ESE370: CircuitLevel Modeling, Design, and Optimization for Digital Systems Lec 6: September 18, 2017 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 semiconductor 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 (MOS) Capacitor! Metal gate! Oxide insulator separating gate from semiconductor " Ideally: no conduction from gate to semiconductor! Semiconductor between source and drain! See why gate input is capacitive? gate drain source semiconductor! Charge distribution and field?! How much charge on plates? gate drain source semiconductor 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 semiconductor to form a channel " Semiconductor can be switched between conducting and not conducting " Hence FieldEffect Transistor gate drain source semiconductor! Contacts: Conductors # metalic " Connect to metal wires that connect transistors 7 8 Fabrication Dimensions! Start with Silicon wafer! Dope silicon! Grow Oxide (SiO 2 )! Deposit Metal! Photoresist mask and 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 Why does metal conduct? 14 http://commons.wikimedia.org/wiki/file:periodic_table_of_elements_showing_electron_shells.svg Conduction Gold Silver Copper Zinc Penn ESE 370 Fall 2017 Khanna 15 Atomic States Valence and Conduction Bands Except at absolute 0 " There is always free energy Causes electrons to hop around " Energy gap between states determines energy required " Must have enough energy to change level (state) " 16 Thermal Energy Quantized Energy Levels (bands) " Electrons move Must be able to remove electron from atom or molecule 17.when there is enough energy to change states 18 3
Silicon Atom Silicon! How many valence electrons?! 4 valence electrons " Inner shells filled " Only outer shells contribute to chemical interactions 19 20 SiliconSilicon Bonding Silicon Lattice! Can form covalent bonds with 4 other silicon atoms! Forms into crystal lattice 21 22 Silicon Ingot Silicon Lattice! Cartoon twodimensional view 1 impurity atom per 10 billion silicon atoms 23 24 4
Outer Orbital? Energy?! What happens to outer shell in Silicon lattice?! What does this say about energy to move electron? 25 26 Energy State View Energy State View Conduction Band all states empty Energy Energy Valance Band all states filled Valance Band all states filled 27 28 Energy State View Band Gap and Conduction Energy Conduction Band all states empty Band Gap Insulator 8ev Metal OR Valance Band all states filled 29 30 5
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 31 32 Doping Doping! Add impurities to Silicon Lattice! Add impurities to Silicon Lattice " Replace a Si atom at a lattice site with another " Replace a Si atom at a lattice site with another! Add a Group 15 element! Add a Group 15 element " E.g. P (Phosphorus) " E.g. P (Phosphorus) " How many valence electrons? " How many valence electrons? 33 34 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 35 36 6
Doped Band Gaps Localized! Addition of donor electrons makes more metallic " Easier to conduct Semiconductor! Donor 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 37 38 Electron Conduction Electron Conduction 39 40 Capacitor Charge MOS Field?! Remember capacitor charge! What does capacitor field do to the Donordoped semiconductor channel? gate drain source semiconductor 41 42 7
MOS Field?! What does capacitor field do to the Donordoped semiconductor channel? MOS Field?! What does capacitor field do to the Donordoped semiconductor channel? + + V cap >0 + + V cap >0 = + V gs >0 Conducts 43 44 MOS Field Effect Doping! Charge on capacitor " Attract or repel charges to form channel " Modulates conduction " Positive " Attracts carriers " Negative? " Enables conduction " Repel carriers " Disable conduction +! What happens if we replace Si atoms with group 13 atom instead? " E.g. B (Boron) " Valance band electrons? 45 46 Doping Doping with B! What happens if we replace Si atoms with group 13 atom instead? " E.g. B (Boron) " Valance band electrons?! 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 B 47 48 8
Doped Band Gaps Hole Conduction! Addition of acceptor sites makes more metallic " Easier to conduct Semiconductor 0.045ev 1.1ev E A 49 50 Field Effect? Field Effect?! Effect of positive field on Acceptordoped Silicon?! Effect of positive field on Acceptordoped Silicon? + + V cap >0 51 52 Field Effect? Field Effect?! Effect of positive field on Acceptordoped Silicon?! Effect of negative field on Acceptordoped Silicon? + + V cap >0 + + V cap <0 = + V gs >0 No conduction 53 54 9
Field Effect? MOSFETs! Effect of negative field on Acceptordoped Silicon?! Donor doping " Excess electrons " Negative or Ntype material + + V cap <0 " NFET! Acceptor doping " Excess holes " Positive or Ptype material " PFET = + V gs >0 Conduction 55 56 Big Ideas: MOSFET Admin! Semiconductor can act like metal or insulator! Use electric field to modulate conduction state of semiconductor! HW due Wednesday! Midterm 1 " Monday Oct 2 79 pm " Let me know of conflicts ASAP + 57 58 10