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We have analyzed the device behavior of a poly-Si thin-film transistor (TFT) with a lightly doped drain (LDD) structure using 2-D device simulation. It is found that the reason that the on current does not fall very much is that the electron channel oozes from the channel region to the LDD region and the electric current paths spread to the entire LDD region. On the other hand, the reason that the off current is effectively reduced is undoubtedly that the electric field at the interface between the channel and LDD regions is weakened. However, it should be noted that the depletion region is formed at the drain edge owing to the “pseudo” space-charge region in the channel region, where the carrier density is much lower than in the other channel region, although the net space-charge does not exist.