X-ray sensitivity of photoconductors: application to stabilized a-Se

The x-ray sensitivity of a high-resistivity photoconductor sandwiched betwe en two parallel plate electrodes and operating under a constant field is an alysed by considering charge carrier generation that follows the x-ray phot on absorption profile and taking into account both electron and hole trappi ng phenomena but neglecting recombination, bulk space charge and diffusion effects. The amount of collected charge in the external circuit due to dist ributed generation of electrons and holes through the detector is calculate d by integrating the Hecht collection efficiency with Ramo's theorem across the sample thickness. The results of the model allow the x-ray sensitivity to be calculated as a function of the applied held, detector thickness and electron and hole ranges (mu tau), given the field and energy dependence o f the electron and hole pair creation energy, W+/-, and the energy spectrum of incident x-ray radiation. The sensitivity model was applied to stabiliz ed a-Se that is currently used as a successful x-ray photoconductor in the recently developed fiat panel x-ray image detectors. Recent free electron-h ole pair creation energy versus electric field data at room temperature and appropriate electron and hole drift mobilities were used to calculate the sensitivity for monoenergetic x-rays at 20 and at 60 keV. For the 20 keV ra diation, it was shown that a typical detector thickness of 200 mum (4 x att enuation depth at 20 keV) with currently attainable electron and hole trapp ing parameters in a-Se was operating optimally, the sensitivity of which ca n only be increased by further increasing the applied field. With the recei ving electrode positively biased, the sensitivity was much more dependent o n the hole lifetime than electron Lifetime. The absence of hole transport r esults in a reduction in sensitivity by a factor of about 4.4, whereas the absence of electron transport results in a sensitivity degradation of only 22%. The ratio of hole trapping limited sensitivity to electron trapping li mited sensitivity is about 0.3. For a detector of thickness 200 mum operati ng at 10 V mum(-1), the maximum sensitivity is about 220 pC cm(-2) mR(-1), and this sensitivity degrades by more than 10% when either the electron lif etime falls below similar to 20 mus or the hole lifetime falls below simila r to5 mus. When the hole lifetime is very short so that the sensitivity is substantially reduced, the sensitivity versus thickness dependence at a giv en field exhibits a maximum tan optimal thickness) that is less than that f or full absorption. In the case of 60 keV x-ray photons, it is more useful to examine the sensitivity as a function of detector thickness given the pr actical bias voltage limit. The sensitivity versus thickness behaviour for a given bias voltage exhibits a maximum, that is an optimal thickness, that is less than that for nearly full absorption. Electron lifetimes longer th an similar to 200 mus and hole lifetimes longer than similar to 10 mus do n ot significantly affect the sensitivity.