Field effect transistor (FET) test devices were fabricated on hydrogenated amorphous silicon (a-Si:H) films grown by capacitively coupled r.f. glow discharge of silane. From the FET characteristics, the flat-band voltages were estimated as a function of the a-Si:H film thickness. A systematic decrease in the flat-band voltage with thickness was observed both in the dark as well as under illumination. The density of states around the midgap NF was also estimated following the theory put forward by Harm et al. A reduction in NF with film thickness was observed both in dark and illuminated conditions. The results have been explained on the basis of the better homogeneity of thicker films leading to a reduction in defect states and a shift in the Fermi level towards the conduction band. © 1991.