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Digital Integrated Circuits A Design Perspective

Digital Integrated Circuits A Design Perspective. The Devices. July 30, 2002. |V. |. GS. A Switch!. An MOS Transistor. What is a Transistor?. The MOS Transistor. Polysilicon. Aluminum. MOS Transistors - Types and Symbols. D. D. G. G. S. S. Depletion. NMOS. Enhancement. NMOS.

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Digital Integrated Circuits A Design Perspective

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  1. Digital Integrated CircuitsA Design Perspective The Devices July 30, 2002

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

  3. The MOS Transistor Polysilicon Aluminum

  4. 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

  5. Threshold Voltage: Concept

  6. The Threshold Voltage

  7. The Body Effect

  8. -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

  9. Transistor in Linear

  10. Pinch-off Transistor in Saturation

  11. Current-Voltage RelationsLong-Channel Device

  12. 5 u = 10 sat ) s / m ( n u x = 1.5 x (V/µm) c Velocity Saturation Constant velocity Constant mobility (slope = µ)

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

  14. -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

  15. -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

  16. -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

  17. Transistor Model for Manual Analysis

  18. The Transistor as a Switch

  19. The Transistor as a Switch • The resistance inversely proportional to W/L • Once VDD approaches VT, the resistance dramatically increases.

  20. The Transistor as a Switch • Req-p ~ 2.5 Req-n • Electron mobility ~ 2.5 Hole mobility, depending on supply voltage and doping concentration

  21. MOS Capacitances • Gate capacitance • Diffusion (Junction) capacitance

  22. 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

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

  24. 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

  25. Capacitances in 0.25 mm CMOS process

  26. 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 VDS from 0 to 0.5V

  27. RD = LD/W * R + RC Parasitic Resistances

  28. Latch-up • Tyrister alike, n-p-n-p • To prevent, • minimize Rwell, Rpsubs • Guard ring

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