In the fabrication of thin film transistors (TFTs), little attention has been paid to the polycrystalline silicon thin films prepared at low temperatures where the glass substrates are adopted so far. Since the film quality is not sufficient to achieve high mobility, e.g., over 50 cm2/Vs in spite of high benefit in their industrial fabrication. We have fabricated bottom gate TFTs with poly-Si and poly- Si1-xGex thin films deposited at 450°C by newly developed low-temperature LPCVD technique and characterized electrical characteristics of the TFTs: disilane and a small amount of either germanium tetrafluoride or fluorine were used as material gases and helium as carrier gas. Thermal annealing for dopant activation and atomic hydrogen treatment for defect passivation were carried out. We found that the defect elimination process is important for improving TFT performance significantly. Finally the mobility of p-channel and n-channel TFTs have attained 36.3-54.4 cm2/Vs and 57 cm2/Vs, respectively.