This article present a multi-level model on the doping mechanism and defect state in the gap of a-Si:H films, and calcnlate the doping efficiency η using the method of statistical physics. It was found that the results from the model and calculation not only cover all arguments of Street et al., but also make a distinction between heavy and light doping. The η of light doping agrees with usual results, only heavy doping obeys η∝?-1/2low.