The effect of phosphorus doping in hydrogenated amorphous silicon (Si:H) and hydrogenated amorphous silicon nitride (a-SiN:H) for the electrical performances of hydrogenated amorphous silicon thin film transistors (a-Si:H TFTs) was studied. The phosphorus-doped layers in the a-Si:H and a-SiN:H with various concentrations were deposited by plasma-enhanced chemical vapor deposition (PECVD). We measured the electrical characteristics of a-Si:H TFTs fabricated by one or both phosphorus doping in a-Si:H and a-SiN:H, and then compared these results to conventional a-Si:H TFT. The sheet resistance and surface roughness of the a-SiN:H layers with various phosphorus doping concentrations were investigated to verify the effects of phosphorus doping on the electrical characteristics of the a-Si:H TFTs.