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Penn ESE370 Fall2011 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 9: September 26, 2011 MOS Model.

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Presentation on theme: "Penn ESE370 Fall2011 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 9: September 26, 2011 MOS Model."— Presentation transcript:

1 Penn ESE370 Fall2011 -- DeHon 1 ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 9: September 26, 2011 MOS Model

2 Today MOS Structure Basic Idea Semiconductor Physics –Metals, insulators –Silicon lattice –Band Gaps –Doping Penn ESE370 Fall2011 -- DeHon 2

3 MOS Metal Oxide Semiconductor Penn ESE370 Fall2011 -- 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 Fall2011 -- DeHon 4 channel gate srcdrain

5 Penn ESE370 Fall2011 -- DeHon 5 channel gate srcdrain Capacitor Charge distribution and field?

6 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 Fall2011 -- DeHon 6 channel gate srcdrain

7 Source/Drain Contacts Penn ESE370 Fall2011 -- DeHon 7 Contacts: Conductors  metalic –Connect to metal wires that connect

8 Fabrication Start with Silicon wafer Dope Grow Oxide (SiO 2 ) Deposit Metal Mask/Etch to define where features go Penn ESE370 Fall2011 -- DeHon 8 http://www.youtube.com/watch?v=35jWSQXku74 Cartoon fab seq.: t=2min—4min

9 Penn ESE370 Fall2011 -- DeHon 9 Dimensions Channel Length (L) Channel Width (W) Oxide Thickness (T ox ) Process named by minimum length –45nm  L=45nm

10 Semiconductor Physics Penn ESE370 Fall2011 -- DeHon 10

11 Conduction Metal – conducts Insulator – does not conduct Semiconductor – can act as either Penn ESE370 Fall2011 -- DeHon 11

12 Why metal conduct? (periodic table) Penn ESE370 Fall2011 -- DeHon 12 http://chemistry.about.com/od/imagesclipartstructures/ig/Science-Pictures/Periodic-Table-of-the-Elements.htm

13 Conduction Electrons move Must be able to “remove” electron from atom or molecule Penn ESE370 Fall2011 -- DeHon 13

14 Atomic States Quantized Energy Levels Must have enough energy to change level (state) Penn ESE370 Fall2011 -- DeHon 14

15 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 Fall2011 -- DeHon 15

16 Silicon Atom How many valence electrons? Penn ESE370 Fall2011 -- DeHon 16

17 Silicon 4 valence electrons –Inner shells filled –Only outer shells contribute to chemical interactions Penn ESE370 Fall2011 -- DeHon 17

18 Silicon-Silicon Bonding Can form covalent bonds with 4 other silicon atoms Penn ESE370 Fall2011 -- DeHon 18

19 Silicon Lattice Forms into crystal lattice Penn ESE370 Fall2011 -- DeHon 19 http://www.webelements.com/silicon/crystal_structure.html

20 Silicon Lattice Cartoon two-dimensional view Penn ESE370 Fall2011 -- DeHon 20

21 Outer Orbital? What happens to outer shell in Silicon lattice? Penn ESE370 Fall2011 -- DeHon 21

22 Energy? What does this say about energy to move electron? Penn ESE370 Fall2011 -- DeHon 22

23 State View Penn ESE370 Fall2011 -- DeHon 23 Valance Band – all states filled Energy

24 State View Penn ESE370 Fall2011 -- DeHon 24 Valance Band – all states filled Energy Conduction Band– all states empty

25 Band Gap and Conduction Penn ESE370 Fall2011 -- DeHon 25 EcEc EvEv EvEv EcEc EvEv EcEc OR InsulatorMetal 8ev EvEv EcEc Semiconductor 1.1ev

26 Doping Add impurities to Silicon Lattice –Replace a Si atom at a lattice site with another Penn ESE370 Fall2011 -- DeHon 26

27 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 Fall2011 -- DeHon 27

28 Doping with P Penn ESE370 Fall2011 -- DeHon 28

29 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 Fall2011 -- DeHon 29

30 Doped Band Gaps Addition of donor electrons makes more metallic –Easier to conduct Penn ESE370 Fall2011 -- DeHon 30 EvEv EcEc Semiconductor 1.1ev EDED 0.045ev

31 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 Fall2011 -- DeHon 31

32 Electron Conduction Penn ESE370 Fall2011 -- DeHon 32

33 Electron Conduction Penn ESE370 Fall2011 -- DeHon 33

34 Capacitor Charge Remember capacitor charge Penn ESE370 Fall2011 -- DeHon 34 + + + + - - - - - - - - -

35 MOS Field? What does “capacitor” field do to the doped semiconductor channel? Penn ESE370 Fall2011 -- DeHon 35 - - - Vgs=0 No field + + + + + - - - = Vgs>0 Conducts + + + + - - - Vcap>0

36 + + + + + - - - 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 Fall2011 -- DeHon 36

37 Group 13 What happens if we replace Si atoms with group 13 atom instead? –E.g. B (Boron) –Valance band electrons? Penn ESE370 Fall2011 -- DeHon 37

38 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 Fall2011 -- DeHon 38

39 Hole Conduction Penn ESE370 Fall2011 -- DeHon 39

40 Doped Band Gaps Addition of acceptor sites makes more metallic –Easier to conduct Penn ESE370 Fall2011 -- DeHon 40 EvEv EcEc Semiconductor 1.1ev EAEA 0.045ev

41 Field Effect? Effect of postivive field on Acceptor- doped Silicon? Penn ESE370 Fall2011 -- DeHon 41 + + + Vgs=0 No field + + + + - - - Vcap>0 Vgs>0 No conduction

42 Field Effect? Effect of negative field on Acceptor- doped Silicon? Penn ESE370 Fall2011 -- DeHon 42 + + + +++++ - - - Vgs=0 No field + - - - + + + Vcap<0 Vgs<0 Conduction

43 MOSFETs Donor doping –Excess electrons –Negative or N-type material –NFET Acceptor doping –Excess holes –Positive or P-type material –PFET Penn ESE370 Fall2011 -- DeHon 43

44 Admin Intel Fabrication Talk Wednesday (28 th ) at 4pm (Raisler) HW3 –Largely based on Wed. lecture –Should be able to do SPICE parts now –Will want to read text 3.3.2 Penn ESE370 Fall2011 -- DeHon 44

45 MOSFET Semiconductor can act like metal or insulator Use field to modulate conduction state of semiconductor Penn ESE370 Fall2011 -- DeHon 45 - - - + + + + +

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