The origin of the leakage current in low-pressure chemically vapor deposited polycrystalline silicon (polysilicon) thin-film transistors is investigated by low-frequency noise measurements. The leakage current depends on the structure of the polysilicon layer. When the grain size is relatively large (about 120 nm), the noise spectra show a pure1/fbehavior caused by carrier fluctuation within the space charge region of the drain junction. For smaller grain size (about 50 nm), the observed1/f1.5spectra are attributed to thermal noise of the bulk polysilicon film. ©1997 American Institute of Physics.