Optimum design of power MOSFETs

Three cell geometeries (rectangle, square and hexagon) have been investigated using a lumped R
ds
model. It is demonstrated that for each geometry one can calculate a spacing of the p-well diffusions that minimizes R
ds
. This optimum spacing is a function of the vertical and lateral p-well dimensions, the desired breakdown voltage, and, in some cases, the average current density in the cell. It is shown that in choosing the minimum R
ds
for the typical range of breakdown voltages (50- 1000V) and realizable cell sizes (20 to 40 µm), the square gives a smaller R
ds
than the rectangle, and the hexagon gives a smaller resistance than the square. Therefore, the hexagon is the preferred geometry for these situations.