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COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 PRESENT POSITION Advisor in the Quality Enhancement Cell COMSATS CIIT, Islamabad ACADEMIC QUALIFICATIONS Ph.D. 1972 University of Cambridge, UK. M.Sc. 1967 Govt. College Lahore. FIELD OF SPECIALIZATION  Semiconductor Physics  Nuclear Physics Dr. Nasim Zafar 2 Electronics 1 EEE 231 Introduction: This course is an elective course for our BS students in the Department of Electrical Engineering, CIIT, Islamabad. Material emphasis: of the BS undergraduate education. Dr. Nasim Zafar 3 Electronics 1: EEE 231 Course Outline: Solid State Theory, Introduction to Semiconductors Devices, Intrinsic and Extrinsic Semiconductors, Electron Hole Pairs, Distribution of Electrons and Holes in a Semiconductors, P.N. Junction Diode, Forward and Reverse Biasing, of a Diode, V-I Characteristics, Ideal & Practical Diodes, DC Load Line & Quiescent Conditions, Small Signal Analysis of Diodes, Dynamic Resistance, AC Resistance, Capacitance and Switching Response, Diode Circuits & Applications, Rectifiers and Clipping Circuits, Special Diodes and their Applications, Zener Diodes, LED, Photo Diode, Tunnel Diode, Temperature Effects and Derating Curves, BJT Transistors, Biasing Techniques, Common Base, Common Emitter (CE) and Emitter Follower (CC) Configurations, Current Flow Mechanism, Equivalent Circuits, Current Amplification, Power Calculations, Theory of the Operation of the FETs and MOSFETs, Types of FETs, FET Amplifiers and Biasing Techniques, Temperature Effects in BJTs & FETs, Bias Stability, Q Point Variations, Stability Factor Analysis and Control. Dr. Nasim Zafar 4 Electronic 1: EEE 231 Recommended Books: B. G. Streetman, Solid State Electronic Devices, 5th ed., Prentice-Hall. Jasprit Singh, Semiconductor Devices–An Introduction, McGraw-Hill, Inc. (1994). Michael Shur, Physics of Semiconductor Devices, Prentice Hall, Inc. (1990). Dr. Nasim Zafar 5 Additional Text and Reference Books: 1. A. Bar-Lev, Semiconductors and Electronic Devices, Prentice Hal 2. S.M. Sze, Physics of Semiconductor Devices, John Wiley, (1981). 3. A.S. Grove, Physics and Technology of Semiconductor Dev., John Wiley, (1967). 4. J.L. Moll, Physics of Semiconductors, McGraw-Hill, Inc. (1964). 5. R.A. Smith, Semiconductors 2nd ed., Cambridge University Press, (1979). 6. Pierret, Semiconductor Device Fundamentals, Addison Wesley, (1996). Dr. Nasim Zafar Course Objectives:  Provide an introduction into the operating principles of electronic and optical devices, the principles of semiconductor processing  Present the relevant materials science issues in semiconductor processing.  Prepare students (a) for work in semiconductor processing facilities and (b) for graduate studies related to semiconductor processing and materials science topics. Dr. Nasim Zafar 7 Course Outline: 1 Semiconductor Materials-Introduction: • • • Band Theory of Solids Band Gap and Material Classification Semiconductor Materials 2 Charge Carriers and Carrier Transport in Semiconductors: • • • • • • Electrons and Holes in Equilibrium Carrier Densities: Fermi Dirac Distribution Function Generation/Recombination Mobility and Conductivity Continuity Equations Einstein Relation Dr. Nasim Zafar 8 3 PN Junctions: • Fabrication Techniques (abrupt & linearly graded junctions) •P-N Junctions under Equilibrium Conditions: - depletion region width - built–in–potential - Fermi levels and band bending •Junction Breakdown •I -V Characteristics of a PN Junction (biased junctions) 5 Bipolar Junction Transistors (BJT): • Fabrication Techniques • Principles of Transistor Action • Currents Flowing in a Transistor Dr. Nasim Zafar 9 6 Junction Field Effect Transistor (JFET): • Basic JFET Structure • Operation of a JFET • Characteristics of JFET 7 Optoelectronic Devices: Solar Cells • Photodiodes • Semiconductor Lasers • Light Emitting Devices (LEDs) • Dr. Nasim Zafar 10 Outcome: Upon completion of this course, the student will learn: • Understanding of the concept of band gap in semiconductors, to distinguish direct and indirect band gap semiconductors, and to relate the band gap with the wavelength of optical absorption and emission. • Understanding of doping of semiconductors to determine the free carrier concentration • Knowledge of the formation of p-n junctions to explain the diode operation and to draw its I-V characteristics. • Understanding of the operation mechanism of solar cells, LEDs, lasers and FETs, so that can draw the band diagram to explain their I-V characteristics and functionalities. Dr. Nasim Zafar 11 •Understanding of the operation mechanism of solar cells, LEDs, lasers and FETs, so that can draw the band diagram to explain their I-V characteristics and functionalities. •Ability to describe major growth techniques of bulk, thin film, and nanostructured semiconductors. •Basic knowledge of doping, purification, oxidation, gettering, diffusion, implantation, metallization, lithography and etching in semiconductor processing. Dr. Nasim Zafar 12 Electronics1: EEE231 Lecture No. 1 In this lecture we will cover the following topics: 1. Semiconductor Materials-Introduction:  The quantization concept  Band Theory of Solids  Band Gap and Material Classification  Semiconductor Materials Material emphasis: of the BS undergraduate education. Dr. Nasim Zafar 13 The quantization of Electron Energy States Dr. Nasim Zafar 14 Quantization Concept Quantum Mechanics  discrete energy levels That the radiation (i.e. electromagnetic waves) is emitted and absorbed as discrete energy quanta - photons. The energy of each photon is related to the wavelength of the radiation: E=h=hc/ where h = Planck’s constant (h = 6.63  1034 Js)  = frequency (Hz = s1) c = speed of light (3  108 m/s)  = wavelength (m) 15 Example Our eye is very sensitive to green light. The corresponding wavelength is 0.555 m or 5550 Å or 555 nm. What is the energy of each photon?  34 8 6.62  10 Js  3  10 m/s E = h = = 3.57  10–19 J  6 0.555  10 m These energies are very small and hence are usually measured using a new energy unit called electron Volts 1 eV = 1.6  1019 CV = 1.6  1019 J 16 A new unit of energy Since the energies related to atoms and photons are very small, (EGREEN LIGHT = 3.57  1019 J), we have defined a new unit of energy called “electron Volt” or “eV” One eV is the energy acquired by an electron when accelerated by a 1.0 V potential difference.  1V + 1 eV = 1.6 10–19 J Energy acquired by the electron is qV. Since q is 1.6  1019 C, the energy is 1.6  1019 J. Define this as 1 eV. Therefore, EGREEN LIGHT = 2.23eV 1 eV = 1 1.610–19 CV = 1.610–19 J 17 Bohr in 1913 hypothesized that electrons in hydrogen was restricted to certain discrete levels. This comes about because the electron waves can have only have certain wavelengths, i. e. n = 2r, where r is the orbit radius.  Quantization Based on this, one can show that: EH   m0 q 4 2 (40  n) h where   2 2  and m0 q 4 8  02 n 2 h 2 for n 1, 2, 3... h Planck' s constant 18 Bohr’s Hydrogen Atom Model and Electron Energy Levels 19 Band Theory of Solids Dr. Nasim Zafar 20 Energy Band Model An isolated atom has its own electronic structure with n = 1, 2, 3 ... shells. When atoms come together, their shells overlap. The energy level scheme in multi-electron atom , like Si is more complex, but intuitively similar. Consider Silicon: Si has 4 electrons in its outermost shell. When a large number of atoms come together, as in solids to form a crystal, these shells overlap and form bands. We do not consider the inner shell electrons since they are too tightly coupled to the inner core atom, and do not participate in anything. Configuration for Ge is identical to that of Si, except that the core has 28 electrons. 21 Survey of the Periodic Table Semiconductor Materials Formed from Atoms in Various Columns Group IV Elements  • Valence electron configuration: ns2 np2 [n = 2, C; n = 3, Si; n = 4, Ge; n = 5, Sn] [n  = 2, C; n = 3, Si; n = 4, Ge; n = 5, Sn] WHAT IS A SEMICONDUCTOR? B - Ch 1, Y - Ch 1, S - Ch 1 Conductivity/Resistivity Definition (σ = conductivity, ρ = resistivity) Metals: Good Conductors! 103 ≤ σ ≤ 108 (Ω-cm)-1; Semiconductor/Semimetals: 10-8 ≤ σ ≤ 103 (Ω-cm)-1; 10-8 ≤ ρ ≤ 10-3 Ω-cm 10-3 ≤ ρ ≤ 108 Ω-cm NOTE THE HUGE RANGE!! Insulators: σ ≤ 10-8 (Ω-cm)-1; ρ ≥ 108 Ω-cm No rigid boundaries! More Semiconductor Characteristics • In pure materials (very rare): The electrical conductivity σ  exp(cT) T = Kelvin Temperature, c = constant • Impure materials (most): – The electrical conductivity σ depends strongly on impurity concentrations. • “Doping” means to add impurities to change σ – The electrical conductivity σ can be changed by light or electron radiation & by injection of electrons at contacts – Transport of charge can occur by the motion of electrons or holes (defined later). 26 Bond model Consider a semiconductor Ge, Si, or C Ge, Si, and C have four nearest neighbors, each has 4 electrons in outer shell Each atom shares its electrons with its nearest neighbor. This is called a covalent bonding No electrons are available for conduction in this covalent structure, so the material is and should be an insulator at 0K 27 Qualitative Picture of Holes (from Seeger’s book) Idealized, 2D, “diamond” lattice for e- & e+ conduction Insulators, semiconductors, and metals 29 SEMICONDUCTOR: Bandgap Definition • Semiconductor: ~ Small bandgap insulator (define bandgap Eg in detail later). Strictly speaking, it must be capable of being doped (define doping in detail later). Typical Bandgaps • Semiconductors: 0 ~ ≤ Eg ≤ ~ 3 eV • Metals & Semimetals: Eg = 0 eV • Insulators: Eg ≥ 3 eV • Exception  Diamond: Eg = ~ 6 eV, is usually an insulator, but it can be doped & used as a semiconductor! • Also, sometimes there is confusing terminology like GaAs: Eg = 1.5 eV is sometimes called semi-insulating! The Best Known Semiconductor is Silicon (Si) • However, there are HUNDREDS (maybe THOUSANDS) of others! • • • • • • Elemental: Si, Ge, C (diamond) Binary compounds: GaAs, InP, . Organic compounds: (CH)n (polyacetyline) Magnetic semiconductors: CdxMn1-xTe, … Ferroelectric semiconductors: SbI, … Superconducting compounds: GeTe, SrTiO3, .. (“High Tc materials”) Group IV Crystalline Materials Elemental Semiconductors formed from atoms in Column IV • C (carbon): Different Crystal Phases Diamond Structure: Diamond! Insulator or semiconductor Graphite: A metal. The most common carbon solid. Fullerenes: Based on Buckminsterfullerene. “Bucky Balls”, Nanotubes, Insulator, Semiconductor or Metal depending on preparation. Clathrates: Possible new forms of C solids? Semiconductor or semimetal, compounds,… Recent Research!! • Si (silicon): Different Crystal Phases Diamond Structure: A Semiconductor. The most common Si solid. Clathrates: “New” forms of Si solids. Semiconductor, Semimetal, Compounds,…. Recent Research Group IV Crystalline Materials • Ge (germanium): Different Crystal Phases Diamond Structure: A Semiconductor. The most common Ge solid. Clathrates: “New” forms of Ge solids. Semiconductor, Semimetal, Compounds,…. Recent Research • Sn (tin): Different Crystal Phases Diamond Structure: Gray tin or α-Sn. A Semimetal Body Centered Tetragonal Structure: White tin or β-Sn. A Metal, The most common Sn solid. Clathrates: “New” forms of Sn solids. Semiconductor, Semimetal, Compounds,…. Recent Research • Pb (lead): Face Centered Cubic Structure: A Metal. Group IV Materials Bandgaps & Near-Neighbor Distances for Solids in Lattices with the Diamond Structure Decreasing Bandgap Eg correlates with Increasing Nearest Neighbor Bond Length d Atom Eg (eV) C 6.0 Si 1.1 Ge 0.7 2.44 Sn (a semimetal) 0.0 Pb (a metal) 0.0 Not diamond structure! d (Å) 2.07 2.35 2.80 1.63 Elemental Semiconductors • Mainly, these are from Column IV elements – C (diamond), Si, Ge, Sn (gray tin or α-Sn) Tetrahedrally bonded in the diamond crystal structure. Each atom has 4 nearest-neighbors. Bonding: sp3 covalent bonds. • Also! Some Column V & Column VI elements are semiconductors! P, A 3-fold coordinated lattice. S, Se, Te 5-fold coordinated lattices. Semiconductor models The subatomic particles responsible for charge transport in metallic wires – electrons The subatomic particles responsible for charge transport in semiconductors – electrons & holes 36 Semiconductor Conductivity Two charge carriers! – Electrons  e- & Holes  e+ What is a hole? – Qualitative definition for now! – Quantitative definition later! Holes: Usually treated as “positively charged electrons”. – How is this possible? – Are holes really particles? Doped Semiconductors • Intrinsic and Extrinsic Semiconductors. • Electron Hole Pairs. • Distribution of Electrons and Holes in a Semiconductors. Dr. Nasim Zafar 38 Doped Materials: Materials with Impurities! More interesting & useful! • Consider idealized carbon (diamond) lattice (could be any Group IV material). C : (Group IV) valence = 4 • Replace one C with a phosphorous. P : (Group V) valence = 5 • 4 e-  go to the 4 bonds • 5th e- ~ is almost free to move in the lattice (goes to the conduction band; is weakly bound). • P donates 1 e- to the material  P is a DONOR (D) impurity • Again, consider an idealized C (diamond) lattice. • C : (Group IV) valence = 4 • Replace one C with a boron.B : (Group III) valence = 3 • B needs one e- to bond to 4 neighbors. • B can capture e- from a C  e+ moves to C (a mobile hole is created) • B accepts 1 e- from the material  B is an ACCEPTOR (A) impurity Terminology • “Compensated material”  ND = NA • “n-type material”  ND > NA (n dominates p: n > p • “p-type material”  NA > ND (p dominates n: p > n ) ) T Dependences of e- & e+ Concentrations • Define: • n  concentration (cm-3) of ep  concentration (cm-3) of e+ np = CT3 exp[- Eg /(kBT)] • In a pure material: n = p  ni (np = ni2) • ni  “Intrinsic carrier concentration” ni = C1/2T3/2exp[- Eg /(2kBT)] • At T = 300K Si : Eg= 1.2 eV, ni =~ 1.5 x 1010 cm-3 Ge : Eg = 0.67 eV, ni =~ 3.0 x 1013 cm-3 Summary: • Quantization of electron energy states In isolated atoms: discrete energy states. – In solids: Energy Bands. – Transport of charge can occur by the motion of electrons or holes. – Doping increases electrical conductivity of semiconductors