This work describes the deposition, annealing and characterisation of semi-insulating oxygen-doped silicon films at temperatures compatible with polysilicon circuitry on glass. The semi-insulating layers are deposited by the plasma enhanced chemical vapour deposition technique from silane (SiH4), nitrous oxide (N2O) and helium (He) gas mixtures at a temperature of 350 degrees C. The as-deposited films are then furnace annealed at 600 degrees C which is the maximum process temperature. Raman analysis shows the as-deposited and annealed films to be completely amorphous. The most important deposition variable is the N2O/SiH4 gas ratio. By varying the N2O/SiH4 ratio the conductivity of the annealed films can be accurately controlled, for the first time, down to a minimum of approximate to 10(-7) Omega(-1) cm(-1) where they exhibit a T--1/4 temperature dependence indicative of a hopping conduction mechanism, Helium dilution of the reactant gases is shown to improve both film uniformity and reproducibility. A model for the microstructure of these semi-insulating amorphous oxygen-doped silicon films is proposed to explain the observed physical and electrical properties.