping influences trap dynamics and charge transport in these X-ray photoconductors. Complementing the experiments, a semi-analytical carrier packet drift analysis (CPDA) and a numerical model were developed to examine charge collection efficiency of a radiation detector under non-uniform electric fields. These models account for space-charge effects from ionized dopants and trapped photogenerated carriers, allowing simulation of internal electric field profiles and CCE across a range of material parameters and operating conditions. The simulations show that field non-uniformity causes significant deviations from the idealized Hecht collection efficiency formula, and in most cases CCE deteriorates compared to the uniform-field scenario. However, for certain ratios of carrier lifetime to transit time, when X-ray absorption occurs near the incident electrode, non-uniform fields can even improve charge collection. The theoretical predictions were validated against published data from perovskite X-ray detectors, demonstrating good agreement for general radiographic applications. Together, the experimental and modelling results advance the understanding of charge transport and charge collection in photoconductors, guiding the design of high-performance X-ray radiation detectors.