The Hecht model describes the charge collection efficiency of semiconductor detectors using the mean free path of the charge carriers. While the fits to data are very good for non-irradiated detectors, modifications to the model are necessary to take into account the structural changes in the detectors induced by their exposure to high particle fluences. A modified model is presented. In this model, the mean free path depends on the shape of the electric field and on the charge carrier lifetimes. The lifetimes were measured experimentally from the front- and back-illuminations of the detectors by 660nm laser light and by a particles from an 241Am source. This new Hecht model was successfully fitted to alpha and beta charge collection efficiencies of standard and oxygenated silicon detectors after their irradiation by 10MeV protons with fluences varying from 10^11 to 3x10^14 p/cm2.