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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. Forward Bias Typically avoided in Digital ICs

  7. Reverse Bias The Dominant Operation Mode

  8. Models for Manual Analysis

  9. Junction Capacitance

  10. Diffusion Capacitance

  11. Secondary Effects 0.1 ) A ( 0 D I –0.1 –25.0 –15.0 –5.0 0 5.0 V (V) D Avalanche Breakdown

  12. Diode Model

  13. SPICE Parameters

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

  15. The MOS Transistor Polysilicon Aluminum

  16. 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

  17. Threshold Voltage: Concept

  18. The Threshold Voltage

  19. The Body Effect

  20. -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

  21. Transistor in Linear

  22. Pinch-off Transistor in Saturation

  23. Current-Voltage RelationsLong-Channel Device

  24. A model for manual analysis

  25. -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

  26. 5 u = 10 sat ) s / m ( n u x = 1.5 x (V/µm) c Velocity Saturation Constant velocity Constant mobility (slope = µ)

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

  28. -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

  29. -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

  30. G S D B A unified modelfor manual analysis

  31. -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

  32. -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

  33. Transistor Model for Manual Analysis

  34. The Transistor as a Switch

  35. The Transistor as a Switch

  36. The Transistor as a Switch

  37. MOS CapacitancesDynamic Behavior

  38. Dynamic Behavior of MOS Transistor

  39. 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

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

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

  42. Measuring the Gate Cap

  43. 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

  44. Junction Capacitance

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

  46. Capacitances in 0.25 mm CMOS process

  47. The Sub-Micron MOS Transistor • Threshold Variations • Subthreshold Conduction • Parasitic Resistances

  48. V V T T Threshold Variations Low V threshold Long-channel threshold DS VDS L Threshold as a function of Drain-induced barrier lowering the length (for low V ) (for low L ) DS

  49. -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

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

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