Modulation transfer function of photoconductive x-ray image detectors: effects of charge carrier trapping

Abstract

An analytical expression for calculating the modulation transfer function (MTF) due to distributed carrier trapping in the bulk of the photoconductor of a direct conversion pixellated x-ray image detector is derived using the trapped charge distribution across the photoconductor. The analytical expressions of trapped charge distributions are also derived by solving the continuity equation for both types of carriers (electrons and holes). The MTF of photoconductive x-ray detectors is analysed in terms of normalized parameters, namely (a) the normalized x-ray absorption depth (absorption depth/photoconductor thickness) and (b) normalized carrier schubwegs (schubweg/thickness). Trapping of the carriers that move towards the pixel electrodes degrades the MTF performance, whereas trapping of the carriers that move away from the pixels improves the sharpness of the x-ray image. The MTF model is applied to polycrystalline CdZnTe detectors and is fitted to recently published experimental results. The theoretical model shows very good agreement with reported experimental data.

Effect of repeated x‐ray exposure on the resolution of amorphous selenium based x‐ray imagers

“…6. It has been found previously 10 that the trapping of the carriers that move toward the pixel electrodes ͑electrons in this case͒ degrades the high frequency components of the MTF, whereas trapping of the oppositely charged carriers that move away from the pixels ͑holes in this case͒ improves the high frequency components of the MTF curve. The high frequency components of the MTF increases with accumulated exposure ͑Fig.…”

“…Thus, the trapping of holes ͑i.e., charges which drift toward the bias electrode͒ generates a MTF CC that increases with frequency. 10 Hence, charge carriers of opposite signs have a complementary effect on the frequency dependence MTF CC of the imager, and appear to cancel each other. However, the extent of the cancellation depends on the depth at which the x rays are absorbed and the lifetime of the carriers.…”

“…The MTF reduction or enhancement due to trapping of both electrons and holes as a function of carrier lifetimes and absorption depth of x rays are described in detail in Ref. 10. It is important to note that when a free carrier recombines with a trapped charge of the opposite sign ͑that is, trapped in a previous exposure͒, its effect on the overall MTF of the imager is the same as that of a free carrier being trapped in the bulk.…”

“…10 The subscripts t and b represent carriers that drift towards the top and bottom electrodes, respectively. Under positive bias, q t ͑x͒ = p t ͑x͒ − n t ͑x͒ and Q b represent holes.…”

“…͑It was, however, successfully applied to CdZnTe based AMFPIs to explain their MTF. 10 ͒ Nonetheless, the KK model 10 predicted that the trapping of the carriers that move toward the pixel electrodes degrades the MTF performance, whereas the trapping of the other type of carriers that move away from the pixels improves the high frequency components and thus enhances the sharpness of the x-ray image. In this paper, we further develop and enhance the KK model 10 by including repeated exposure cycles and the recombination of drifting carriers with oppositely charged trapped carriers to extract the MTF ͑in Sec.…”