Abstract

Flat-panel detectors (FPDs) are digital detectors that are widely utilised in medical applications such as general radiography and mammography. They are exposed to electromagnetic excitation to produce digital images of internal body organs. The electromagnetic radiation (optical or X-rays) creates electrons and holes in the photoconductor layer. These photogenerated electrons and holes drift under the influence of the applied electric field and constitute a photocurrent. Apart from the photocurrent, there is an undesirable current known as the lag signal detected in the devices after the removal of the excitation. The lag signal causes image artifacts in the digital image output that can lead to inaccurate or misleading medical diagnosis. Generally, the photocurrent and lag signals in FPDs are analyzed through experimental means. To the best of our knowledge, a complete mathematical model does not exist in literature to represent the entire current profile, which includes the photocurrent and the lag signal, for exponential carrier generation in FPDs.
This thesis is concerned with developing a mathematical model for transient photocurrent and lag signal in FPDs for X-ray and optical excitation by considering charge carrier trapping and detrapping in the energy distributed defect states under exponentially distributed carrier generation across the photoconductor. The model for the transient and steady-state carrier distributions and hence the photocurrent has been developed by solving the carrier continuity equation for both holes and electrons. The lag signal is modeled by solving the trapping rate equations considering the thermal release and trap filling effects. The model is applied to amorphous selenium (a-Se) detectors for both chest radiography and mammography. The dependence of the lag signal on various factors such as X-ray exposure, applied electric field, and temperature is analyzed. The lag signal is found to be more prominent in chest radiographic detector than in mammographic detectors. Moreover, the transient rise and decay of the photocurrent profile as a function of time is studied. The quick rise and decay parts, and then the slow rise and decay parts of the photocurrent profile are due to the hole and electron transports, respectively. The model calculations are compared with the published experimental data and show a good agreement within the limits of experimental error. The satisfactory fittings of the experimental data with the model reveals the origin of the residual signal in the detector, which could be helpful for predicting the lag signal and designing the readout circuit and finding the ways to reduce lag.