In this work 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 GaAs detectors reduce the effect of charge sharing significantly. Our study concludes that one of advantage's 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 mum thick epitaxial GaAs detector absorbs 52% of the photons, while a 300 pin thick Silicon detector absorbs only 8% of the photons (30keV source). In addition to the superior stopping power, the GaAs detector has 5 times lower charge diffusion, resulting in superior spatial and energy resolution.