Amorphous hydrogenated silicon (a‐Si:H) was deposited by SiH4decomposition on a hot tungsten filament. The substrate temperature was held at 400 °C for all samples, maintaining conditions where material combining a low defect density and a low hydrogen content is obtained. A systematic study of the effects of gas pressure, substrate‐to‐filament distance, and filament temperature on film properties is presented, allowing insight into the growth condition required for this material as well as the significance of secondary gas phase reactions. Material of good optoelectronic quality is obtained at high growth rates. The stability with respect to light degradation was compared to typical plasma deposited films. Conditions for the transition from amorphous to microcrystalline films, observed under gas phase dilution with hydrogen, were investigated. Byinsituellipsometry and atomic force microscopy the nucleation and film morphology were shown to be significantly different from those for plasma‐chemical vapor deposition material. ©1996 American Institute of Physics.