[1] Satellite Venture Business Lab., The University of Tokushima, 2-1 Minami-josanjima, Tokushima 770-8506, Japan
[2] Dept. of Elec. and Electronic Eng., The University of Tokushima, 2-1 Minami-josanjima, Tokushima 770-8506, Japan
[3] Nitride Semiconductor Co., Ltd., 115-7 Itayajima, Akinokami, Seto-cho, Naruto, Tokushima 771-0360, Japan