It is known that MILC polysilicon thin-film transistors (TFTs) show excellent electrical properties except for a relatively high leakage current. In this work, a lightly doped drain structure (LDD) was prepared in an n-type MILC poly silicon TFT and the effect of the LDD length on the leakage current has been systematically investigated. It turned out that the LDD is mainly responsible for the low leakage current, and no more than 0.5 µm of the lightly doped region is necessary to lower the leakage current to less than 3 × 10−11A at VD = 10 V. No appreciable effect on the leakage current can be found by an LDS (lightly doped source) but an LDS length of more than 1.5 µm caused a reduction of the on-current to below 10−5 A, whereas the on-current is typically more than 10−4 A in a conventional MILC TFT.