The morphology of polycrystalline films grown by low‐pressure chemical‐vapor deposition (LPCVD) is investigated by transmission electron microscopy (TEM) as a function of the film thickness, the deposition pressure, and the level of contamination. An orientation filtering mechanism, due to the growth‐velocity competition in the early stage of growth, is responsible for the preferred orientation of the films. The size of the crystallites, the surface roughness, and the type of the structural defects are investigated by combined cross‐sectional and plane‐view TEM analysis. In polycrystalline silicon thin‐film transistors (TFTs), the influence of surface roughness scattering on the mobility is investigated by measuring the effective electron mobility under high effective normal field at 295 and 77 K. Although the surface curvature is increased when the deposition pressure is decreased, the surface roughness scattering is constant in the deposition pressure range from 40 to 0.5 mTorr. By decreasing the deposition pressure from 40 to 10 mTorr, although the grain size increases, the TFT performance degrades due to the following factors: (a) the increase of the grain‐boundary trap density which is related to the change of the mode of growth at 10 mTorr; and (b) the increase of impurity contamination in the environment of the LPCVD system with constant silane flow rate at all pressures. At a deposition pressure of 0.5 mTorr the TFT performance is improved indicating that the grain size is the prevailing key factor.