The combined effect of the spacing between adjacent drain junctions, epilayer thickness, and resistivity in VDMOS transistors on both its breakdown voltage and

product is investigated. It is shown that an increase in the breakdown voltage results as the junctions spacing is reduced. That gives a significant difference in the

product when compared with results where this effect is ignored. A design optimization study is carried out to determine the parameters of a VDMOS transistor at 1000-V breakdown.