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ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems

ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems. Day 10: September 29, 2010 MOS Transistors Details. Last Time. Focused on I vs V relationships Effective resistance Drive. Today. Capacitance Gate Source/Drain Contact More threshold dependence V DS.

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ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems

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  1. ESE370:Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 10: September 29, 2010 MOS Transistors Details

  2. Last Time • Focused on I vs V relationships • Effective resistance • Drive

  3. Today • Capacitance • Gate • Source/Drain Contact • More threshold dependence • VDS

  4. Theme • Refining model • Exploring next level of complexity

  5. Capacitance gate drain src channel • First order: looks like a capacitor • Today: • Like resistance, it is not constant • Capacitance not just to src (drain)

  6. Threshold • Threshold decreases with VDS VT VDS

  7. Capacitance Setup

  8. Capacitance • Argued looked like a capacitor to the channel • …but the channel isn’t really one of our terminals • Don’t connect directly to it.

  9. Capacitance • Four Terminals • How many combinations • 4 things taken 2 at a time

  10. Capacitances • GS, GB, GD, SB, DB, SD

  11. Moving Plates? • What is distance from gate to conductor? • Depletion? • Strong Inversion?

  12. Capacitance Decomposition

  13. Overlap • What is the capacitive implication of gate/src and gate/drain overlap?

  14. Overlap • Length of overlap?

  15. Overlap Capacitance

  16. Overlap Capacitance

  17. Capacitance in Strong Inversion(easy case) • Looks like parallel plate Gate – Channel • What is CGC? • What is CGB?

  18. Capacitance in Strong Inversion • Looks like parallel plate Gate – Channel • What is CGC? • CGB=0

  19. Capacitance in Strong Inversion • But channel isn’t a terminal • Split evenly with source and drain

  20. Capacitance in Strong Inversion • Add in Overlap capacitance

  21. Capacitance Subthreshold • Need to refine model • What showed on Day 9 not quite right • Channel doesn’t start depleted • Starts with substrate doping

  22. Channel Evolution Subthreshold

  23. Capacitance Depletion • What happens to capacitance here? • Capacitor plate distance?

  24. Capacitance Depletion • Capacitance becomes Gate-Body • Capacitance drops

  25. Capacitance vs VGS • G CGC CGCS=CGCD CGCB

  26. Saturation Capacitance?

  27. Saturation Capacitance? • Source end of channel in inversion • Destination end of channel close at threshold • Capacitance shifts to source • Total capacitance reduced

  28. Saturation Capacitance CGC CGCS CGCD VDS/(VGS-VT)

  29. Contact Capacitance

  30. Contact Capacitance • n+ contacts are formed by doping = diffusion • Depletion under contact • Contact-Body capacitance • Depletion around perimeter of contact • Also contact-Body capacitance

  31. Contact/Diffusion Capacitance • Cj – diffusion depletion • Cjsw – sidewall capacitance • LS – length of diffusion LS

  32. Capacitance Roundup • CGS=CGCS+CO • CGD=CGCD+CO • CGB=CGCB • CSB=Cdiff • CDB=Cdiff

  33. One Implication

  34. Step Response? Rsmall Rlarge

  35. Step Response

  36. Impact of CGD • What does CGD do to the switching response here?

  37. Impact of CGD

  38. Threshold

  39. Threshold • Describe VT as a constant • Induce enough electron collection to invert channel

  40. VDS impact • In practice, VDS impacts state of channel

  41. VDS impact • Increasing VDS, already depletes portions of channel

  42. VDS impact • Increasing VDS, already depletes portions of channel • Need less charge, less voltage to invert

  43. Drain-Induced Barrier Lowering (DIBL) VT VDS

  44. DIBL Impact

  45. In a Gate? • What does it impact most? • Which device, which state/operation?

  46. In a Gate • VDS largest for off device • Easier to turn on

  47. In a Gate • VDS largest for off device • Easier to turn on • Leak more

  48. In a Gate • VDS largest for off device • Easier to turn on • Leak more

  49. Admin • HW3 due Friday

  50. Ideas • Capacitance • To every terminal • Voltage dependent • Threshold • Voltage dependent • Generally do manual analysis without CGC VT CGCS VDS CGCB

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