ESE370: CircuitLevel Modeling, Design, and Optimization for Digital Systems Lec 6: September 14, 2015 MOS Model You are Here: Transistor Edition! Previously: simple models (0 and 1 st order) " Comfortable with basic functions and circuits! This week and next (4 lectures) " Detailed discussion " MOSFET phenomenology " Don t Blink!! Rest of term " Implications of the MOS device 2 Today MOS! MOS Structure! Basic Fabrication! Semiconductor Physics " Metals, insulators " Silicon lattice " Band gaps " Doping " Field Effects! Metal Oxide Semiconductor 3 4 MOS Capacitor! Metal gate! Oxide insulator separating gate from " Ideally: no conduction from gate to! Semiconductor between source and drain! See why gate input capacitive? gate drain source! Charge distribution and field?! How much charge on plates? gate drain source 5 6 1
Idea Source/Drain Contacts! Semiconductor can behave as metal or insulator! Voltage on gate induces an electrical field! Induced field attracts (repels) charge in to form a channel " Semiconductor can be switched between conducting and not conducting " Hence FieldEffect Transistor gate drain source! Contacts: Conductors # metalic " Connect to metal wires that connect transistors 7 8 Fabrication Dimensions! Start with Silicon wafer! Dope! Grow Oxide (SiO 2 )! Deposit Metal! Mask/Etch to define where features go! Channel Length (L)! Channel Width (W)! Oxide Thickness (T ox )! Process named by minimum length " 22nm # L=22nm http://www.youtube.com/watch?v=35jwsqxku74 Time Code: 2:004:30 9 10 Conduction Semiconductor Physics! Metal conducts! Insulator does not conduct! Semiconductor can act as either 11 12 2
Why does metal conduct? Why does metal conduct?! (periodic table) http://chemistry.about.com/od/imagesclipartstructures/ig/sciencepictures/periodictableoftheelements.htm 13 http://commons.wikimedia.org/wiki/file:periodic_table_of_elements_showing_electron_shells.sv 14 Conduction! Electrons move! Must be able to remove electron from atom or molecule Atomic States! Quantized Energy Levels (bands) " Valence and Conduction Bands! Must have enough energy to change level (state) 15 16 Thermal Energy Silicon Atom! Except at absolute 0! How many valence electrons? " There is always free energy " Causes electrons to hop around ".when there is enough energy to change states " Energy gap between states determines energy required 17 18 3
Silicon SiliconSilicon Bonding! 4 valence electrons! Can form covalent bonds with 4 other silicon atoms " Inner shells filled " Only outer shells contribute to chemical interactions 19 20 Silicon Lattice Silicon Ingot! Forms into crystal lattice 1 impurity atom per 10 billion silicon atoms 21 Penn ESE370 Fall Fall2015 Khanna Khanna 22 Silicon Lattice! Cartoon twodimensional view Outer Orbital?! What happens to outer shell in Silicon lattice? 23 24 4
Energy? Energy State View! What does this say about energy to move electron? Energy Valance Band all states filled 25 26 Energy State View Energy State View Conduction Band all states empty Conduction Band all states empty Energy Energy Band Gap Valance Band all states filled Valance Band all states filled 27 28 Band Gap and Conduction Doping! Add impurities to Silicon Lattice " Replace a Si atom at a lattice site with another Insulator Metal 8ev OR Semiconductor 1.1ev 29 30 5
Doping 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?! 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? 31 32 Doping with P 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 33 34 Doped Band Gaps Localized! Addition of donor electrons makes more metallic " Easier to conduct Semiconductor! Electron is localized! Won t go far if no low energy states nearby! Increasing doping concentration " Ratio of P atoms to Si atoms " Decreases energy to conduct 0.045ev 1.1ev E D 35 36 6
Electron Conduction Electron Conduction 37 38 Capacitor Charge MOS Field?! Remember capacitor charge! What does capacitor field do to the Donordoped channel? gate drain source 39 40 MOS Field? MOS Field?! What does capacitor field do to the Donordoped channel?! What does capacitor field do to the Donordoped channel? + V cap >0 + V cap >0 = V gs >0 Conducts 41 42 7
MOS Field Effect! Charge on capacitor " Attract or repel charges to form channel " Modulates conduction " Positive " Attracts carriers " Enables conduction " Negative? " Repel carriers " Disable conduction Group 13! What happens if we replace Si atoms with group 13 atom instead? " E.g. B (Boron) " Valance band electrons? B 43 44 Doping with B Hole Conduction! 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 45 46 Doped Band Gaps Field Effect?! Addition of acceptor sites makes more metallic! Effect of positive field on Acceptordoped Silicon? " Easier to conduct Semiconductor 0.045ev 1.1ev E A 47 48 8
Field Effect?! Effect of positive field on Acceptordoped Silicon? Field Effect?! Effect of positive field on Acceptordoped Silicon? + + V cap >0 + V cap >0 = V gs >0 No conduction 49 50 Field Effect? Field Effect?! Effect of negative field on Acceptordoped Silicon?! Effect of negative field on Acceptordoped Silicon? + + V cap <0 + V cap <0 = V gs >0 Conduction 51 52 MOSFETs MOSFET! Donor doping " Excess electrons " Negative or Ntype material " NFET! Acceptor doping " Excess holes " Positive or Ptype material " PFET! Semiconductor can act like metal or insulator! Use field to modulate conduction state of 53 54 9