Material Limit of Power Devices—Applied to Asymmetric 2-D Superjunction MOSFET

In spite of the reporting of several mathematical approaches dealing with the behavior of the superjunction MOSFET's specific resistance, a study for the asymmetrical pillar (when the width of the n-pillar and the p-pillar are not the same at a given cellpitch) has not been carried out yet. When the width of one of the pillar (say n-pillar) is modified, the doping concentration (say donor) should be changed to maintain a charge balance condition. This in turn, changes the width of the depletion region, due to the parasitic JFET effect and as a result the effective on-state conduction path. This raises the question whether the best tradeoff between the specific on-state resistance and the breakdown voltage could be achieved by employing the conventional assumption of the same width of the n and p pillars. This paper clarifies the best option for the width of each pillar when designing a superjunction MOSFET and adapts the figures of merit to take into account the asymmetrical superjunction cell.