Transcript ppt - SEAS
ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems Day 9: September 24, 2012 MOS Model 1 Penn ESE370 Fall2012 -- DeHon You are Here • Previously: simple models – Comfortable with basic functions and circuits • This week – Detail semiconductor, MOSFET phenomenology Don’t Blink! • Rest of terms – Implications 2 Penn ESE370 Fall2012 -- DeHon Today • MOS Structure • Basic Idea • Semiconductor Physics – Metals, insulators – Silicon lattice – Band Gaps – Doping 3 Penn ESE370 Fall2012 -- DeHon MOS • Metal Oxide Semiconductor 4 Penn ESE370 Fall2012 -- DeHon MOS gate 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? 5 Penn ESE370 Fall2012 -- DeHon Capacitor gate drain src channel • Charge distribution and field? 6 Penn ESE370 Fall2012 -- DeHon gate Idea drain src channel • 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 7 Penn ESE370 Fall2012 -- DeHon Source/Drain Contacts • Contacts: Conductors metalic – Connect to metal wires that connect transistors 8 Penn ESE370 Fall2012 -- 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 Penn ESE370 Fall2012 -- DeHon Cartoon fab seq.: t=2min—4min 9 Dimensions • Channel Length (L) • Channel Width (W) • Oxide Thickness (Tox) • Process named by minimum length – 45nm L=45nm 10 Penn ESE370 Fall2012 -- DeHon Semiconductor Physics 11 Penn ESE370 Fall2012 -- DeHon Conduction • Metal – conducts • Insulator – does not conduct • Semiconductor – can act as either 12 Penn ESE370 Fall2012 -- DeHon Why metal conduct? • (periodic table) http://chemistry.about.com/od/imagesclipartstructures/ig/Science-Pictures/Periodic-Table-of-the-Elements.htm 13 Penn ESE370 Fall2012 -- DeHon Conduction • Electrons move • Must be able to “remove” electron from atom or molecule 14 Penn ESE370 Fall2012 -- DeHon Atomic States • Quantized Energy Levels • Must have enough energy to change level (state) 15 Penn ESE370 Fall2012 -- 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 16 Penn ESE370 Fall2012 -- DeHon Silicon Atom • How many valence electrons? 17 Penn ESE370 Fall2012 -- DeHon Silicon • 4 valence electrons – Inner shells filled – Only outer shells contribute to chemical interactions 18 Penn ESE370 Fall2012 -- DeHon Silicon-Silicon Bonding • Can form covalent bonds with 4 other silicon atoms 19 Penn ESE370 Fall2012 -- DeHon Silicon Lattice • Forms into crystal lattice http://www.webelements.com/silicon/crystal_structure.html 20 Penn ESE370 Fall2012 -- DeHon Silicon Lattice • Cartoon two-dimensional view 21 Penn ESE370 Fall2012 -- DeHon Outer Orbital? • What happens to outer shell in Silicon lattice? 22 Penn ESE370 Fall2012 -- DeHon Energy? • What does this say about energy to move electron? 23 Penn ESE370 Fall2012 -- DeHon Energy State View Valance Band – all states filled 24 Penn ESE370 Fall2012 -- DeHon State View Energy Conduction Band– all states empty Valance Band – all states filled 25 Penn ESE370 Fall2012 -- DeHon Band Gap and Conduction Insulator Metal Ec 8ev Ec Ev OR Ev Ev Ec Semiconductor 1.1ev Ec Ev 26 Penn ESE370 Fall2012 -- DeHon Doping • Add impurities to Silicon Lattice – Replace a Si atom at a lattice site with another 27 Penn ESE370 Fall2012 -- DeHon 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? 28 Penn ESE370 Fall2012 -- DeHon Doping with P 29 Penn ESE370 Fall2012 -- DeHon 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 30 Penn ESE370 Fall2012 -- DeHon Doped Band Gaps • Addition of donor electrons makes more metallic – Easier to conduct Semiconductor 0.045ev 1.1ev Ec ED Ev 31 Penn ESE370 Fall2012 -- 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 32 Penn ESE370 Fall2012 -- DeHon Electron Conduction 33 Penn ESE370 Fall2012 -- DeHon Electron Conduction 34 Penn ESE370 Fall2012 -- DeHon Capacitor Charge • Remember capacitor charge ++++++++ - - - - - - - - - 35 Penn ESE370 Fall2012 -- DeHon MOS Field? • What does “capacitor” field do to the doped semiconductor channel? Vgs=0 No field - - - = + +++++ + + + - - - Vcap>0 Vgs>0 Conducts -----36 Penn ESE370 Fall2012 -- 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 Fall2012 -- DeHon ------ ------ 37 Group 13 • What happens if we replace Si atoms with group 13 atom instead? – E.g. B (Boron) – Valance band electrons? 38 Penn ESE370 Fall2012 -- 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 39 Penn ESE370 Fall2012 -- DeHon Hole Conduction 40 Penn ESE370 Fall2012 -- DeHon Doped Band Gaps • Addition of acceptor sites makes more metallic – Easier to conduct Semiconductor Ec 0.045ev 1.1ev EvEA 41 Penn ESE370 Fall2012 -- DeHon Field Effect? • Effect of positive field on Acceptordoped Silicon? Vgs=0 No field + + + + + + + + + + - - - Vcap>0 Vgs>0 No conduction 42 Penn ESE370 Fall2012 -- DeHon Field Effect? • Effect of negative field on Acceptordoped Silicon? Vgs=0 No field + + + + - - +++++ Penn ESE370 Fall2012 -- DeHon - - + + + 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 44 Penn ESE370 Fall2012 -- DeHon Admin • Tuesday 11am: Chandrasakan Talk – Ultra Low Power Systems • HW3 – Should be able to do all now • HW4 – Will be on stuff this week 45 Penn ESE370 Fall2012 -- DeHon MOSFET • Semiconductor can act like metal or insulator • Use field to modulate conduction state of semiconductor -----+++++ 46 Penn ESE370 Fall2012 -- DeHon