University:
Massachusetts Institute of Technology
Course:
6.012 | Microelectronic Devices and Circuits
Academic year:

2023
Views:

438

Pages:

19
Author:

Aylin Miranda

Lecture 9 - MOSFET (I) MOSFET I-V Characteristics October 6, 2005 Contents: 1. MOSFET: cross-section, layout, symbols 2. Qualitative operation 3. I-V characteristics Reading assignment: Howe and Sodini, Ch. 4, §§4.1-4.3 Announcements: Quiz 1: 10/13, 7:30-9:30 PM, (lectures #1-9); open book; must have calculator. Key questions • How can carrier inversion be exploited to make a transistor? • How does a MOSFET work? • How does one construct a simple first-order model for the current-voltage characteristics of a MOSFET? 1. MOSFET: layout, cross-section, symbols body polysilicon gate source drain gate n+ p+ n+ n+ p inversion layer channel n gate oxide gate length p+ p n+ n+ n+ gate width n n+ STI edge Key elements: • inversion layer under gate (depending on gate voltage) • heavily-doped regions reach underneath gate ⇒ inversion layer electrically connects source and drain • 4-terminal device: body voltage important 2 Circuit symbols Two complementary devices: • n-channel device (n-MOSFET) on p-Si substrate (uses electron inversion layer) • p-channel device (p-MOSFET) on n-Si substrate (uses hole inversion layer) IDn G + D + IDn VDS > 0 B D _ G + VBS VGS _ _ S− (a) n-channel MOSFET B S VSG S + S + G VSB _ B VSD > 0 −IDp D− G B −IDp (b) p-channel MOSFET D 2. Qualitative operation Water analogy of MOSFET: • Source: water reservoir • Drain: water reservoir • Gate: gate between source and drain reservoirs VDS VGS ID G n+ S D VGS n+ n+ inversion layer water VDS depletion region p B source gate drain Want to understand MOSFET operation as a function of: • gate-to-source voltage (gate height over source water level) • drain-to-source voltage (water level difference between reservoirs) Initially consider source tied up to body (substrate or back). Three regimes of operation: 2 Cut-off regime: • MOSFET: VGS < VT , VGD < VT with VDS > 0. • Water analogy: gate closed; no water can flow regardless of relative height of source and drain reservoirs. VGS VT , VGD > VT , with VDS > 0. • Water analogy: gate open but small difference in height between source and drain; water flows. VGS>VT VGD>VT G n+ S D n+ n+ inversion layer everywhere depletion region p Electrons drift from source to drain ⇒ electrical current! • VGS ↑ → |Qn| ↑ → ID ↑ • VDS ↑ → |Ey | ↑ → ID ↑ ID small VDS ID VGS>VT small VDS VDS 0 0 0 VDS VT VGS 2 Saturation regime: • MOSFET: VGS > VT , VGD < VT (VDS > 0). • Water analogy: gate open; water flows from source to drain, but free-drop on drain side ⇒ total flow independent of relative reservoir height! VGS>VT VGDVT small VDS VDS 0 0 0 VDS This is the linear or triode regime. VT VGS In general, ID = W VDS µnCox (VGS − − VT )VDS L 2 Equation valid if VGS − Vc(y) ≥ VT at every y. Worst point is y = L, where Vc (y) = VDS , hence, equation valid if VGS − VDS ≥ VT , or: VDS ≤ VGS − VT ID VDS=VGS-VT VGS VGS=VT 0 0 VDS term responsible for bend over of ID : − VDS 2 To understand why ID bends over, must understand first channel debiasing: |Qn(y)| VDS=0 Cox(VGS-VT) VDS>0 0 0 y L |Ey(y)| VDS>0 VDS=0 0 0 y L Vc(y) VDS VDS>0 VDS=0 0 y L 0 VGS-Vc(y) VDS=0 VGS VDS local gate overdrive VDS>0 VT 0 L y Along channel from source to drain: y ↑ → Vc(y) ↑ → |Qn(y)| ↓ → |Ey (y)| ↑ Local ”channel overdrive” reduced closer to drain. Impact of VDS : |Qn(y)| VDS=0 Cox(VGS-VT) VDS 0 0 L y |Ey(y)| VDS VDS=0 0 0 L y Vc(y) VDS VDS VDS=0 0 L 0 y VGS-Vc(y) VDS=0 VDS VGS VDS local gate overdrive VT 0 L y As VDS ↑, channel debiasing more prominent ⇒ ID rises more slowly with VDS 3µm n-channel MOSFET Output characteristics (VGS = 0 − 4 V, ∆VGS = 0.5 V ): Zoom close to origin (VGS = 0 − 2 V, ∆VGS = 0.25 V ): Transfer characteristics (VDS = 0 − 100 mV, ∆VDS = 20 mV ): Key conclusions • The MOSFET is a field-effect transistor: – the amount of charge in the inversion layer is controlled by the field-effect action of the gate – the charge in the inversion layer is mobile ⇒ conduction possible between source and drain • In the linear regime: – VGS ↑⇒ ID ↑: more electrons in the channel – VDS ↑⇒ ID ↑: stronger field pulling electrons out of the source • Channel debiasing: inversion layer ”thins down” from source to drain ⇒ current saturation as VDS approaches: VDSsat = VGS − VT