1. National Institute of Advanced Industrial Science and Technology (AIST), Device Technology Research Institute, 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan
2. National Institute of Advanced Industrial Science and Technology (AIST), Advanced Power Electronics Research Center (ADPERC), 1-8-31 Midoriokai, Ikeda, Osaka 563-8577, Japan