A method for fabricating a semiconductor transistor device. An epitaxial layer is grown on a semiconductor substrate. A gate trench is formed in the epitaxial layer. A spacer is formed on a sidewall of the gate trench. A recess is formed at the bottom of the gate trench. A thermal oxidation process is performed to form an oxide layer in the recess. The oxide layer completely fills the recess. The spacer is then removed. A gate oxide layer is formed on the exposed sidewall of the gate trench. A gate is then formed into the gate trench.