Mos device with isolated drain and method for fabricating the same

A metal-oxide-semiconductor (MOS) device with an isolated drain, comprising:

• a semiconductor substrate having a first conductivity type;
• a first well region embedded in a first portion of the semiconductor substrate, having a second conductivity type opposite to the first conductivity type;
• a second well region disposed in a second portion of the semiconductor substrate, overlying the first well region and having the first conductivity type;
• a third well region disposed in a third portion of the semiconductor substrate, overlying the first well region and adjacent to the second well region, having the second conductivity type;
• a fourth well region disposed in a fourth portion of the semiconductor substrate between the first and third well regions, having the first conductivity type;
• a gate stack formed over the semiconductor substrate, covering a portion of the second and third well regions;
• a source region disposed in a portion of the second well region, having the second conductivity type;
• and
• a drain region disposed in a portion of the third well region, having the second conductivity type.