The effects of nitrogen trifluoride (NF3) in the reactant gas stream on the properties of plasma-enhanced chemical-vapor-deposited (PECVD) semi-insulating polysilicon (SIPOS) films were investigated. The addition of NF3 to the reactant gas stream resulted in a significant increase in the deposition rate. The presence of Si-F species in the SIPOS was revealed by Fourier transform infrared and X-ray photoelectron spectroscopy. A gradual transformation of the Si-F species into Si-O-x is observed in fluorinated SIPOS films deposited with a higher nitrous oxide flow rate, or alternatively, in films containing a higher oxygen content. The fluorinated SIPOS films exhibit both a larger magnitude and temperature dependence in their current densities than unfluorinated films and also exhibit two activation energies, one below and one above 230 K. However, a lower forward current density is obtained for a fluorinated SIPOS heterojunction compared with an unfluorinated SIPOS heterojunction. Furthermore, thermionic emission was found to be the predominant conduction mechanism in heterojunctions of fluorinated SIPOS on p-type crystalline silicon.