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Digital Integrated Circuits A Design Perspective

Digital Integrated Circuits A Design Perspective. Jan M. Rabaey Anantha Chandrakasan Borivoje Nikolic. The Devices. July 30, 2002. Goal of this chapter. Present intuitive understanding of device operation Introduction of basic device equations Introduction of models for manual analysis

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Digital Integrated Circuits A Design Perspective

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  1. Digital Integrated CircuitsA Design Perspective Jan M. Rabaey Anantha Chandrakasan Borivoje Nikolic The Devices July 30, 2002

  2. Goal 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. B A Al SiO 2 p n Cross-section of pn -junction in an IC process A Al A p n B B One-dimensional representation diode symbol The Diode Mostly occurring as parasitic element in Digital ICs

  4. Depletion Region

  5. Diode Current

  6. Models for Manual Analysis

  7. Diode Model

  8. SPICE Parameters

  9. |V | GS A Switch! An MOS Transistor What is a Transistor?

  10. The MOS Transistor Polysilicon Aluminum

  11. MOS Transistors -Types and Symbols D D G G S S Depletion NMOS Enhancement NMOS D D G G B S S NMOS with PMOS Enhancement Bulk Contact

  12. Threshold Voltage: Concept

  13. The Threshold Voltage

  14. The Body Effect

  15. -4 x 10 6 VGS= 2.5 V 5 Resistive Saturation 4 VGS= 2.0 V Quadratic Relationship (A) 3 VDS = VGS - VT D I 2 VGS= 1.5 V 1 VGS= 1.0 V 0 0 0.5 1 1.5 2 2.5 V (V) DS Current-Voltage RelationsA good ol’ transistor

  16. Transistor in Linear

  17. Pinch-off Transistor in Saturation

  18. Current-Voltage RelationsLong-Channel Device

  19. A model for manual analysis

  20. -4 x 10 2.5 VGS= 2.5 V Early Saturation 2 VGS= 2.0 V 1.5 Linear Relationship (A) D I VGS= 1.5 V 1 VGS= 1.0 V 0.5 0 0 0.5 1 1.5 2 2.5 V (V) DS Current-Voltage RelationsThe Deep-Submicron Era

  21. 5 u = 10 sat ) s / m ( n u x = 1.5 x (V/µm) c Velocity Saturation Constant velocity Constant mobility (slope = µ)

  22. Perspective I D Long-channel device V = V GS DD Short-channel device V V - V V DSAT GS T DS

  23. -4 x 10 -4 x 10 6 2.5 5 2 4 1.5 (A) 3 (A) D D I I 1 2 0.5 1 0 0 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 V (V) V (V) GS GS ID versus VGS linear quadratic quadratic Long Channel Short Channel

  24. -4 -4 x 10 x 10 2.5 6 VGS= 2.5 V VGS= 2.5 V 5 2 Resistive Saturation VGS= 2.0 V 4 VGS= 2.0 V 1.5 (A) (A) 3 D D VDS = VGS - VT I I VGS= 1.5 V 1 2 VGS= 1.5 V VGS= 1.0 V 0.5 1 VGS= 1.0 V 0 0 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5 V (V) V (V) DS DS ID versus VDS Long Channel Short Channel

  25. G S D B A unified modelfor manual analysis NB For PMOS, use Vmax=max(…) instead of Vmin

  26. -4 x 10 2.5 VDS=VDSAT 2 VelocitySaturated 1.5 Linear 1 VDSAT=VGT 0.5 VDS=VGT Saturated 0 0 0.5 1 1.5 2 2.5 Simple Model versus SPICE (A) D I V (V) DS

  27. -4 x 10 0 -0.2 -0.4 (A) D I -0.6 -0.8 -1 -2.5 -2 -1.5 -1 -0.5 0 V (V) DS A PMOS Transistor VGS = -1.0V VGS = -1.5V VGS = -2.0V Assume all variables negative! VGS = -2.5V

  28. Transistor Model for Manual Analysis

  29. The Transistor as a Switch

  30. The Transistor as a Switch

  31. The Transistor as a Switch

  32. Unified Model Examples

  33. MOS CapacitancesDynamic Behavior

  34. Dynamic Behavior of MOS Transistor

  35. Polysilicongate Source Drain W x x + + n n d d Gate-bulk L d overlap Top view Gate oxide t ox + + n n L Cross section The Gate Capacitance

  36. Gate Capacitance Cut-off Resistive Saturation Most important regions in digital design: saturation and cut-off

  37. Gate Capacitance Capacitance as a function of the degree of saturation Capacitance as a function of VGS (with VDS = 0)

  38. Measuring the Gate Cap

  39. Diffusion Capacitance Channel-stop implant N 1 A Side wall Source W N D Bottom x Side wall j Channel L Substrate N S A

  40. Junction Capacitance

  41. Linearizing the Junction Capacitance Replace non-linear capacitance by large-signal equivalent linear capacitance which displaces equal charge over voltage swing of interest

  42. Capacitances in 0.25 mm CMOS process

  43. -2 10 Linear -4 10 -6 Quadratic 10 (A) D I -8 10 Exponential -10 10 VT -12 10 0 0.5 1 1.5 2 2.5 V (V) GS Sub-Threshold Conduction The Slope Factor S is DVGS for ID2/ID1 =10 Typical values for S: 60 .. 100 mV/decade

  44. Sub-Threshold ID vs VGS VDS from 0 to 0.5V

  45. Sub-Threshold ID vs VDS VGS from 0 to 0.3V

  46. Summary of MOSFET Operating Regions • 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

  47. Parasitic Resistances

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