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

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

MOS Metal Oxide Semiconductor Penn ESE370 Fall DeHon 3

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 Fall DeHon 4 channel gate srcdrain

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

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 Fall DeHon 6 channel gate srcdrain

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

Fabrication Start with Silicon wafer Dope Grow Oxide (SiO 2 ) Deposit Metal Mask/Etch to define where features go Penn ESE370 Fall DeHon 8 Cartoon fab seq.: t=2min—4min

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

Semiconductor Physics Penn ESE370 Fall DeHon 10

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

Why metal conduct? (periodic table) Penn ESE370 Fall DeHon 12

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

Atomic States Quantized Energy Levels Must have enough energy to change level (state) Penn ESE370 Fall DeHon 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 Fall DeHon 15

Silicon Atom How many valence electrons? Penn ESE370 Fall DeHon 16

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

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

Silicon Lattice Forms into crystal lattice Penn ESE370 Fall DeHon 19

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

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

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

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

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

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

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

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 Fall DeHon 27

Doping with P Penn ESE370 Fall DeHon 28

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 Fall DeHon 29

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

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 Fall DeHon 31

Electron Conduction Penn ESE370 Fall DeHon 32

Electron Conduction Penn ESE370 Fall DeHon 33

Capacitor Charge Remember capacitor charge Penn ESE370 Fall DeHon

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

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 Fall DeHon 36

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

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 Fall DeHon 38

Hole Conduction Penn ESE370 Fall DeHon 39

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

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

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

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

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 Penn ESE370 Fall DeHon 44

MOSFET Semiconductor can act like metal or insulator Use field to modulate conduction state of semiconductor Penn ESE370 Fall DeHon