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Berkeley ELENG 42 - Lecture Notes

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1Copyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003EECS 42 Introduction to Electronics for Computer ScienceLecture By Yun ChiuLecture # 23 Diodes and Diode CircuitsHandout of Monday Lecture. A) Basic Semiconductor MaterialsB) Diode Current and EquationC) Diode Circuitshttp://inst.EECS.Berkeley.EDU/~ee42/Wed: MOS Operation and I vs. V ModelsCopyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003MotivationDigital Circuits, Logic, D/A, etcc We need a “smart switch,” i.e., an electronically controlled switchd We need a “gain element” –for example, to make comparators.BONUS: MOS is very simple in conceptThe device of our dreams exists ! ⇒MOS transistor• a terrific switch• low power•smartThis week: Basic Semiconductors, Diodes, MOS transistorNext week: MOS and CMOS Fabrication2Copyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003Game Plan1. Learn a little more about semiconductors and pn junction diodes2. Consider the I vs. V model of diodes and their uses in circuits3. Learn about MOSFET Operation as a voltage controlled resistor4. Learn a little about the MOSFET I-V characteristics 5. Learn enough about the fabrication process for MOS integrated circuits so that we can visualize the layout of actual CMOS circuitsHere is how we begin:Thus we begin with a very brief review of semiconductors and dopingCopyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003Conductors, Insulators and SemiconductorsSolids with “free electrons” – that is electrons not directly involved in the inter-atomic bonding- are the familiar metals (Cu, Al, Fe, Au, etc). Solids with no free electrons are the familiar insulators (glass, quartz crystals, ceramics, etc.) Silicon is an insulator, but at higher temperatures some of the bonding electrons can get free and make it a little conducting –hence the term “semiconductor”Pure silicon is a poor conductor (and a poor insulator). It has 4 valence electrons, all of which are needed to bond with nearest neighbors. No free electrons.3Copyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003Electronic Bonds in Silicon2-D picture of perfect crystal of pure silicon; double line is a Si-Si bond with each line representing an electronTwo electrons in each bondSi ion(charge+4 q)Actual structure is 3-dimensional tetrahedral- just like carbon bonding in organic and inorganic materials.Essentially no free electrons, and no conduction - insulatorCopyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003How to get conduction in Si?For the first approach controlled impurities, “dopants”, are added to Si:orWe must either:1) Chemically modify the Si to produce free carriers (permanent) or2) Electrically “induce” them by the field effect (switchable)(Extra electrons produce “free electrons” for conduction.)Add group V elements (5 bonding electrons vs four for Si), such as phosphorus or arsenicDeficiency of electrons results in “free holes”Add group III elements (3 bonding electrons), such as boron4Copyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003Doping Silicon with Donors (n-type)Donors donate mobile electrons (and thus “n-type” silicon)Example: add arsenic (As) to the silicon crystal:Immobile (stuck) positively charged arsenic ion after 5th electron leftAsMobile electrondonated by As ionThe extra electron with As, “breaks free” and becomes a free electron for conductionCopyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003Doping with Acceptors (p-type) BMobile hole con-tributed by B ionand later pathImmobile (stuck) negative boron ion after accepting electron from neighboring bondGroup III element (boron, typically) is added to the crystalThe “hole” which is a missing bonding electron, breaks free fromthe B acceptor and becomes a roaming positive charge, free to carry current in the semiconductor. It is positively charged.5Copyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003If the lower floor is full and top one is empty, no traffic is possible. Analog of an insulator. All electrons are locked up.Shockley’s Parking Garage Analogy for Conduction in SiTwo-story parking garage on a hill:Copyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003If one car is moved upstairs, it can move AND THE HOLE ON THE LOWER FLOOR CAN MOVE. Conduction is possible. Analog to warmed-up semiconductor. Some electrons get free (and leave “holes” behind).Shockley’s Parking Garage Analogy for Conduction in SiTwo-story parking garage on a hill:6Copyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003If an extra car is “donated” to the upper floor, it can move. Conduction is possible. Analog to N-type semiconductor.(An electron donor is added to the crystal, creating free electrons).Shockley’s Parking Garage Analogy for Conduction in SiTwo-story parking garage on a hill:Copyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R. NeureutherVersion Date 04/25/03EECS 42 Intro. electronics for CS Spring 2003If a car is removed from the lower floor, it leaves a HOLE which can move. Conduction is possible. Analog to P-type semiconductor. (Acceptors are added to the crystal, “consuming” bonding electrons,creating free holes).Shockley’s Parking Garage Analogy for Conduction in SiTwo-story parking garage on a hill:7Copyright 2001, Regents of University of CaliforniaLecture 23: 04/28/03 A.R.


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Berkeley ELENG 42 - Lecture Notes

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