Patent ID: 7605428

Claim:
A depletion mode MOSFET formed in a semiconductor substrate of a first conductivity type with a background doping concentration, the substrate not comprising an epitaxial layer, the MOSFET comprising: a field oxide layer at a surface of the substrate, the field oxide layer having first and second openings formed therein, the first and second openings being separated by a segment of the field oxide layer; a source region of a second conductivity type located adjacent the surface of the substrate in the first opening in the field oxide layer; a drain region of the second conductivity type located adjacent the surface of the substrate in the second opening in the field oxide layer; a deep drain well adjoining and being located directly below the drain region, the deep drain well comprising at least two regions having different peak doping concentrations, the at least two regions comprising an upper dopant region and a lower dopant region, the upper dopant region being located directly below the drain region and directly above the lower dopant region, the drain region having a peak doping concentration greater than a peak doping concentration of either the upper dopant region or the lower dopant region, the lower dopant region having a peak doping concentration greater than a peak doping concentration of the upper dopant region; a gate and gate dielectric layer overlying a channel region of the substrate, the channel region being located adjacent the surface of the substrate in the first opening in the field oxide layer; and a drift region of the second conductivity type, the drift region abutting the deep drain well and extending from the deep drain well laterally under the segment of the field oxide layer and into the first opening in the field oxide layer, the drift region being conformal with the segment of the field oxide layer such that the drift region comprises a shallow portion and a deep portion, the shallow portion being located in an area directly below the segment of the field oxide layer and extending downward from the surface of the substrate to a shallow lower boundary, the deep portion being located in an area directly below the first opening in the field oxide layer and extending downward from the surface of the substrate to a deep lower boundary, the deep lower boundary being at a greater depth in the substrate than the shallow lower boundary; wherein the doping concentration of the channel region is the background doping concentration.