1. Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan
2. Department of Mathematic and Physical Sciences, R.O.C. Air Force Academy, Kaohsiung 820, Taiwan
3. Department of Electronic Engineering, National Kaohsiung Normal University, Kaohsiung 802, Taiwan
4. Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan