1. Penn ESE370 Fall2010 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 8: September 24, 2010 MOS Model

2. Today MOS Structure Basic Idea Semiconductor Physics –Metals, insulators –Silicon lattice –Band Gaps –Doping Penn ESE370 Fall2010 -- DeHon 2

3. MOS Metal Oxide Semiconductor Penn ESE370 Fall2010 -- DeHon 3

4. MOS Metal – gate Oxide – insulator separating gate from channel –Ideally: no conduction from gate to channel Semiconductor – between source and drain See why input capacitive? Penn ESE370 Fall2010 -- DeHon 4 channel gate srcdrain

5. 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 Fall2010 -- DeHon 5 channel gate srcdrain

6. Source/Drain Contacts Penn ESE370 Fall2010 -- DeHon 6

7. Fabrication Start with Silicon wafer Dope Grow Oxide Deposit Metal Mask/Etch to define where features go Penn ESE370 Fall2010 -- DeHon 7 http://jas.eng.buffalo.edu/education/fab/NMOS/nmos.html

8. Penn ESE534 Spring 2010 -- DeHon 8 Dimensions Channel Length (L) Channel Width (W) Oxide Thickness (T ox ) Process named by minimum length –45nm  L=45nm

9. Semiconductor Physics Penn ESE534 Spring 2010 -- DeHon 9

10. Conduction Metal – conducts Insulator – does not conduct Semiconductor – can act as either Penn ESE370 Fall2010 -- DeHon 10

11. Why metal conduct? (periodic table) Penn ESE370 Fall2010 -- DeHon 11 http://chemistry.about.com/od/imagesclipartstructures/ig/Science-Pictures/Periodic-Table-of-the-Elements.htm

12. Conduction Electrons move Must be able to “remove” electron from atom or molecule Penn ESE370 Fall2010 -- DeHon 12

13. Atomic States Quantized Energy Levels Must have enough energy to change level (state) Penn ESE370 Fall2010 -- DeHon 13

14. 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 Fall2010 -- DeHon 14

15. Silicon 4 valence electrons –Inner shells filled –Only outer shells contribute to chemical interactions Penn ESE370 Fall2010 -- DeHon 15

16. Silicon-Silicon Bonding Can form covalent bonds with 4 other silicon atoms Penn ESE370 Fall2010 -- DeHon 16

17. Silicon Lattice Forms into crystal lattice Penn ESE370 Fall2010 -- DeHon 17 http://www.webelements.com/silicon/crystal_structure.html

18. Silicon Lattice Cartoon two-dimensional view Penn ESE370 Fall2010 -- DeHon 18

19. Outer Orbital? What happens to outer shell in Silicon lattice? Penn ESE370 Fall2010 -- DeHon 19

20. Energy? What does this say about energy to move electron? Penn ESE370 Fall2010 -- DeHon 20

21. State View Penn ESE370 Fall2010 -- DeHon 21 Valance Band – all states filled Energy

22. State View Penn ESE370 Fall2010 -- DeHon 22 Valance Band – all states filled Energy Conduction Band– all states empty

23. Band Gap and Conduction Penn ESE370 Fall2010 -- DeHon 23 EcEc EvEv EvEv EcEc EvEv EcEc OR InsulatorMetal 8ev EvEv EcEc Semiconductor 1.1ev

24. Doping Add impurities to Silicon Lattice –Replace a Si atom at a lattice site with another Penn ESE370 Fall2010 -- DeHon 24

25. Doping Add impurities to Silicon Lattice –Replace a Si atom at a lattice site with another E.g. add a Group V element –E.g. P (Phosphorus) –How many valence electrons? Penn ESE370 Fall2010 -- DeHon 25

26. Doping with P Penn ESE370 Fall2010 -- DeHon 26

27. 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 Fall2010 -- DeHon 27

28. Doped Band Gaps Addition of donor electrons makes more metallic –Easier to conduct Penn ESE370 Fall2010 -- DeHon 28 EvEv EcEc Semiconductor 1.1ev EDED 0.045ev

29. 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 Fall2010 -- DeHon 29

30. Electron Conduction Penn ESE370 Fall2010 -- DeHon 30

31. Electron Conduction Penn ESE370 Fall2010 -- DeHon 31

32. Capacitor Charge What does charge look like in a capacitor? Penn ESE370 Fall2010 -- DeHon 32 + + + + - - - - - - - - -

33. MOS Field? What does “capacitor” field do to the doped semiconductor channel? Penn ESE370 Fall2010 -- DeHon 33 + + + + + - - -

34. + + + + + - - - 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 Penn ESE370 Fall2010 -- DeHon 34

35. Group III What happens if we replace Si atoms with group III atom instead? –E.g. B (Boron) –Valance band electrons? Penn ESE370 Fall2010 -- DeHon 35

36. 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 Fall2010 -- DeHon 36

37. Hole Conduction Penn ESE370 Fall2010 -- DeHon 37

38. Doped Band Gaps Addition of acceptor sites makes more metallic –Easier to conduct Penn ESE370 Fall2010 -- DeHon 38 EvEv EcEc Semiconductor 1.1ev EAEA 0.045ev

39. Field Effect? Effect of field on Acceptor-doped Silicon? Penn ESE370 Fall2010 -- DeHon 39 +++++ - - -

40. MOSFETs Donor doping –Excess electrons –Negative or N-type material –NFET Acceptor doping –Excess holes –Positive or P-type material –PFET Penn ESE370 Fall2010 -- DeHon 40

41. Admin HW3 out Penn ESE370 Fall2010 -- DeHon 41

42. MOSFET Semiconductor can act like metal or insulator Use field to modulate conduction state of semiconductor Penn ESE370 Fall2010 -- DeHon 42 - - - + + + + +