A field effect transistor (FET) includes body regions of a first conductivity type over a semiconductor region of a second conductivity type. The body regions form p-n junctions with the semiconductor region. Source regions of the second conductivity type extend over the body regions. The source regions form p-n junctions with the body regions. Gate electrodes extend adjacent to but are insulated from the body regions by a gate dielectric. A carbon-containing region extends in the semiconductor region below the body regions.