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VLSI Devices

VLSI Devices. Intuitive understanding of device operation Fundamental analytic models Manual Models Spice Models Secondary and deep-sub-micron effects Junction Diode and FET Resistor and Capacitor. B. A. Al. SiO. 2. p. n. Cross-section of. pn. -junction in an IC process. A. Al.

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VLSI Devices

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  1. VLSI Devices • Intuitive understanding of device operation • Fundamental analytic models • Manual Models • Spice Models • Secondary and deep-sub-micron effects • Junction Diode and FET • Resistor and Capacitor

  2. 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 Occurs as parasitic element in Digital ICs

  3. Depletion Region

  4. ) 2 W ( n p p n0 L p n p0 0 -W W x 1 2 p-region n-region diffusion Forward Bias Forward Bias usually avoided in Digital ICs

  5. Reverse Bias Diode Isolation Mode

  6. Diode Current

  7. Models for Manual Analysis

  8. Junction Capacitance

  9. Diffusion Capacitance (Forward Bias)

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

  11. Diode Model (Manual Analysis)

  12. SPICE Parameters • Transit time models charge storage

  13. A Switch! What is a Transistor? • Resistor is poor model in saturation– current source • Source and Drain are symmetric • N-channel: Source is most negative of the two • P-channel: Source is most positive of the two • Four Modes: • Off (leakage current only) • Sub-Threshold (exponential) • Linear (Resistive) • Saturation (Current Source)

  14. The MOS Transistor Polysilicon Aluminum

  15. 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

  16. Threshold Voltage: Concept

  17. The Threshold Voltage

  18. The Body Effect

  19. -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 Relation

  20. Transistor in Linear

  21. Pinch-off Region Transistor in Saturation

  22. Current-Voltage RelationsLong-Channel Device

  23. A model for manual analysis

  24. -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 Relations:Deep-Submicron FET

  25. 5 u = 10 sat ) s / m ( n u x = 1.5 x (V/µm) c Velocity Saturation Constant velocity Constant mobility (slope = µ)

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

  27. -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

  28. -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

  29. G S D B A unified modelfor manual analysis

  30. -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

  31. -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 VGS = -2.5V

  32. Transistor Model for Manual Analysis

  33. The Transistor as a Switch

  34. The Transistor as a Switch

  35. The Transistor as a Switch

  36. MOS CapacitancesDynamic Behavior

  37. Dynamic Behavior of MOS Transistor

  38. 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

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

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

  41. 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

  42. Junction Capacitance

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

  44. MOS Capacitances in 0.25/0.5 mm CMOS processes

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

  46. 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

  47. -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

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

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

  50. 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

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