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In this paper, we present a numerical study of charge sharing in photon-counting X-ray imaging detectors. The study is based on charge transport simulations combined with a system level Monte Carlo simulation code to calculate the energy resolution of different pixel detector configurations. Our simulations show that the charge sharing is very sensitive to the electric field distribution in the device, and that the higher doping levels used in the GaAs detector reduce the effect of charge sharing significantly. Our study concludes that one of advantages in using very heavy semiconductor materials in X-ray imaging detectors is the possibility to suppress charge sharing utilizing structures with much higher electric field. A 100-μm-thick epitaxial GaAs detector absorbs 52% of the photons, while a 300-μm-thick silicon (detector absorbs only 8% of the photons (30 keV source). In addition to the superior stopping power, the GaAs detector has five times lower charge diffusion, resulting in superior spatial and energy resolution.