A method of manufacturing a semiconductor device is provided. The method includes providing a semiconductor substrate, forming a plurality of fins on the semiconductor substrate, forming a plurality of shallow trench isolation (STI) structures on the semiconductor substrate on opposite sides of the fins, forming a dummy gate on the fins, forming gate spacers on opposite sides of the dummy gate, etching a first portion of the STI structures disposed outside a gate region, the first portion having a first predetermined thickness, forming an interlayer dielectric over the semiconductor substrate, removing the dummy gate, etching a second portion of the STI structures disposed in the gate region, the second portion having a second predetermined thickness, and forming a high-k dielectric layer and a metal gate in an area where the dummy gate is removed.