A field effect transistor according to the present invention includes a semiconductor layer including a groove, an insulating film formed on an upper surface of the semiconductor layer and having an opening above the groove and a gate electrode buried in the opening to be in contact with side surfaces and a bottom surface of the groove and having parts being in contact with an upper surface of the insulating film on both sides of the opening, wherein the gate electrode has a T-shaped sectional shape in which a width of an upper end is larger than a width of the upper surface of the insulating film.

By directing an ion beam with a beam divergence on a process surface of a semiconductor substrate, parallel electrode trenches are formed in the semiconductor substrate. A center axis of the directed ion beam is tilted to a normal to the process surface at a tilt angle , wherein at least one of the tilt angle and the beam divergence is not equal to zero. The semiconductor substrate is moved along a direction parallel to the process surface during formation of the electrode trenches. A conductive electrode is formed in the electrode trenches, wherein first sidewalls of the electrode trenches are tilted to the normal by a first slope angle 1 with 1=(+0/2) and second sidewalls are tilted to the normal by a second slope angle 2 with 2=(/2).