Downloaded 33 times
Uploaded byvidhya DS
Chapter3
This document discusses the operation and modeling of semiconductor devices used in digital integrated circuits, including diodes, MOS transistors, and their parasitic components. It covers device physics concepts like depletion regions, threshold voltage, carrier transport equations, and capacitances. Models are presented for manual analysis and SPICE simulation of diodes and MOSFETs in different regions of operation. Emerging effects in deep-submicron transistors like velocity saturation and threshold variations are also examined.
More Related Content
Chapter3
- 1. © Digital IntegratedCircuits2nd Devices Digital IntegratedDigital Integrated CircuitsCircuits A Design PerspectiveA Design Perspective The DevicesThe Devices Jan M. Rabaey Anantha Chandrakasan Borivoje Nikolic July 30, 2002
- 2. © Digital IntegratedCircuits2nd Devices Goal of this chapterGoal of this chapter Present intuitive understanding of device operation Introduction of basic device equations Introduction of models for manual analysis Introduction of models for SPICE simulation Analysis of secondary and deep-sub- micron effects Future trends
- 3. © Digital IntegratedCircuits2nd Devices The DiodeThe Diode n p p n B A SiO2 Al A B Al A B Cross-section of pn-junction in an IC process One-dimensional representation diode symbol Mostly occurring as parasitic element in Digital ICs
- 4. © Digital IntegratedCircuits2nd Devices Depletion RegionDepletion Region hole diffusion electron diffusion p n hole drift electron drift Charge Density Distance x+ - Electrical xField x Potential V ξ ρ W2-W1 ψ0 (a) Current flow. (b) Charge density. (c) Electric field. (d) Electrostatic potential.
- 5. © Digital IntegratedCircuits2nd Devices Diode CurrentDiode Current
- 6. © Digital IntegratedCircuits2nd Devices Forward BiasForward Bias x pn0 np0 -W1 W2 0 pn(W2) n-regionp-region Lp diffusion Typically avoided in Digital ICs
- 7. © Digital IntegratedCircuits2nd Devices Reverse BiasReverse Bias x pn0 np0 -W1 W20 n-regionp-region diffusion The Dominant Operation Mode
- 8. © Digital IntegratedCircuits2nd Devices Models for Manual AnalysisModels for Manual Analysis VD ID = IS(eVD/φT – 1) + – VD + – + – VDon ID (a) Ideal diode model (b) First-order diode model
- 9. © Digital IntegratedCircuits2nd Devices Junction CapacitanceJunction Capacitance
- 10. © Digital IntegratedCircuits2nd Devices Diffusion CapacitanceDiffusion Capacitance
- 11. © Digital IntegratedCircuits2nd Devices Secondary EffectsSecondary Effects –25.0 –15.0 –5.0 5.0 VD (V) –0.1 ID(A) 0.1 0 0 Avalanche Breakdown
- 12. © Digital IntegratedCircuits2nd Devices Diode ModelDiode Model ID RS CD + - VD
- 13. © Digital IntegratedCircuits2nd Devices SPICE ParametersSPICE Parameters
- 14. © Digital IntegratedCircuits2nd Devices What is a Transistor?What is a Transistor? VGS ≥ VT R o n S D A Switch! |VGS| An MOS Transistor
- 15. © Digital IntegratedCircuits2nd Devices The MOS TransistorThe MOS Transistor Polysilicon Aluminum
- 16. © Digital IntegratedCircuits2nd Devices MOS Transistors -MOS Transistors - Types and SymbolsTypes and Symbols D S G D S G G S D D S G NMOS Enhancement NMOS PMOS Depletion Enhancement B NMOS with Bulk Contact
- 17. © Digital IntegratedCircuits2nd Devices Threshold Voltage: ConceptThreshold Voltage: Concept n+n+ p-substrate DS G B VGS + - Depletion Region n-channel
- 18. © Digital IntegratedCircuits2nd Devices The Threshold VoltageThe Threshold Voltage
- 19. © Digital IntegratedCircuits2nd Devices The Body EffectThe Body Effect -2.5 -2 -1.5 -1 -0.5 0 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 V BS (V) V T (V)
- 20. © Digital IntegratedCircuits2nd Devices Current-Voltage RelationsCurrent-Voltage Relations A good ol’ transistorA good ol’ transistor Quadratic Relationship 0 0.5 1 1.5 2 2.5 0 1 2 3 4 5 6 x 10 -4 VDS (V) ID (A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V Resistive Saturation VDS = VGS - VT
- 21. © Digital IntegratedCircuits2nd Devices Transistor in LinearTransistor in Linear n+n+ p-substrate D S G B VGS xL V(x) +– VDS ID MOS transistor and its bias conditions
- 22. © Digital IntegratedCircuits2nd Devices Transistor in SaturationTransistor in Saturation n+n+ S G VGS D VDS > VGS - VT VGS - VT +- Pinch-off
- 23. © Digital IntegratedCircuits2nd Devices Current-Voltage RelationsCurrent-Voltage Relations Long-Channel DeviceLong-Channel Device
- 24. © Digital IntegratedCircuits2nd Devices A model for manual analysisA model for manual analysis
- 25. © Digital IntegratedCircuits2nd Devices Current-Voltage RelationsCurrent-Voltage Relations The Deep-Submicron EraThe Deep-Submicron Era Linear Relationship -4 VDS (V) 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 x 10 ID (A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V Early Saturation
- 26. © Digital IntegratedCircuits2nd Devices Velocity SaturationVelocity Saturation ξ (V/µm)ξc = 1.5 υn(m/s) υsat = 105 Constant mobility (slope = µ) Constant velocity
- 27. © Digital IntegratedCircuits2nd Devices PerspectivePerspective ID Long-channel device Short-channel device VDS VDSAT VGS - VT VGS = VDD
- 28. © Digital IntegratedCircuits2nd Devices IIDD versus Vversus VGSGS 0 0.5 1 1.5 2 2.5 0 1 2 3 4 5 6 x 10 -4 VGS(V) ID(A) 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 x 10 -4 VGS(V) ID(A) quadratic quadratic linear Long Channel Short Channel
- 29. © Digital IntegratedCircuits2nd Devices IIDD versus Vversus VDSDS -4 VDS(V) 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 x 10 ID(A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V 0 0.5 1 1.5 2 2.5 0 1 2 3 4 5 6 x 10 -4 VDS(V) ID(A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V Resistive Saturation VDS = VGS - VT Long Channel Short Channel
- 30. © Digital IntegratedCircuits2nd Devices A unified modelA unified model for manual analysisfor manual analysis S D G B
- 31. © Digital IntegratedCircuits2nd Devices Simple Model versus SPICESimple Model versus SPICE 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 x 10 -4 VDS (V) ID (A) Velocity Saturated Linear Saturated VDSAT=VGT VDS=VDSAT VDS=VGT
- 32. © Digital IntegratedCircuits2nd Devices A PMOS TransistorA PMOS Transistor -2.5 -2 -1.5 -1 -0.5 0 -1 -0.8 -0.6 -0.4 -0.2 0 x 10 -4 VDS (V) ID(A) Assume all variables negative! VGS = -1.0V VGS = -1.5V VGS = -2.0V VGS = -2.5V
- 33. © Digital IntegratedCircuits2nd Devices Transistor ModelTransistor Model for Manual Analysisfor Manual Analysis
- 34. © Digital IntegratedCircuits2nd Devices The Transistor as a SwitchThe Transistor as a Switch VGS ≥ VT R o n S D ID VDS VGS = VDD VDD/2 VDD R0 Rmid ID VDS VGS = VDD VDD/2 VDD R0 Rmid
- 35. © Digital IntegratedCircuits2nd Devices The Transistor as a SwitchThe Transistor as a Switch 0.5 1 1.5 2 2.5 0 1 2 3 4 5 6 7 x 10 5 V DD (V) R eq (Ohm)
- 36. © Digital IntegratedCircuits2nd Devices The Transistor as a SwitchThe Transistor as a Switch
- 37. © Digital IntegratedCircuits2nd Devices MOS CapacitancesMOS Capacitances Dynamic BehaviorDynamic Behavior
- 38. © Digital IntegratedCircuits2nd Devices Dynamic Behavior of MOS TransistorDynamic Behavior of MOS Transistor DS G B CGDCGS CSB CDBCGB
- 39. © Digital IntegratedCircuits2nd Devices The Gate CapacitanceThe Gate Capacitance tox n+ n+ Cross section L Gate oxide xd xd L d Polysilicon gate Top view Gate-bulk overlap Source n+ Drain n+ W
- 40. © Digital IntegratedCircuits2nd Devices Gate CapacitanceGate Capacitance S D G CGC S D G CGC S D G CGC Cut-off Resistive Saturation Most important regions in digital design: saturation and cut-off
- 41. © Digital IntegratedCircuits2nd Devices Gate CapacitanceGate Capacitance WLCox WLCox 2 2WLCox 3 CGC CGCS VDS /(VGS-VT ) CGCD 0 1 CGC CGCS = CGCD CGC B WLCox WLCox 2 VGS Capacitance as a function of VGS (with VDS = 0) Capacitance as a function of the degree of saturation
- 42. © Digital IntegratedCircuits2nd Devices Measuring the Gate CapMeasuring the Gate Cap 2 1.52 1 2 0.5 0 3 4 5 6 7 8 9 10 3 102 16 2 VGS (V) VGS GateCapacitance(F) 0.5 1 1.5 22 2 I
- 43. © Digital IntegratedCircuits2nd Devices Diffusion CapacitanceDiffusion Capacitance Bottom Sidewall Sidewall Channel Source ND Channel-stop implant NA1 Substrate NA W xj LS
- 44. © Digital IntegratedCircuits2nd Devices Junction CapacitanceJunction Capacitance
- 45. © Digital IntegratedCircuits2nd Devices Linearizing the Junction CapacitanceLinearizing the Junction Capacitance Replace non-linear capacitance by large-signal equivalent linear capacitance which displaces equal charge over voltage swing of interest
- 46. © Digital IntegratedCircuits2nd Devices Capacitances in 0.25Capacitances in 0.25 µµm CMOSm CMOS processprocess
- 47. © Digital IntegratedCircuits2nd Devices The Sub-Micron MOS TransistorThe Sub-Micron MOS Transistor Threshold Variations Subthreshold Conduction Parasitic Resistances
- 48. © Digital IntegratedCircuits2nd Devices Threshold VariationsThreshold Variations VT L Long-channel threshold Low VDS threshold Threshold as a function of the length (for low VDS) Drain-induced barrier lowering (for low L) VDS VT
- 49. © Digital IntegratedCircuits2nd Devices Sub-Threshold ConductionSub-Threshold Conduction 0 0.5 1 1.5 2 2.5 10 -12 10 -10 10 -8 10 -6 10 -4 10 -2 VGS (V) ID(A) VT Linear Exponential Quadratic Typical values for S: 60 .. 100 mV/decade The Slope Factor ox DnkT qV D C C neII GS +=1,~ 0 S is ∆VGS for ID2/ID1 =10
- 50. © Digital IntegratedCircuits2nd Devices Sub-ThresholdSub-Threshold IIDD vsvs VVGSGS VDS from 0 to 0.5V −= − kT qV nkT qV D DSGS eeII 10
- 51. © Digital IntegratedCircuits2nd Devices Sub-ThresholdSub-Threshold IIDD vsvs VVDSDS ( )DS kT qV nkT qV D VeeII DSGS ⋅+ −= − λ110 VGS from 0 to 0.3V
- 52. © Digital IntegratedCircuits2nd Devices Summary of MOSFET OperatingSummary of MOSFET Operating RegionsRegions Strong Inversion VGS >VT Linear (Resistive) VDS <VDSAT Saturated (Constant Current) VDS ≥VDSAT Weak Inversion (Sub-Threshold) VGS ≤VT Exponential in VGS with linear VDS dependence
- 53. © Digital IntegratedCircuits2nd Devices Parasitic ResistancesParasitic Resistances W LD Drain Drain contact Polysilicon gate DS G RS RD VGS,eff
- 54. © Digital IntegratedCircuits2nd Devices Latch-upLatch-up
- 55. © Digital IntegratedCircuits2nd Devices Future PerspectivesFuture Perspectives 25 nm FINFET MOS transistor