We report on new results of the effects of ppm level phosphine doping in hydrogenated amorphous silicon. The results show that the dark conductivity changes from 5 × 10 −11 to 1 × 10 −3 Scm −1 with ppm phosphine doping. The results can be explained by the low defect state density near mid-gap in a-Si:H and the shift of the Fermi level by doping. These new results mean that the electronic properties of a-Si:H can be tuned by very fine doping without introducing large density of defect states. The application of these results in the photoconductive type image sensors is discussed.