1State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China
2Key Laboratory of MEMS of the Ministry of Education, School of Electronic Science and Engineering, Southeast University, Nanjing 210096, P. R. China
3National Key Laboratory of Application Specific Integrated Circuit, Hebei Semiconductor Research Institute, Shijiazhuang 050051, P. R. China