We have studied the effect of Cl incorporation on the performance of amorphous silicon thin film transistors (a-Si:H(:Cl) TFTs). The off-state leakage current of a-Si:H(:Cl) TFTs under light illumination is much lower than that of a-Si:H TFTs, because the photoconductivity of a-Si:H(:Cl) is much lower than that of conventional a-Si:H. The a-Si:H(:Cl) films deposited between [SiH2Cl2]/[SiH4]=0.04 and[SiH2Cl2]/[SiH4]=0.12show p-type conduction, leading to the much lower photoconductivity. The TFT using a-Si:H(:Cl) deposited with [SiH2Cl2]/[SiH4]=0.04 exhibits a field effect mobility of 0.41 cm2/V s and a threshold voltage of 5.56 V; however the off-state leakage current under light illumination is two orders of magnitude smaller than that of a conventional a-Si:H TFT. ©1997 American Institute of Physics.