Mosfet having a drift region with a graded doping profile and methods of manufacturing thereof

A method of producing a vertical power semiconductor transistor device, the method comprising:

• forming a drain region of a first conductivity type, a drift region of the first conductivity type, a body region of a second conductivity type opposite the first conductivity and which is separated from the drain region by the drift region, and a source region of the first conductivity type and which is separated from the drift region by the body region;
• forming a gate trench which extends through the source region and the body region and into the drift region, the gate trench including a gate electrode;
• forming a field electrode in the gate trench or in a separate trench, wherein the trench that includes the field electrode terminates before reaching the drain region such that a bottom of the trench that includes the field electrode is separated from the drain region by a section of the drift region;
• establishing a generally linearly graded first doping profile in the drift region and which increases from the body region toward the bottom of the trench that includes the field electrode;
• and
• establishing a graded second doping profile in the drift region and which increases at a greater rate than the first doping profile from an end of the first doping profile toward the drain region.