In x-and γ-ray semiconductor detectors, the distribution of the internal electric field strongly affects their charge-collection properties. In planar detectors, the electric field distribution depends not only on the semiconductor characteristics, but mainly on the nature of the electrical contacts. This is an issue in CdTe and CdZnTe (CZT) detectors, where many efforts have been devoted to the study and the improvement of contacts, with the aim of suppressing dark current to low values and enhancing the collection of the photo-generated carriers.