Characteristics of hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs) with various phosphorus doping concentrations in a-SiN:H layer have been investigated. The a-Si:H TFTs with a heterostructure of the phosphorus-contained a-SiN:H and intrinsic a-Si:H layers have been fabricated by a plasma enhanced chemical vapor deposition using SiH(4), PH(3), and NH(3) gases. The mobility, the threshold and the surface roughness of the devices have been investigated and compared with the conventional amorphous silicon thin film transistors.