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We have proposed and fabricated a self-aligned polysilicon thin-film transistor (poly-Si TFT) with a thick dielectric layer at the gate edges near the source and drain. A T-shaped polysilicon gate was successfully formed by the damascene process used in VLSI interconnection technology. During the on state, an inversion layer is induced by the subgate as a drain so that the on current is still high and the poly-Si region under the subgate behaves as an offset, reducing the off-state leakage current during the off-state. As the subgate dielectric becomes 3.5 times thicker than the main gate oxide, the minimum off-state leakage current of the new TFT is decreased from 1.4×10-10 to 1.3×10-11 without sacrifice of the on current. In addition, the on-off current ratio is significantly improved.