A semiconductor device and a method of forming the same are disclosed. The semiconductor device includes a semiconductor substrate; a fin extending from the semiconductor substrate; a first charged dielectric layer covering a bottom portion of the fin, the first charged dielectric layer having net fixed first-type charges; a second charged dielectric layer covering the first charged dielectric layer, the second charged dielectric layer having net fixed second-type charges, the second-type charges being opposite to the first-type charges; and a gate structure engaging a top portion of the fin.