The present invention provides a semiconductor device (20) comprising a trench (5) formed in a semiconductor substrate formed of a stack (4) of layers (1,2,3), a layer (6) of a first, grown dielectric material covering sidewalls and bottom of the trench (5), the layer (6) including one or more notches (13) at the bottom of the trench (5) and one or more spacers (14) formed of a second, deposited dielectric material to fill the one or more notches (13) in the layer (6) formed of the first, grown dielectric material. The semiconductor device (20) according to the present invention shows improved breakdown voltage and on-resistance. The present invention furthermore provides a method for the manufacturing of such semiconductor devices (20).