Since the first ideal specific resistance model by Fujihira in 1997 and the first commercial superjunction MOSFET by Infineon technology in 1998, the technology and the understanding of superjunction devices have been gradually progressed. Although Fujihira’s ideal model was developed to estimate the specific on-state resistance at a given cell pitch as a function of the breakdown voltage, the model does not work for sub micro cell pitches because it does not consider the parasitic junction field effect transistor (JFET) presented in the superjunction. Fujihira’s model assumes the limit of the minimum cell pitch, by saying that the specific resistance can be decreased indefinitely with decreasing the cell pitch. In 2018, the first universal model for the specific on-state resistance of a superjunction MOSFET including the parasitic JFET effect was derived by H. Kang and F. Udrea. This model employs the classical JFET theory into the superjunction structure and clearly presents the true limit of the cell pitch of superjunction MOSFETs, as well as the specific resistance for different semiconductor materials. The classical JFET theory can be also applied to various superjunction structures and the detailed derivation process is described in Chapter 2. In Chapter 3, an advance device structure, three-dimensional (3-D) superjunction MOSFET, is introduced. To understand the superior performance of 3-D superjunction MOSFET, radial Poisson equation is employed. From theses mathematical calculation, it can be clearly seen that the 3-D superjunction is able to decrease the specific on-state resistance by half of the 2-D. In Chapter 4, based on the depletion process of superjunction during the switching, the inner circuit models for capacitance are provided. From the inner circuit models, typical capacitance curves with respect to the applied bias are drawn and they are expressed in terms of material parameters.