1. Penn ESE370 Fall2014 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 9: September 17, 2014 MOS Model

2. You are Here Previously: simple models –Comfortable with basic functions and circuits This week and next (4 lectures) –Detail semiconductor, MOSFET phenomenology  Don’t Blink! Rest of terms –Implications Penn ESE370 Fall2014 -- DeHon 2

3. Today MOS Structure Basic Idea Semiconductor Physics –Metals, insulators –Silicon lattice –Band Gaps –Doping Penn ESE370 Fall2014 -- DeHon 3

4. MOS Metal Oxide Semiconductor Penn ESE370 Fall2014 -- DeHon 4

5. MOS Metal – gate Oxide – insulator separating gate from channel –Ideally: no conduction from gate to channel Semiconductor – between source and drain See why gate input capacitive? Penn ESE370 Fall2014 -- DeHon 5 channel gate srcdrain

6. Penn ESE370 Fall2014 -- DeHon 6 channel gate srcdrain Capacitor Charge distribution and field?

7. Idea Semiconductor – can behave as metal or insulator Voltage on gate creates an electrical field Field pulls (repels) charge from channel –Causing semiconductor to switch conduction –Hence “Field-Effect” Transistor Penn ESE370 Fall2014 -- DeHon 7 channel gate srcdrain

8. Source/Drain Contacts Penn ESE370 Fall2014 -- DeHon 8 Contacts: Conductors  metalic –Connect to metal wires that connect transistors

9. Fabrication Start with Silicon wafer Dope Grow Oxide (SiO 2 ) Deposit Metal Mask/Etch to define where features go Penn ESE370 Fall2014 -- DeHon 9 http://www.youtube.com/watch?v=35jWSQXku74 Cartoon fab seq.: t=2min—4min

10. Penn ESE370 Fall2014 -- DeHon 10 Dimensions Channel Length (L) Channel Width (W) Oxide Thickness (T ox ) Process named by minimum length –22nm  L=22nm

11. Semiconductor Physics Penn ESE370 Fall2014 -- DeHon 11

12. Conduction Metal – conducts Insulator – does not conduct Semiconductor – can act as either Penn ESE370 Fall2014 -- DeHon 12

13. Why metal conduct? (periodic table) Penn ESE370 Fall2014 -- DeHon 13 http://chemistry.about.com/od/imagesclipartstructures/ig/Science-Pictures/Periodic-Table-of-the-Elements.htm

14. Why metal conduct? Penn ESE370 Fall2014 -- DeHon 14 http://commons.wikimedia.org/wiki/File:Periodic_Table_of_Elements_showing_Electron_Shells.svg

15. Conduction Electrons move Must be able to “remove” electron from atom or molecule Penn ESE370 Fall2014 -- DeHon 15

16. Atomic States Quantized Energy Levels Must have enough energy to change level (state) Penn ESE370 Fall2014 -- DeHon 16

17. Thermal Energy Except at absolute 0 –There is always free energy –Causes electrons to hop around ….when enough energy to change states –Energy gap between states determines energy required Penn ESE370 Fall2014 -- DeHon 17

18. Silicon Atom How many valence electrons? Penn ESE370 Fall2014 -- DeHon 18

19. Silicon 4 valence electrons –Inner shells filled –Only outer shells contribute to chemical interactions Penn ESE370 Fall2014 -- DeHon 19

20. Silicon-Silicon Bonding Can form covalent bonds with 4 other silicon atoms Penn ESE370 Fall2014 -- DeHon 20

21. Silicon Lattice Forms into crystal lattice Penn ESE370 Fall2014 -- DeHon 21 http://www.webelements.com/silicon/crystal_structure.html

22. Silicon Lattice Cartoon two-dimensional view Penn ESE370 Fall2014 -- DeHon 22

23. Outer Orbital? What happens to outer shell in Silicon lattice? Penn ESE370 Fall2014 -- DeHon 23

24. Energy? What does this say about energy to move electron? Penn ESE370 Fall2014 -- DeHon 24

25. State View Penn ESE370 Fall2014 -- DeHon 25 Valance Band – all states filled Energy

26. State View Penn ESE370 Fall2014 -- DeHon 26 Valance Band – all states filled Energy Conduction Band– all states empty

27. Band Gap and Conduction Penn ESE370 Fall2014 -- DeHon 27 EcEc EvEv EvEv EcEc EvEv EcEc OR InsulatorMetal 8ev EvEv EcEc Semiconductor 1.1ev

28. Doping Add impurities to Silicon Lattice –Replace a Si atom at a lattice site with another Penn ESE370 Fall2014 -- DeHon 28

29. Doping Add impurities to Silicon Lattice –Replace a Si atom at a lattice site with another E.g. add a Group 15 element –E.g. P (Phosphorus) –How many valence electrons? Penn ESE370 Fall2014 -- DeHon 29

30. Doping with P Penn ESE370 Fall2014 -- DeHon 30

31. Doping with P End up with extra electrons –Donor electrons Not tightly bound to atom –Low energy to displace –Easy for these electrons to move Penn ESE370 Fall2014 -- DeHon 31

32. Doped Band Gaps Addition of donor electrons makes more metallic –Easier to conduct Penn ESE370 Fall2014 -- DeHon 32 EvEv EcEc Semiconductor 1.1ev EDED 0.045ev

33. Localized Electron is localized Won’t go far if no low energy states nearby Increase doping concentration –Fraction of P’s to Si’s –Decreases energy to conduct Penn ESE370 Fall2014 -- DeHon 33

34. Electron Conduction Penn ESE370 Fall2014 -- DeHon 34

35. Electron Conduction Penn ESE370 Fall2014 -- DeHon 35

36. Capacitor Charge Remember capacitor charge Penn ESE370 Fall2014 -- DeHon 36 + + + + - - - - - - - - -

37. MOS Field? What does “capacitor” field do to the doped semiconductor channel? Penn ESE370 Fall2014 -- DeHon 37 - - - Vgs=0 No field + + + + + - - - = Vgs>0 Conducts + + + + - - - Vcap>0

38. + + + + + - - - MOS Field Effect Charge on capacitor –Attract or repel charge in channel –Change the donors in the channel –Modulates conduction –Positive Attracts carriers –Enables conduction –Negative? Repel carriers –Disable conduction Penn ESE370 Fall2014 -- DeHon 38

39. Group 13 What happens if we replace Si atoms with group 13 atom instead? –E.g. B (Boron) –Valance band electrons? Penn ESE370 Fall2014 -- DeHon 39

40. Doping with B End up with electron vacancies -- Holes –Acceptor electron sites Easy for electrons to shift into these sites –Low energy to displace –Easy for the electrons to move Movement of an electron best viewed as movement of hole Penn ESE370 Fall2014 -- DeHon 40

41. Hole Conduction Penn ESE370 Fall2014 -- DeHon 41

42. Doped Band Gaps Addition of acceptor sites makes more metallic –Easier to conduct Penn ESE370 Fall2014 -- DeHon 42 EvEv EcEc Semiconductor 1.1ev EAEA 0.045ev

43. Field Effect? Effect of positive field on Acceptor- doped Silicon? Penn ESE370 Fall2014 -- DeHon 43 + + + Vgs=0 No field + + + + - - - Vcap>0 Vgs>0 No conduction

44. Field Effect? Effect of negative field on Acceptor- doped Silicon? Penn ESE370 Fall2014 -- DeHon 44 + + + +++++ - - - Vgs=0 No field + - - - + + + Vcap<0 Vgs<0 Conduction

45. MOSFETs Donor doping –Excess electrons –Negative or N-type material –NFET Acceptor doping –Excess holes –Positive or P-type material –PFET Penn ESE370 Fall2014 -- DeHon 45

46. MOSFET Semiconductor can act like metal or insulator Use field to modulate conduction state of semiconductor Penn ESE370 Fall2014 -- DeHon 46 - - - + + + + +

47. Admin HW 3 due tomorrow New Ketterer combo on Piazza Friday: back here for lecture –MOS Transistor Basics HW4 is out Penn ESE370 Fall2014 -- DeHon 47