High voltage field effect transistors with superjunctions and method of making the same

A field effect transistor, comprising:

• a semiconductor channel having a doping of a first conductivity type;
• a gate structure overlying the semiconductor channel and comprising a stack of a gate dielectric and a gate electrode;
• a source region and a drain region having a doping of a second conductivity type and laterally spaced from the gate structure and from each other along a first horizontal direction, the second conductivity type being an opposite of the first conductivity type;
• a source-side extension region laterally extending between the source region and a first bottom surface segment of the gate dielectric, having a doping of the second conductivity type, and including a source-side-extension plate portion and source-side-extension rail portions that overlie the source-side-extension plate portion;
• source-side counter-doped rails having a doping of the first conductivity type, embedded in the source-side extension region, and interlaced with the source-side-extension rail portions along a second horizontal direction to provide a first superjunction structure;
• a drain-side extension region laterally extending between the drain region and a second bottom surface segment of the gate dielectric, having a doping of the second conductivity type, and including a drain-side-extension plate portion and drain-side-extension rail portions that overlie the drain-side-extension plate portion;
• and
• drain-side counter-doped rails having a doping of the first conductivity type, embedded in the drain-side extension region, and interlaced with the drain-side-extension rail portions along the second horizontal direction to provide a second superjunction structure.