1. Penn ESE370 Fall2010 -- DeHon 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 Penn ESE370 Fall2010 -- DeHon 2

3. Today Capacitance –Gate –Source/Drain Contact More threshold dependence –V DS Penn ESE370 Fall2010 -- DeHon 3

4. Theme Refining model –Exploring next level of complexity Penn ESE370 Fall2010 -- DeHon 4

5. channel gate srcdrain Capacitance First order: looks like a capacitor Today: –Like resistance, it is not constant –Capacitance not just to src (drain) Penn ESE370 Fall2010 -- DeHon 5

6. Threshold Threshold decreases with V DS Penn ESE370 Fall2010 -- DeHon 6 VTVT V DS

7. Capacitance Setup Penn ESE370 Fall2010 -- DeHon 7

8. Capacitance Argued looked like a capacitor to the channel …but the channel isnt really one of our terminals –Dont connect directly to it. Penn ESE370 Fall2010 -- DeHon 8

9. Capacitance Four Terminals How many combinations –4 things taken 2 at a time Penn ESE370 Fall2010 -- DeHon 9

10. Capacitances GS, GB, GD, SB, DB, SD Penn ESE370 Fall2010 -- DeHon 10

11. Moving Plates? What is distance from gate to conductor? –Depletion? –Strong Inversion? Penn ESE370 Fall2010 -- DeHon 11

12. Capacitance Decomposition Penn ESE370 Fall2010 -- DeHon 12

13. Overlap What is the capacitive implication of gate/src and gate/drain overlap? Penn ESE370 Fall2010 -- DeHon 13

14. Overlap Length of overlap? Penn ESE370 Fall2010 -- DeHon 14

15. Overlap Capacitance Penn ESE370 Fall2010 -- DeHon 15

16. Overlap Capacitance Penn ESE370 Fall2010 -- DeHon 16

17. Capacitance in Strong Inversion (easy case) Looks like parallel plate Gate – Channel –What is C GC ? –What is C GB ? Penn ESE370 Fall2010 -- DeHon 17

18. Capacitance in Strong Inversion Looks like parallel plate Gate – Channel –What is C GC ? –C GB =0 Penn ESE370 Fall2010 -- DeHon 18

19. Capacitance in Strong Inversion But channel isnt a terminal –Split evenly with source and drain Penn ESE370 Fall2010 -- DeHon 19

20. Capacitance in Strong Inversion Add in Overlap capacitance Penn ESE370 Fall2010 -- DeHon 20

21. Capacitance Subthreshold Need to refine model –What showed on Day 9 not quite right Channel doesnt start depleted –Starts with substrate doping Penn ESE370 Fall2010 -- DeHon 21

22. Channel Evolution Subthreshold Penn ESE370 Fall2010 -- DeHon 22

23. Capacitance Depletion What happens to capacitance here? –Capacitor plate distance? Penn ESE370 Fall2010 -- DeHon 23

24. Capacitance Depletion Capacitance becomes Gate-Body Capacitance drops Penn ESE370 Fall2010 -- DeHon 24

25. Capacitance vs V GS Penn ESE370 Fall2010 -- DeHon 25 G C GC C GCS = C GCD C GCB

26. Saturation Capacitance? Penn ESE370 Fall2010 -- DeHon 26

27. Saturation Capacitance? Penn ESE370 Fall2010 -- DeHon 27 Source end of channel in inversion Destination end of channel close at threshold Capacitance shifts to source –Total capacitance reduced

28. Saturation Capacitance Penn ESE370 Fall2010 -- DeHon 28 C GC C GCS C GCD V DS /(V GS -V T )

29. Contact Capacitance Penn ESE370 Fall2010 -- DeHon 29

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 Penn ESE370 Fall2010 -- DeHon 30

31. Contact/Diffusion Capacitance C j – diffusion depletion C jsw – sidewall capacitance L S – length of diffusion Penn ESE370 Fall2010 -- DeHon 31 LSLS

32. Capacitance Roundup C GS =C GCS +C O C GD =C GCD +C O C GB =C GCB C SB =C diff C DB =C diff Penn ESE370 Fall2010 -- DeHon 32

33. One Implication Penn ESE370 Fall2010 -- DeHon 33

34. Step Response? Penn ESE370 Fall2010 -- DeHon 34 R small R large

35. Step Response Penn ESE370 Fall2010 -- DeHon 35

36. Impact of C GD What does CGD do to the switching response here? Penn ESE370 Fall2010 -- DeHon 36

37. Impact of C GD Penn ESE370 Fall2010 -- DeHon 37

38. Threshold Penn ESE370 Fall2010 -- DeHon 38

39. Threshold Describe V T as a constant Induce enough electron collection to invert channel Penn ESE370 Fall2010 -- DeHon 39

40. V DS impact In practice, V DS impacts state of channel Penn ESE370 Fall2010 -- DeHon 40

41. V DS impact Increasing V DS, already depletes portions of channel Penn ESE370 Fall2010 -- DeHon 41

42. V DS impact Increasing V DS, already depletes portions of channel Need less charge, less voltage to invert Penn ESE370 Fall2010 -- DeHon 42

43. Drain-Induced Barrier Lowering (DIBL) Penn ESE370 Fall2010 -- DeHon 43 VTVT V DS

44. DIBL Impact Penn ESE370 Fall2010 -- DeHon 44

45. In a Gate? What does it impact most? –Which device, which state/operation? Penn ESE370 Fall2010 -- DeHon 45

46. In a Gate V DS largest for off device –Easier to turn on Penn ESE370 Fall2010 -- DeHon 46

47. In a Gate V DS largest for off device –Easier to turn on –Leak more Penn ESE370 Fall2010 -- DeHon 47

48. In a Gate V DS largest for off device –Easier to turn on –Leak more Penn ESE370 Fall2010 -- DeHon 48

49. Admin HW3 due Friday Penn ESE370 Fall2010 -- DeHon 49

50. Ideas Capacitance –To every terminal –Voltage dependent Threshold –Voltage dependent Generally do manual analysis without Penn ESE370 Fall2010 -- DeHon 50 VTVT V DS C GC C GCS C GCB