tailieunhanh - Smart Material Systems and MEMS - Vijay K. Varadan Part 3

Tham khảo tài liệu 'smart material systems and mems - vijay k. varadan part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 54 Smart Material Systems and MEMS a b c P- Silicon diaphragm P2 Contacts for sense lines Vacuu cover nclosur Resonant microbeam Drive ine Silicon Proof mass Silicon flexure Figure Resonant microbeam system a showing cross-sectional views of the polysilicon beam attached to a silicon diaphragm b or silicon flexure c along with d a schematic of the related microbeam test circuit 15 . Reprinted from Sensors Actuators A 35 Zook J D Burns D W Guckel H Sniegowski J J Engelstad R L and Feng Z Characteristics of polysilicon resonant microbeams pp. 51-59 Copyright 1992 with permission from Elsevier First we will discuss an accelerometer consisting of a proof mass suspended over an FET with the gate electrode of the device attached to the suspended structure. The anchors of the meander support are elevated to suspend the beam above the gate region Figure . This arrangement provides a gap between the gate and the insulator layer thus keeping the threshold voltage for the FET constant 16 . The meander beams attached to this system are configured such that the electrode moves in the direction shown in Figure . This motion of the gate electrode changes the transistor drain current without affecting the current density through the channel. The sensitivity S of this device is given by the following d n S dW A m where ỖID is the change in drain current and dW is the change in the depth to which the gate is overlapping the channel. Sensors for Smart Systems 55 a Gate I ị Source Drain W T L Channel Anchor b Structural layer----- Channel Drain------ Substrate ------------Gate electrode Insulator Source Figure Schematics of a movable-gate field effect transistor a top view b cross-sectional view. For a typical n-channel FET the drain current is given by ID 2L 2 VGS V VDS V2S for VDS VGS VT 3-19 and Id 2l Vgs Vt 2 for VDS Vgs Vt where VGS and VDS are the gate-to-source and drain-to-source voltages VT is the threshold voltage at which the channel begins to .

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