We have studied the effects of adding small amounts of Mg on the electrical and thermal properties of a‐Se films. The addition of Mg in the ppm range has resulted in significant changes in the charge transport and stability. The doped samples have shown to be excellent n ‐like layers which have electron ranges much greater than hole ranges, μeτe ≫ μhτh. The n ‐like layers block the injection of holes from the positive electrode which results in a lower dark current in a‐Se based X‐ray detectors. The Differential scanning calorimetry studies have shown that a‐Se doped with lower concentrations of Mg is more stable than samples with higher concentrations. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)