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

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 5 channel gate srcdrain

Source/Drain Contacts Penn ESE370 Fall DeHon 6

Fabrication Start with Silicon wafer Dope Grow Oxide Deposit Metal Mask/Etch to define where features go Penn ESE370 Fall DeHon 7

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

Semiconductor Physics Penn ESE534 Spring DeHon 9

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

Why metal conduct? (periodic table) Penn ESE370 Fall DeHon 11

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

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

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 14

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

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

Silicon Lattice Forms into crystal lattice Penn ESE370 Fall DeHon 17

Silicon Lattice Cartoon two-dimensional view Penn ESE370 Fall DeHon 18

Outer Orbital? What happens to outer shell in Silicon lattice? Penn ESE370 Fall DeHon 19

Energy? What does this say about energy to move electron? Penn ESE370 Fall DeHon 20

State View Penn ESE370 Fall DeHon 21 Valance Band – all states filled Energy

State View Penn ESE370 Fall DeHon 22 Valance Band – all states filled Energy Conduction Band– all states empty

Band Gap and Conduction Penn ESE370 Fall DeHon 23 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 24

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 Fall DeHon 25

Doping with P Penn ESE370 Fall DeHon 26

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 27

Doped Band Gaps Addition of donor electrons makes more metallic –Easier to conduct Penn ESE370 Fall DeHon 28 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 29

Electron Conduction Penn ESE370 Fall DeHon 30

Electron Conduction Penn ESE370 Fall DeHon 31

Capacitor Charge What does charge look like in a capacitor? Penn ESE370 Fall DeHon

MOS Field? What does “capacitor” field do to the doped semiconductor channel? Penn ESE370 Fall DeHon

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 Fall DeHon 34

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

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 36

Hole Conduction Penn ESE370 Fall DeHon 37

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

Field Effect? Effect of field on Acceptor-doped Silicon? Penn ESE370 Fall DeHon

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 40

Admin HW3 out Penn ESE370 Fall DeHon 41

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