The growth of a-Si:H and the resulting weak bond and defect formation mechanism are analyzed in terms of the adsorbed SiH3 model of growth. It is found that this model describes the surface processes well, but it needs further development to correctly describe: the temperature dependence of the formation of defects and weak bonds, since the surface defect density decreases monotonically with temperature and does not show a minimum near 250 degrees C. We show that the experimentally observed increase in hydrogen content, weak bond and defect density at lower deposition temperatures can be accounted for by a hydrogen evolution reaction from H-2* sites.