The nanostructure and morphology of MoTa films deposited on glass substrates as gate metals are studied by combined scanning force microscopy and plan-view transmission electron microscopy (TEM). Hydrogenated amorphous silicon bottom-gate thin-film transistors (TFTs) are used to investigate whether the deposition of each subsequent layer of metal or dielectric is a conformal process. The improvement in the electron mobility of TFTs is discussed in relation to the surface roughness of the bottom-gate metal. The conformability of the nanometer-scale multilayered structure of the TFTs is confirmed by means of cross-sectional TEM. The electron mobility of the TFTs increase with a decrease in the surface roughness of the bottom-gate metal. The improvement in the TFT performance indicates that the smooth surface of the bottom-gate metal is the key factor.