A semiconductor device includes a semiconductor substrate having a gate trench penetrating through an active area and a trench isolation region surrounding the active area. The gate trench exposes a sidewall of the active area and a sidewall of the trench isolation region. The sidewall of the trench isolation region includes a void. A first gate dielectric layer conformally covers the sidewall of the active area and the sidewall of the trench isolation region. The void in the sidewall of the trench isolation region is filled with the first gate dielectric layer. A second gate dielectric layer is grown on the sidewall of the active area. A gate is embedded in the gate trench.