Semiconductor device and method of fabricating the same

A semiconductor device includes an epitaxial layer, at least one gate trench, and at least one trench gate structure. The gate trench includes a lower gate trench and an upper gate trench, and a width of the lower gate trench is less than a width of the upper gate trench. The trench gate structure is disposed in the gate trench, and the trench gate structure includes a bottom gate structure, a middle gate structure, and a top gate structure. The thickness of the second gate dielectric layer of the middle gate structure is less than the thickness of the first gate dielectric layer of the bottom gate structure. The thickness of the third gate dielectric layer of the top gate structure is less than the thickness of the second gate dielectric layer of the middle gate structure. The first, second, and third gate electrodes are separated from each other.