1. a Functional Devices Laboratories, SHARP Corporation, 273-1 Kashiwa, Kashiwa, Chiba 277, Japan. Former Address: Central Research Laboratories, SHARP Corporation , 2613-1 Ichinomoto-cho, Tenri, Nara , 632 , Japan
2. b Department of Electrical Engineering, Faculty of Engineering , Nagaoka University of Technology , Kamitomioka 1603-1, Nagaoka, Niigata , 940-21 , Japan