I. Introduction
Recent improvements in crystalline quality and productive technologies of free-standing GaN substrates have accelerated developments of the vertical structure GaN power devices, which can extract the GaN's full potentials for the power-conversion applications. GaN p-n junction diodes with high breakdown voltages around 3 kV showed the larger Baliga's figure of merits (FOM) than those of SiC Schottky-barrier diodes [1]–[3].