An attempt is made to analyze and summarize the literature data on hybrid x-ray GaAs pixel detectors designed for medical imaging. Criteria are stated for selecting a semiconductor material. It is shown that GaAs is currently preferable to the other semiconductor materials. Detector figures of merit used to select a semiconductor material and a process technology are listed. Major types of x-ray detector based on bulk-grown or epitaxial GaAs are described, namely, the planar Schottky, the planar p-i-n, and the 3D detector. The influence is considered of different process steps (contact formation, passivation coating, hybridization, etc.) on detector figures of merit. Monolithic technology is briefly discussed.