We have found experimentally that high-purity Ge low-energy X-ray detectors have a relatively thick entrance window which renders them practically use less below -2.S KcV. A simple X-ray fluorescence experinient establishes clearly that the window is physically in the Ge iraterial itself. Experiments with detectors made from "tifi'erent Ge crystals, <*nd with Schottky barrier contacts of different metals indicate that the effect is due to a basic property of tre transport of electrons near a

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... face. Theoretical considerations and a Monte Carlo calcula tion show that the window is caused by the escape of warm eJcctrtns which are the end product of a phcto event. The a?an free path of the electrons becomes longer as the,-lose energy by optical phonon collis ions and they can be trapped at th<: surface before they are pici'-d up by the electrK field.