X-ray detectors are fabricated with non-intentionally doped thick epitaxial GaAs layers grown by a chemical technique. They are p(+)/l/n(+) structures in which the highly doped p- and n-type layers are made by ion implantation on both faces of the i epilayers. In order to understand the behaviour of charge collection, we modelize the capacitance-voltage characteristics of such structures, which are then compared with experimental data. (c) 2005 Elsevier B.V. All rights reserved.