Authors: M.Z. Kabir, S.-A. Imam
Affilation: Concordia University, Canada
Pages: 68 - 71
Keywords: amorphous selenium, avalanche detectors, dark current, interface states, carrier injections, thermal generation
Amorphous selenium (a-Se) is currently the only commercially viable X-ray photoconductor in direct conversion detectors. However, the a-Se detector is not perfect and it has low conversion gain (X-ray to free electron-hole pair, EHP, generation) compared to other potential photoconductors such as PbO. The overall noise can be even higher than the signal strength in some parts of a-Se detectors in low dose imaging and thus severely affect the diagnostic features of the image. The avalanche multiplication of holes in a-Se at the field of 70-110 V/um may increase the signal strength and improve the signal to noise ratio. However, the dark current in avalanche detectors can be high and very sensitive to temperature variation or the exposure. In this paper, we have performed a detailed analysis on quantitative dark current contributions from the bulk thermal generation, transient carrier depletion, and the carrier injections from the electrodes. We have developed analytical expressions for the above dark current components considering impact ionization of holes. We compare the model with the published experimental dark currents at various fields to determine the magnitudes of various dark current components. This work is extremely important to optimize the operating field and a-Se thickness.