Influence of phosphorus doping in the a-Si:H layer with various deposition time and phosphine gas flow rate in hydrogenated amorphous silicon thin film transistor (a-Si:H TFT) has been investigated. The a-SiN:H layer, the phosphorus doped a-Si:H layer and the n(+) a-Si:H layer were deposited by plasma enhanced chemical vapor deposition (PECVD) using SiH4, PH3, Ar, H-2 and NH3 gases. Proper phosphorus doping in the a-Si:H layer affected the channel of the a-Si:H TFT and improved the characteristics. Also, the field effect mobility was increased from 0.22 to 0.61 cm(2)/V.s. The characteristics of phosphorus doping in the a-Si:H TFT were investigated and compared with the conventional a-Si:H TFT.