1. Day 9: September 18, 2013 MOS Model
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems
Day 9: September 18, 2013
MOS Model
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2. You are Here Previously: simple models This week and next (4 lectures)
Comfortable with basic functions and circuits
This week and next (4 lectures)
Detail semiconductor, MOSFET phenomenology  Don’t Blink!
Rest of terms
Implications
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3. Today MOS Structure Basic Idea Semiconductor Physics
Metals, insulators
Silicon lattice
Band Gaps
Doping
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4. MOS
Metal Oxide Semiconductor
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5. MOS Metal – gate Oxide – insulator separating gate from channel
drain
src
channel
Metal – gate
Oxide – insulator separating gate from channel
Ideally: no conduction from gate to channel
Semiconductor – between source and drain
See why input capacitive?
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6. Capacitor Charge distribution and field? gate drain src channel
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7. Idea Semiconductor – can behave as metal or insulator
channel
gate
src
drain
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
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8. Source/Drain Contacts
Contacts: Conductors  metalic
Connect to metal wires that connect transistors
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9. Fabrication Start with Silicon wafer Dope Grow Oxide (SiO2)
Deposit Metal
Mask/Etch to define where features go
Cartoon fab seq.: t=2min—4min
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10. Dimensions Channel Length (L) Channel Width (W) Oxide Thickness (Tox)
Process named by minimum length
32nm  L=32nm
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11. Semiconductor Physics
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12. Conduction Metal – conducts Insulator – does not conduct
Semiconductor – can act as either
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13. Why metal conduct? (periodic table)
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14. Conduction Electrons move
Must be able to “remove” electron from atom or molecule
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15. Atomic States Quantized Energy Levels
Must have enough energy to change level (state)
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16. 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
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17. Silicon Atom
How many valence electrons?
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18. Silicon 4 valence electrons Inner shells filled
Only outer shells contribute to chemical interactions
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19. Silicon-Silicon Bonding
Can form covalent bonds with 4 other silicon atoms
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20. Silicon Lattice Forms into crystal lattice
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21. Silicon Lattice Cartoon two-dimensional view
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22. Outer Orbital? What happens to outer shell in Silicon lattice?
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23. Energy? What does this say about energy to move electron?
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24. State View Energy Valance Band – all states filled
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25. State View Conduction Band– all states empty Energy
Valance Band – all states filled
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26. Band Gap and Conduction
Insulator
Metal Ec Ec Ev
8ev OR Ev Ev Ec Ev Ec
Semiconductor
1.1ev
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27. Doping Add impurities to Silicon Lattice
Replace a Si atom at a lattice site with another
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28. Doping Add impurities to Silicon Lattice E.g. add a Group 15 element
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?
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29. Doping with P
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30. Doping with P End up with extra electrons Not tightly bound to atom
Donor electrons
Not tightly bound to atom
Low energy to displace
Easy for these electrons to move
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31. Doped Band Gaps Addition of donor electrons makes more metallic
Easier to conduct
Semiconductor Ec
0.045ev ED
1.1ev Ev
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32. 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
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33. Electron Conduction
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34. Electron Conduction
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35. Capacitor Charge Remember capacitor charge + + + + + + + +
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36. MOS Field?
What does “capacitor” field do to the doped semiconductor channel?
Vgs=0
No field +
Vcap>0 =
Vgs>0
Conducts
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37. 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
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38. Group 13
What happens if we replace Si atoms with group 13 atom instead?
E.g. B (Boron)
Valance band electrons?
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39. 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
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40. Hole Conduction
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41. Doped Band Gaps Addition of acceptor sites makes more metallic
Easier to conduct
Semiconductor Ec
1.1ev EA
0.045ev Ev
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42. + Field Effect? Effect of positive field on Acceptor-doped Silicon?
Vgs=0
No field +
Vcap>0
Vgs>0
No conduction
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43. + Field Effect? Effect of negative field on Acceptor-doped Silicon?
Vgs=0
No field +
Vcap<0
+++++
Vgs<0
Conduction
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44. MOSFETs Donor doping Acceptor doping Excess electrons
Negative or N-type material
NFET
Acceptor doping
Excess holes
Positive or P-type material
PFET
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45. MOSFET Semiconductor can act like metal or insulator
Use field to modulate conduction state of semiconductor
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46. Admin HW 3 due tomorrow Friday: back here for lecture HW4 is out
MOS Transistor Basics
HW4 is out
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