Low-frequency drain current fluctuation noise was measured in a series of polycrystalline silicon thin-film transistors (polysilicon TFTs), characterized by an average grain size Lg and in-grain defect density Din. In one type of polysilicon TFT, Din is low (about 5×108 cm-2) and Lg varies from about 67 to 145 nm. In another type of polysilicon TFT, Lg remains constant (about 2.5 μm) and Din varies from about 5×1012 to 5×1010 cm-2. It is demonstrated that the noise originates from fluctuations of carriers due to carrier capture/release processes: (a) in traps located near the polysilicon/SiO2 interface and (b) in traps located at the grain boundaries. The polysilicon/SiO2 interface is distinguished from that of the crystalline silicon/SiO2 by comprising the in-grain traps, in addition to the oxide traps. The proposed carrier number fluctuation model, sufficient to explain noise in polysilicon TFTs, is closely related to the structural parameters Lg and Din of the polysilicon layer. © 2002 American Institute of Physics.