DOCSLIB.ORG

Electrical Behavior • Askeland and Phule , the Science and Engineering of Materials, 4Th Topic 3 Ed., Chapter 18

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Electrical Behavior • Askeland and Phule , the Science and Engineering of Materials, 4Th Topic 3 Ed., Chapter 18 Reading assignment • Chung, Multifunctional cement- based Materials, Ch. 2. Electrical behavior • Askeland and Phule , The Science and Engineering of Materials, 4th Topic 3 Ed., Chapter 18. Supplementary reading Shackelford, Materials Science for Engineers, 6th Ed., Ch. 15. 1 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™is a trademark used herein under license. is a trademark used herein under license. ™ Figure 18.2 (a) Charge carriers, such as Learning electrons, are deflected by atoms or defects and take an irregular path through a conductor. The average rate at which the carriers move is the drift velocity v. (b) Valence electrons in the metallic bond move 2003 Brooks/Cole, a division of Thomson Learning, Inc. easily. (c) Covalent bonds must be broken in © semiconductors and insulators for an electron to be able to move. (d) Entire ions must diffuse to carry charge in many ionically bonded materials. Mean free path – The average distance that electrons can move without being scattered by other atoms. 2 Current charge I = . time 1 coulomb 1 ampere = . sec Current density ~ I J = A Current density – The current flowing through per unit cross- sectional area. 3 Electrical resistance V R = l I R µ l I (I/A) s = = A A V (V / l) 1 l ~ J R = s = s A (V / l) where s is the electrical conductivity Electric field – The voltage gradient or volts per unit length. dV V = - Drift velocity dx l Electric field v µ E dV E = - dx V dV v = m E = - = E l dx where µ is the mobility ~ J s = E 4 • Drift velocity - The average • Current density - The current flowing through per unit rate at which electrons or cross-sectional area. • Electric field - The voltage gradient or volts per unit other charge carriers move length. • Drift velocity - The average rate at which electrons or through a material under the other charge carriers move through a material under influence of an electric or the influence of an electric or magnetic field. • Mobility - The ease with which a charge carrier moves magnetic field. through a material. • Dielectric constant - The ratio of the permittivity of a • Mobility - The ease with material to the permittivity of a vacuum, thus which a charge carrier moves describing the relative ability of a material to polarize and store a charge; the same as relative permittivity. through a material. I = qnvA v = m ~ I qnvA J = = = qnv E A A ~ s = q n m J qnv s = = E E Flux dn Flux J n = - D n dp dx J p = -Dp dx Current density ~ Current density Jn = (- q) J n ~ æ dp ö dp ~ æ dn ö Jp = q J p = q ç - D p ÷ = -q D p . J n = (- q) ç - D n ÷ dx dx è dx ø è ø dn = q D n . dx 5 Energy levels of an isolated atom Einstein relationship D D p kT n = = mn m p q ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™is a trademark used herein under license. 6 Energy bands of solid sodium is a trademark used herein under license. ™ Learning 2003 Brooks/Cole, a division of Thomson Learning, Inc. © Energy bands of an insulator is a trademark used herein under license. ™ Learning 2003 Brooks/Cole, a division of Thomson Learning, Inc. © • Valence band - The energy levels filled by electrons in their lowest energy states. • Conduction band - The unfilled energy levels into which electrons can be excited to provide conductivity. • Energy gap (Bandgap) - The energy between the top of the valence band and the bottom of the conduction band that a charge carrier must obtain before it can transfer a charge. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™is a trademark used herein under license. 7 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™is a trademark used herein under license. 8 • Holes are in the Holes - Unfilled energy valence band. levels in the valence band. Because • Conduction electrons move to fill electrons are in the these holes, the holes conduction band. move and produce a current. Electrical conduction through a composite material consisting of Radiative recombination - three components (1, 2 and 3) that are in a parallel configuration Recombination of holes and 1 2 3 electrons that leads to - emission of light; this occurs in direct bandgap materials. l V I + 9 Resistance due to component i Total resistance l l R i = r i R = r|| A i (A1 + A 2 + A 3 ) Current through component i Total current VA V V (A 1 + A 2 + A 3 ) i I = = I i = R r ||l r i l Total current through the composite V æ A A A ö V V æ A A A ö 1 2 3 I = I + I + I = 1 + 2 + 3 ç + + ÷ = (A + A + A ) 1 2 3 ç ÷ ç ÷ 1 2 3 l ç r r r ÷ l r r r r l è 1 2 3 ø è 1 2 3 ø || Electrical conduction through a composite material consisting of three components (1, 2 and 3) that are in a series configuration - 1 1 A 1 A = 1 + 2 + r r (A + A + A ) r (A + A + A ) || 1 1 2 3 2 1 2 3 1 1 A3 2 V I r3 (A1 + A 2 + A3 ) 3 1 1 1 = f1 + f2 + f3 , + r1 r2 r3 Rule of Mixtures Area A Total resistance Vi = IR i L + L + L L ( 1 2 3 ) i R = r = Ir i ^ A A Total voltage drop Total voltage drop I V = IR V = (r L + r L + r L ) A 1 1 2 2 3 3 (L + L + L ) = Ir 1 2 3 ^ A 10 Total voltage drop I I V = (r1 L1 + r 2 L 2 + r 3 L 3 ) (r L + r L + r L ) A A 1 1 2 2 3 3 V = IR (L + L + L ) 1 2 3 L + L + L = Ir ( ) ^ A = Ir 1 2 3 ^ A çæ r L + r L + r L ÷ö è 1 1 2 2 3 3 ø r = Conduction through a ^ L + L + L composite material with an ( 1 2 3 ) insulating matrix and short conductive fibers = ? 1f 1 + ? 2 f 2 + ? 3 f 3 Rule of Mixtures Percolation threshold is a trademark used herein under license. ™ Minimum volume fraction of Learning conductive fibers (or particles) for adjacent fibers (or particles) to touch each other and form a 2003 Brooks/Cole, a division of Thomson Learning, Inc. © continuous conductive path. 11 Contact resistance Conduction through 1 Rc µ an interface A rc Rc = A where ?c is the contact resistivity Energy bands of an intrinsic semiconductor Intrinsic silicon Without thermal With thermal excitation excitation Without thermal With thermal excitation excitation Electrical conductivity of a semiconductor s = q n mn + q p mp , For an intrinsic semiconductor (n = p), where q = magnitude of the charge of an electron, n = number of conduction electrons per unit volume, p = number of holes per unit volume, m = mobility of conduction electrons, s = q n( m + m ) . n n p and mp = mobility of conduction holes. 12 • Intrinsic semiconductor - A semiconductor in Current density due to both which properties are controlled by the element an electric field or compound that makes the semiconductor and a concentration gradient and not by dopants or impurities. ~ dn • Extrinsic semiconductor - A semiconductor J n = qn m n E + qD n . prepared by adding dopants, which determine dx the number and type of charge carriers. ~ dp • Doping - Deliberate addition of controlled J p = qp m p E - qD p . dx amounts of other elements to increase the number of charge carriers in a semiconductor. ~ ~ ~ J = Jn + Jp . Extrinsic semiconductor (doped with an electron donor) Without thermal With thermal excitation excitation ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™is a trademark used herein under license. Extrinsic semiconductor Energy bands (doped with an electron acceptor) Extrinsic semiconductor Intrinsic (doped with an electron semiconductor donor) Without thermal With thermal excitation excitation 13 Energy bands Extrinsic semiconductor Intrinsic (doped with an electron ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™is a trademark used herein under license. semiconductor acceptor) Defect semiconductor (excess semiconductor Zn1+xO) Zn+ ion serves as an electron donor. Energy bands of Zn1+xO Defect semiconductor (deficit semiconductor Ni 1-xO) Ni3+ ion serves as an electron acceptor. 14 Energy bands of Ni1-xO c Consider an n-type semiconductor being n = illuminated. l Illumination increases conduction E µ n electrons and holes by equal number, E = h n (Photon energy) since electrons and holes are generated where h = Planck’s constant in pairs. = 6.6262 x 10-34 J.s Thus, the minority carrier concentration (pn) is affected much more than the Photon energy must be at least equal to the energy majority carrier concentration (nn). bandgap in order for electrons to be excited from the valence band to the conduction band. 15 • Temperature Effect - When the temperature of a metal increases, thermal energy causes the atoms to vibrate • Effect of Atomic Level Defects - ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™is a trademark used herein under license. Imperfections in crystal structures scatter electrons, reducing the mobility and conductivity of the metal Change of resistivity with temperature for a metal Dr = aDT r where a = temperature coefficient of electrical resistivity ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™is a trademark used herein under license. 16 Matthiessen’s rule – The resistivity of a metallic material is given by the addition of a base resistivity that accounts for the effect of temperature, and a temperature independent term that reflects the effect of atomic level defects, including impurities forming solid solutions.
Load more

Recommended publications
  • CHAPTER 8: Diffusion
    1 Chapter 8 CHAPTER 8: Diffusion Diffusion and ion implantation are the two key processes to introduce a controlled amount of dopants into semiconductors and to alter the conductivity type. Figure 8.1 compares these two techniques and the resulting dopant profiles. In the diffusion process, the dopant atoms are introduced from the gas phase of by using doped-oxide sources. The doping concentration decreases monotonically from the surface, and the in-depth distribution of the dopant is determined mainly by the temperature and diffusion time. Figure 8.1b reveals the ion implantation process, which will be discussed in Chapter 9. Generally speaking, diffusion and ion implantation complement each other. For instance, diffusion is used to form a deep junction, such as an n-tub in a CMOS device, while ion implantation is utilized to form a shallow junction, like a source / drain junction of a MOSFET. Boron is the most common p-type impurity in silicon, whereas arsenic and phosphorus are used extensively as n-type dopants. These three elements are highly soluble in silicon with solubilities exceeding 5 x 1020 atoms / cm3 in the diffusion temperature range (between 800oC and 1200oC). These dopants can be introduced via several means, including solid sources (BN for B, As2O3 for As, and P2O5 for P), liquid sources (BBr3, AsCl3, and POCl3), and gaseous sources (B2H6, AsH3, and PH3). Usually, the gaseous source is transported to the semiconductor surface by an inert gas (e.g. N2) and is then reduced at the surface. 2 Chapter 8 Figure 8.1: Comparison of (a) diffusion and (b) ion implantation for the selective introduction of dopants into a semiconductor substrate.
    [Show full text]
  • Which Pnictogen Is Best?† Cite This: New J
    NJC PAPER Pnictogen bonding with alkoxide cages: which pnictogen is best?† Cite this: New J. Chem., 2019, 43,14305 Henry J. Trubenstein, ‡ Shiva Moaven, ‡ Maythe Vega, Daniel K. Unruh and Anthony F. Cozzolino * Pnictogen bonding is beginning to emerge as a useful supramolecular interaction. The design strategies for these systems are still in the early stages of development and much attention has been focused on the lighter pnictogens. Pnictogen bond donors can have up to three independent sites for binding which can result in triple pnictogen bonding. This has been observed in the self-assembly of antimony alkoxide cages, but not with the lighter congeners. This work reports structural characterization of an analogous arsenic alkoxide cage that engages in a single pnictogen bond and synthetic explorations of Received 14th July 2019, the bismuth congener. DFT calculations are used to evaluate the differences between the structures. Accepted 13th August 2019 Ultimately the partial charge on the pnictogen and the energy of the pnictogen lone pair dictate the DOI: 10.1039/c9nj03648b strength, orientation and number of pnictogen bonds that these cages form. Antimony cages strike the best balance between strength and directionality, allowing them to achieve triple pnictogen bonding rsc.li/njc where the other congeners do not. Introduction or bismuth.15–23 Recently, antimony centred PnBs have been purposefully designed into molecules to actively direct the self- A pnictogen bond (PnB), in analogy to a halogen or chalcogen assembly of reversed bilayer vesicles,24 enable anion binding bond (XB/HaB or ChB), is ‘‘the net attractive interaction between with applications in sensing and transport,25–28 self-assembly an electrophilic region associated with a [pnictogen] atom in a complex architectures through triple pnictogen bonding29 and molecular entity [the PnB donor] and a nucleophilic region in allow for supramolecular catalysis.30 These applications rely on another, or the same, molecular entity [the PnB acceptor].’’1 the predictable formation of PnBs.
    [Show full text]
  • Donor Qubits in Silicon
    04/09/2015 Experiments with spin qubits in siliconDonor and diamond qubits in silicon Gavin W Morley University of Warwick Gavin W Morley, Spin qubits in silicon and diamond, QuICC Warwick, August 2015 Experiments with spin qubits in silicon and diamond: overview • Lecture 1 – Magnetic resonance – Silicon • Lecture 2 – Silicon (cont.) Lab tours • Lecture 3 Friday morning – Diamond Gavin W Morley, Spin qubits in silicon and diamond, QuICC Warwick, August 2015 1 04/09/2015 Experiments with spin qubits in silicon and diamond: overview • Lecture 1 – Magnetic resonance 1. Prepare (spin Hamiltonian) 2. Control (electromagnetic pulses) 3. Measure (spin state readout) – Silicon Gavin W Morley, Spin qubits in silicon and diamond, QuICC Warwick, August 2015 – Magnetic resonance 1. Prepare (spin Hamiltonian) 2. Control (electromagnetic pulses) 3. Measure (spin state readout) Gavin W Morley, Spin qubits in silicon and diamond, QuICC Warwick, August 2015 2 04/09/2015 Magnetic resonance: prepare - put a spin ½ into a magnetic field H Ŝ Hamiltonian = ωS z with energy, ħ ωS = g µB B0 for g-factor, g ~ 2, Bohr magneton µB = e ħ/2me, electron spin S = ½ Energy m = ½ of a s spin Magnetic field, B0 ms = -½ Gavin W Morley, Spin qubits in silicon and diamond, QuICC Warwick, August 2015 Magnetic resonance: prepare - put a spin ½ into a magnetic field Larmor precession at angular frequency ωS Gavin W Morley, Spin qubits in silicon and diamond, QuICC Warwick, August 2015 3 04/09/2015 Magnetic resonance: prepare - put a spin ½ into a magnetic field Larmor precession
    [Show full text]
  • Donor-Acceptor Methods for Band Gap Reduction in Conjugated Polymers: the Role of Electron Rich Donor Heterocycles
    DONOR-ACCEPTOR METHODS FOR BAND GAP REDUCTION IN CONJUGATED POLYMERS: THE ROLE OF ELECTRON RICH DONOR HETEROCYCLES By CHRISTOPHER A. THOMAS A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2002 Copyright 2002 by Christopher A. Thomas All rights reserved. ACKNOWLEDGMENTS I thank my parents, Nancy and Larry, for their continuous support and attempts to understand and encourage me during what has been simultaneously the most enjoyable and most stressful part of my life. They made this document possible by encouraging and par- ticipating in experiences that ensured I would have the backgound and interest in trying to figure out how the world works. The decision about whom to work for in graduate school is one of the events I have agonized the most about in my life. As promised, the graduate advisor-student relationship is a complicated entity consisting of advisor, boss, counselor and friend that can change its active role without warning. Despite this, I am positive that there is no other person that I would rather have worked with during this process. I especially appreciate being given an unusual amount of decision making and direction setting power in the projects I was involved with and the freedom to explore aspects of science that interested me even when they did not overlap cleanly with Professor Reynolds’ research interests or funding. For their major contributions to my enjoyment and interest in chemistry, I thank Joe Carolan for his contagious enthusiasm, and Joel Galanda, who immensely affected the three years of my life he was involved with my chemistry and physical science education.
    [Show full text]
  • Donor-Acceptor Properties of Trivalent Phosphorus and Arsenic Ligands Larry James Vande Griend Iowa State University
    Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1975 Donor-acceptor properties of trivalent phosphorus and arsenic ligands Larry James Vande Griend Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Inorganic Chemistry Commons Recommended Citation Vande Griend, Larry James, "Donor-acceptor properties of trivalent phosphorus and arsenic ligands " (1975). Retrospective Theses and Dissertations. 5764. https://lib.dr.iastate.edu/rtd/5764 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. INFORMATION TO USERS This material was produced from a microfilm copy of the original document. While the most advanced technologcal means to photogaph and reproduce this document have been used, the quality is heavily dependent upon the quality of the original submitted. The following explanation of techniques is provided to help you understand markings or patterns which may appear on this reproduction. 1. The sign or "terget" for pages apparently lacking from the document photographed is "Missing Page(s)". If it was possible to obtain the missing page(s) or section, they are spliced into the film along with adjacent pages. This may have necasitstsd cutung thru an image and du^icating adjacent pages to insure you complete continuity. 2. When an image on the film is obliterated with a large round black mark, it is an indication that the photographer suspected that the copy may have moved during exposure and thus cause a blurred image.
    [Show full text]
  • CHAPTER 1: Semiconductor Materials & Physics
    Chapter 1 1 CHAPTER 1: Semiconductor Materials & Physics In this chapter, the basic properties of semiconductors and microelectronic devices are discussed. 1.1 Semiconductor Materials Solid-state materials can be categorized into three classes - insulators, semiconductors, and conductors. As shown in Figure 1.1, the resistivity of semiconductors, ρ, is typically between 10-2 and 108 Ω-cm. The portion of the periodic table related to semiconductors is depicted in Table 1.1. Figure 1.1: Typical range of conductivities for insulators, semiconductors, and conductors. Semiconductors can be composed of a single element such as silicon and germanium or consist of two or more elements for compound semiconductors. A binary III-V semiconductor is one comprising one element from Column III (such as gallium) and another element from Column V (for instance, arsenic). The common element and compound semiconductors are displayed in Table 1.2. City University of Hong Kong Chapter 1 2 Table 1.1: Portion of the Periodic Table Related to Semiconductors. Period Column II III IV V VI 2 B C N Boron Carbon Nitrogen 3 Mg Al Si P S Magnesium Aluminum Silicon Phosphorus Sulfur 4 Zn Ga Ge As Se Zinc Gallium Germanium Arsenic Selenium 5 Cd In Sn Sb Te Cadmium Indium Tin Antimony Tellurium 6 Hg Pd Mercury Lead Table 1.2: Element and compound semiconductors. Elements IV-IV III-V II-VI IV-VI Compounds Compounds Compounds Compounds Si SiC AlAs CdS PbS Ge AlSb CdSe PbTe BN CdTe GaAs ZnS GaP ZnSe GaSb ZnTe InAs InP InSb City University of Hong Kong Chapter 1 3 1.2 Crystal Structure Most semiconductor materials are single crystals.
    [Show full text]
  • Lecture 7: Extrinsic Semiconductors - Fermi Level
    Lecture 7: Extrinsic semiconductors - Fermi level Contents 1 Dopant materials 1 2 EF in extrinsic semiconductors 5 3 Temperature dependence of carrier concentration 6 3.1 Low temperature regime (T < Ts)................7 3.2 Medium temperature regime (Ts < T < Ti)...........8 1 Dopant materials Typical doping concentrations in semiconductors are in ppm (10−6) and ppb (10−9). This small addition of `impurities' can cause orders of magnitude increase in conductivity. The impurity has to be of the right kind. For Si, n-type impurities are P, As, and Sb while p-type impurities are B, Al, Ga, and In. These form energy states close to the conduction and valence band and the ionization energies are a few tens of meV . Ge lies the same group IV as Si so that these elements are also used as impurities for Ge. The ionization energy data n-type impurities for Si and Ge are summarized in table 1. The ionization energy data for p-type impurities for Si and Ge is summarized in table 2. The dopant ionization energies for Ge are lower than Si. Ge has a lower band gap (0.67 eV ) compared to Si (1.10 eV ). Also, the Table 1: Ionization energies in meV for n-type impurities for Si and Ge. Typical values are close to room temperature thermal energy. Material P As Sb Si 45 54 39 Ge 12 12.7 9.6 1 MM5017: Electronic materials, devices, and fabrication Table 2: Ionization energies in meV for p-type impurities. Typical values are comparable to room temperature thermal energy.
    [Show full text]
  • The Metal-Semiconductor Junction: Review
    Review Energy Bands Carrier Density & Mobility Carrier Transport Generation and Recombination The Metal-Semiconductor Junction: Review Energy band diagram of the metal and the semiconductor before (a) and after (b) contact Barrier Height Built-in Potential M-S Junctions: Thermal Equilibrium Depletion Region Region Depletion Energy band diagram of a metal-semiconductor contact in thermal equilibrium. Under Bias Full depletion approximation Review (a) Charge density, (b) electric field, (c) potential and (d) energy with the full depletion analysis. MOS Capacitors: Review The MOS capacitor consists of a Metal-Oxide-Semiconductor structure Current Transport at the MS Interface The current across a metal-semiconductor junction is mainly due to majority carriers. Three distinctly different mechanisms exist: diffusion of carriers from the semiconductor into the metal, thermionic emission of carriers across the Schottky barrier and quantum-mechanical tunneling through the barrier. Diffusion Current: driving force is distributed over the length of the depletion layer. Thermionic Emission: only energetic carriers, with energy equal to or larger than the conduction band energy at the metal-semiconductor interface, contribute to the current flow. Tunneling: the wave-nature of the electrons, allowing them to penetrate through thin barriers. Diffusion, Thermionic Emission & Tunneling For Diffusion current, the depletion layer is large compared to the mean free path, so that the concepts of drift and diffusion are valid. The current depends exponentially on the applied voltage, Va, and the barrier height, B. Electric-field at MS Junction: The thermionic emission theory assumes that electrons, with an energy larger than the top of the barrier, will cross the barrier provided they move towards the barrier.
    [Show full text]
  • Electrons and Holes in Semiconductors
    Hu_ch01v4.fm Page 1 Thursday, February 12, 2009 10:14 AM 1 Electrons and Holes in Semiconductors CHAPTER OBJECTIVES This chapter provides the basic concepts and terminology for understanding semiconductors. Of particular importance are the concepts of energy band, the two kinds of electrical charge carriers called electrons and holes, and how the carrier concentrations can be controlled with the addition of dopants. Another group of valuable facts and tools is the Fermi distribution function and the concept of the Fermi level. The electron and hole concentrations are closely linked to the Fermi level. The materials introduced in this chapter will be used repeatedly as each new device topic is introduced in the subsequent chapters. When studying this chapter, please pay attention to (1) concepts, (2) terminology, (3) typical values for Si, and (4) all boxed equations such as Eq. (1.7.1). he title and many of the ideas of this chapter come from a pioneering book, Electrons and Holes in Semiconductors by William Shockley [1], published Tin 1950, two years after the invention of the transistor. In 1956, Shockley shared the Nobel Prize in physics for the invention of the transistor with Brattain and Bardeen (Fig. 1–1). The materials to be presented in this and the next chapter have been found over the years to be useful and necessary for gaining a deep understanding of a large variety of semiconductor devices. Mastery of the terms, concepts, and models presented here will prepare you for understanding not only the many semiconductor devices that are in existence today but also many more that will be invented in the future.
    [Show full text]
  • Doping Properties of C, Si, and Ge Impurities in Gan and Aln
    PHYSICAL REVIEW B VOLUME 56, NUMBER 15 15 OCTOBER 1997-I Doping properties of C, Si, and Ge impurities in GaN and AlN P. Bogusławski Department of Physics, North Carolina State University, Raleigh, North Carolina 27695 and Institute of Physics, PAN, 02-668 Warsaw, Poland J. Bernholc Department of Physics, North Carolina State University, Raleigh, North Carolina 27695 ~Received 9 May 1997; revised manuscript received 8 July 1997! Doping properties of substitutional C, Si, and Ge impurities in wurtzite GaN and AlN were studied by quantum molecular dynamics. We considered incorporation of impurities on both cation and anion sublattices. When substituting for cations, Si and Ge are shallow donors in GaN, while Ge becomes a deep donor in AlN. Both impurities are deep acceptors on the N site. Substitutional C cation is a shallow donor in GaN, but a deep one in AlN; C N is a relatively shallow acceptor in both materials. Two effects that potentially quench doping efficiency were investigated. The first one is the transition of a donor from a substitutional position to a DX-like configuration. In crystals with a wurtzite symmetry, there are two possible variants of a DX-like state, and they have substantially different properties. In GaN, DX2 states of both Si and Ge are unstable, or metastable, and thus they do not affect doping efficiency. In contrast, they are stable in AlN, and therefore neither Si nor Ge is a dopant in this material. Estimates obtained for Al xGa 12xN alloys show that the crossover composition for DX stability is much lower for Ge (x.0.3) than for Si (x.0.6).
    [Show full text]
  • Intrinsic and Extrinsic Semiconductors, Fermi-Dirac Distribution Function, the Fermi Level and Carrier Concentrations Zeynep Dilli, Oct
    ENEE 313, Spr. ’09 Supplement I Intrinsic and Extrinsic Semiconductors, Fermi-Dirac Distribution Function, the Fermi level and carrier concentrations Zeynep Dilli, Oct. 2008, rev. Mar 2009 This is a supplement on the concepts of charge carriers, intrinsic and extrinsic semiconductors, carrier concentrations, the Fermi-Dirac distribution function and the Fermi level, density of states and the effective density of states. 1 Review: Charge Carriers in Semiconductors Remember the energy band diagram of a semiconductor, shown in Figure 1, displaying the conduction and valence bands and the bandgap. The electrons in the valence band are bound to the atoms of the crystal. They need to have enough extra energy to go across the forbidden bandgap to get into the energy levels of the conduction band. To quantify electron concentration 1 we count the number of electrons in the conduction band per a 1This is a short way of saying ”conduction band electron concentration”, since only the electrons in the conduction band contribute to conductivity as electrons. Figure 1: 1 unit volume, typically a cm3. This is shown by the symbol n. When the system is in equilibrium, it is shown by n0. When an electron leaves the valence band to go to the conduction band, it is essentially breaking free of its bond, leaving behind an empty state available for another electron in the valence band energy levels. This empty state is called a ”hole” and effectively behaves like a positively charged particle. The number of holes in the valence band per unit volume is called ”hole concentration” and shown by the symbol p.
    [Show full text]
  • Semiconductors
    Semiconductors Diodes and BJ Transistors Lecture 8 Diodes • Typical Diode VI Characteristics – Forward Bias Region i – Reverse Bias Region d – Reverse Breakdown Region + - vd – Forward bias Threshold 5 i d 4 3 2 1 v d 0 -7 -5 -3 -1 1 3 5 7 -1 Reverse -2 breakdown Reverse bias Forward bias region -3 region region -4 -5 VI stands for Voltage Current 2 Zener Diodes • Operated in the breakdown region. • Used for maintain a constant output voltage 3 Basic Semiconductor Electronics • Atomic Structure of Valence-4 elements like Carbon, Silicon, Germanium, etc. – have 4 valence electrons in its outer atomic shell – these atoms form covalent bonds with 4 other atoms in a lattice • When the energy levels of these electrons are raised several of these bonds may become randomly broken and a free electron is created – as a result these electrons are free to move about in the material similar to electron conduction occurs in a metal – in addition to the free electron, a negative particle, a “hole” which is a positive “particle” is created which also moves freely within the material. • As electrons and holes move through the material, they may encounter each other and recombine and, thereby, become electrically neutral • This type of material is called an intrinsic semiconductor 4 Intrinsic Silicon Crystal Complete Lattice +4 +4 +4 +4 +4 +4 +4 +4 +4 5 Intrinsic Silicon Crystal Thermal Energy causes a bond to be broken and a free electron and hole are created +4 +4 +4 +4 +4 +4 +4 +4 +4 6 Doped Semiconductor Material • If we incorporate a small impurity of five or three valence band materials into a 4 valence band lattice, we have created an extrinsic semiconductor which is doped with an impurity • n-type semiconductor – Doping with five valence material (e.g.
    [Show full text]