Day 9: September 18, 2013 MOS Model ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 9: September 18, 2013 MOS Model Penn ESE370 Fall2013 -- DeHon

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 Penn ESE370 Fall2013 -- DeHon

Today MOS Structure Basic Idea Semiconductor Physics Metals, insulators Silicon lattice Band Gaps Doping Penn ESE370 Fall2013 -- DeHon

MOS Metal Oxide Semiconductor Penn ESE370 Fall2013 -- DeHon

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? Penn ESE370 Fall2013 -- DeHon

Capacitor Charge distribution and field? gate drain src channel Penn ESE370 Fall2013 -- DeHon

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 Penn ESE370 Fall2013 -- DeHon

Source/Drain Contacts Contacts: Conductors  metalic Connect to metal wires that connect transistors Penn ESE370 Fall2013 -- DeHon

Fabrication Start with Silicon wafer Dope Grow Oxide (SiO2) Deposit Metal Mask/Etch to define where features go http://www.youtube.com/watch?v=35jWSQXku74 Cartoon fab seq.: t=2min—4min Penn ESE370 Fall2013 -- DeHon

Dimensions Channel Length (L) Channel Width (W) Oxide Thickness (Tox) Process named by minimum length 32nm  L=32nm Penn ESE370 Fall2013 -- DeHon

Semiconductor Physics Penn ESE370 Fall2013 -- DeHon

Conduction Metal – conducts Insulator – does not conduct Semiconductor – can act as either Penn ESE370 Fall2013 -- DeHon

Why metal conduct? (periodic table) http://chemistry.about.com/od/imagesclipartstructures/ig/Science-Pictures/Periodic-Table-of-the-Elements.htm Penn ESE370 Fall2013 -- DeHon

Conduction Electrons move Must be able to “remove” electron from atom or molecule Penn ESE370 Fall2013 -- DeHon

Atomic States Quantized Energy Levels Must have enough energy to change level (state) Penn ESE370 Fall2013 -- DeHon

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 Fall2013 -- DeHon

Silicon Atom How many valence electrons? Penn ESE370 Fall2013 -- DeHon

Silicon 4 valence electrons Inner shells filled Only outer shells contribute to chemical interactions Penn ESE370 Fall2013 -- DeHon

Silicon-Silicon Bonding Can form covalent bonds with 4 other silicon atoms Penn ESE370 Fall2013 -- DeHon

Silicon Lattice Forms into crystal lattice http://www.webelements.com/silicon/crystal_structure.html Penn ESE370 Fall2013 -- DeHon

Silicon Lattice Cartoon two-dimensional view Penn ESE370 Fall2013 -- DeHon

Outer Orbital? What happens to outer shell in Silicon lattice? Penn ESE370 Fall2013 -- DeHon

Energy? What does this say about energy to move electron? Penn ESE370 Fall2013 -- DeHon

State View Energy Valance Band – all states filled Penn ESE370 Fall2013 -- DeHon

State View Conduction Band– all states empty Energy Valance Band – all states filled Penn ESE370 Fall2013 -- DeHon

Band Gap and Conduction Insulator Metal Ec Ec Ev 8ev OR Ev Ev Ec Ev Ec Semiconductor 1.1ev Penn ESE370 Fall2013 -- DeHon

Doping Add impurities to Silicon Lattice Replace a Si atom at a lattice site with another Penn ESE370 Fall2013 -- DeHon

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? Penn ESE370 Fall2013 -- DeHon

Doping with P Penn ESE370 Fall2013 -- DeHon

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 Penn ESE370 Fall2013 -- DeHon

Doped Band Gaps Addition of donor electrons makes more metallic Easier to conduct Semiconductor Ec 0.045ev ED 1.1ev Ev Penn ESE370 Fall2013 -- DeHon

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 Fall2013 -- DeHon

Electron Conduction Penn ESE370 Fall2013 -- DeHon

Electron Conduction Penn ESE370 Fall2013 -- DeHon

Capacitor Charge Remember capacitor charge + + + + + + + + - - - - - - - - - Penn ESE370 Fall2013 -- DeHon

MOS Field? What does “capacitor” field do to the doped semiconductor channel? Vgs=0 No field + + + + - - - Vcap>0 - - - + + + + + - - - - - - = Vgs>0 Conducts Penn ESE370 Fall2013 -- 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 Negative? Repel carriers Disable conduction + + + + + - - - - - - Penn ESE370 Fall2013 -- DeHon

Group 13 What happens if we replace Si atoms with group 13 atom instead? E.g. B (Boron) Valance band electrons? Penn ESE370 Fall2013 -- DeHon

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 Fall2013 -- DeHon

Hole Conduction Penn ESE370 Fall2013 -- DeHon

Doped Band Gaps Addition of acceptor sites makes more metallic Easier to conduct Semiconductor Ec 1.1ev EA 0.045ev Ev Penn ESE370 Fall2013 -- DeHon

+ Field Effect? Effect of positive field on Acceptor-doped Silicon? Vgs=0 No field + + + + - - - Vcap>0 + + + + + + Vgs>0 No conduction Penn ESE370 Fall2013 -- DeHon

+ Field Effect? Effect of negative field on Acceptor-doped Silicon? Vgs=0 No field + - - - + + + Vcap<0 + + + +++++ - - - Vgs<0 Conduction Penn ESE370 Fall2013 -- DeHon

MOSFETs Donor doping Acceptor doping Excess electrons Negative or N-type material NFET Acceptor doping Excess holes Positive or P-type material PFET Penn ESE370 Fall2013 -- DeHon

MOSFET Semiconductor can act like metal or insulator Use field to modulate conduction state of semiconductor - - - - - - + + + + + Penn ESE370 Fall2013 -- DeHon

Admin HW 3 due tomorrow Friday: back here for lecture HW4 is out MOS Transistor Basics HW4 is out Penn ESE370 Fall2013 -- DeHon