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N-type junctionless (JL) planar poly-Si thin-film transistors (TFTs), which contain an in situ heavily phosphorous-doped channel with thickness ranging from 8 to 12 nm, were fabricated and characterized. The devices exhibit superior current drive and good control over performance variability. From C-V characterization, the ionized dopant concentration in the channel is determined to be around 2 × 1019 cm-3 and the fixed charge density to be around -6 × 1012 cm-2. The negative fixed charge density is probably related to the segregation of phosphorous species at the oxide/channel interface. We also observed a reverse short-channel effect from the relationship between the threshold voltage and the channel length. One mechanism considering enhanced phosphorous segregation is proposed to explain this finding.