A semiconductor structure includes: a substrate with at least a trench therein, wherein the trench is filled with an insulation layer; a first polysilicon layer disposed on the insulation layer and covering at least two opposite borders of a top surface of the insulation layer; a second polysilicon layer disposed above the first polysilicon layer and the substrate; and a dielectric layer disposed between the first and second polysilicon layers, wherein the first and second polysilicon layers are respectively shaped as first and second strips.