Bottom gate drain-offset polysilicon thin film transistors (poly-Si TFTs) were fabricated on SiO2 coated Si wafers. After completing gate oxide deposition, we exposed the wafers to air in a clean room. Poly-Si films were deposited on the gate oxides for the active layer of the drain offset poly-Si TFTs with changing the air-exposure time. Threshold voltage shift to positive value and turn-off current raise with increasing the air-exposure time were observed. In this paper, we focused on evaluating the causes of the turn-off current raise with the air-exposure time. The carbons piled up at the poly-Si/SiO2 interface were observed by a SIMS measurement, which is considered to be the origin of negative charges. The concentration of the carbon was remarkably increased by expanding the air-exposure time. The existence of the negative charges in the oxide was also found by a capacitance-voltage measurement. We conclude that the carbons originated from the air in the clean room are the main cause of the threshold voltage and turn-off current variation in the drain-offset poly-Si TFTs. (C) 2004 Elsevier B.V. All rights reserved.