A semiconductor device includes a semiconductor substrate, a gate structure, a first drift region, a first source/drain region, and a gate oxide layer. The gate structure and the gate oxide layer are disposed on the semiconductor substrate. The first drift region is disposed in the semiconductor substrate. The first source/drain region is disposed in the first drift region. At least a part of a first portion of the gate oxide layer is disposed between the gate structure and the semiconductor substrate in a vertical direction. A second portion of the gate oxide layer is disposed between the first portion and the first source/drain region in a horizontal direction. The second portion includes a bottom extending downwards and a first concave top surface located above the bottom. A part of the first drift region is located under the first portion and the second portion of the gate oxide layer.