1. a Electrotechnical Laboratory , Tsukuba , 305-8568 , Japan
2. b Dept. of Electrical Eng. , Dong-A Univ. , Pusan , 604-714 , Korea
3. c Dept. of Chemical Eng. , Dong-A Univ. , Pusan , 604-714 , Korea
4. d Advanced Technology Center, Seiko Instruments Inc. , Chiba , 270-2222 , Japan