Optimum design of power MOSFETs

Three cell geometeries (rectangle, square and hexagon) have been investigated using a lumped Rdsmodel. It is demonstrated that for each geometry one can calculate a spacing of the p-well diffusions that minimizes Rds. 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 Rdsfor the typical range of breakdown voltages (50- 1000V) and realizable cell sizes (20 to 40 µm), the square gives a smaller Rdsthan the rectangle, and the hexagon gives a smaller resistance than the square. Therefore, the hexagon is the preferred geometry for these situations.