Semiconductor structures are provided. A semiconductor structure includes a bottom substrate having a first region and a second region; an insulation layer formed on the bottom substrate in the first region; a top substrate on side surface of the trench and the insulation layer; a first fin portion formed over the insulation layer, and a gate structure crossing the first fin portion. The first fin portion is electrically isolated from the bottom substrate through the insulation layer to reduce the leakage current at the bottom of the first fin portion. The gate structure covers part of side and top surfaces of the first fin portion.