The dark conductivity of phosphorus‐doped amorphous‐silicon alloys (a‐Si:H:F) prepared by theRFplasma decomposition of a gaseous mixture of SiF4, H2and diluted PH3is extremely high; it exceeds 10 (O‐cm)‐1with only a small amount of PH3(∼500 ppm) added in the gas phase. These doping characteristics represent a significant improvement over the doping characteristics ofa‐Si:H alloys prepared by a glow‐discharge of SiH4. The improvement was found to be due to the fact that P‐dopeda‐Si:H:F contains microcrystallites which are embedded in an amorphous network. The percolation process in these two‐phase systems gives rise to high conductivity. We have used transmission electron microscopy (TEM) and diffraction (TED) to determine the critical surface fraction,ρc,of crystallinity at the onset of extended conduction. The measuredρcis approximately 0.46. This percolation limit provides a basis for the analysis of the electrical properties of P‐dopeda‐Si:H:F.