The effects of photo-related stress on the electrical performances of a-SiGe:H thin-film transistors (TFTs) were investigated in comparison with a-Si:H TFTs. Compared with a-Si:H TFTs, the a-SiGe:H TFTs show better stability to the light stress because the number of electrons involved in the creation of dangling bonds are smaller in a-SiGe:H TFTs, resulting in less light-induced degradation. However, a larger threshold voltage shift from the positive gate bias was observed due to the higher number of weak bonds in a-SiGe:H TFTs, which leads to a higher gate bias instability than is observed for a-Si:H TFTs. The temperature dependences of the electrical properties in a-SiGe:H TFTs were observed, and they indicated that a-SiGe:H TFTs follow a thermally activated behavior pattern. Based on the thermally activated behavior, a new model predicting the lifetime of a-SiGe:H TFT image sensors was proposed. The instability of the drain current with respect to the stress time under an electrical bias and light was estimated. Based on the calculated lifetime, the a-SiGe:H TFTs are predicted to be reliable for long-term applications in image sensors.