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Chapter 20 - 1 How are electrical conductance and resistance characterized ? What are the physical phenomena that distinguish conductors, semiconductors,

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Presentation on theme: "Chapter 20 - 1 How are electrical conductance and resistance characterized ? What are the physical phenomena that distinguish conductors, semiconductors,"— Presentation transcript:

1 Chapter How are electrical conductance and resistance characterized ? What are the physical phenomena that distinguish conductors, semiconductors, and insulators? Electrical Properties

2 Chapter Electrical Conduction Resistivity,  and Conductivity,  : -- geometry-independent forms of Ohm's Law conductivity -- Resistivity is a material property & is independent of sample Ohm's Law:  V = I R voltage drop (volts = J/C) C = Coulomb resistance (Ohms) current (amps = C/s) I e - A (cross sect. area) VV L

3 Chapter Room T values (Ohm-m) Selected values from Tables 18.1, 18.3, and 18.4, Callister 7e. Conductivity: Comparison Silver 6.8 x 10 7 Copper 6.0 x 10 7 Iron 1.0 x 10 7 METALS conductors Silicon 4 x Germanium 2 x 10 0 GaAs SEMICONDUCTORS semiconductors = (  - m) Polystyrene < Polyethylene Soda-lime glass 10 Concrete Aluminum oxide < CERAMICS POLYMERS insulators

4 Chapter Energy States: Insulators & Semiconductors Insulators: -- Higher energy states not accessible due to gap (> 2 eV). Energy filled band filled valence band empty band filled states GAP Semiconductors: -- Higher energy states separated by smaller gap (< 2 eV). Energy filled band filled valence band empty band filled states GAP ?

5 Chapter Intrinsic: # electrons = # holes (n = p) --case for pure Si Extrinsic: --n ≠ p --occurs when impurities are added with a different # valence electrons than the host (e.g., Si atoms) Intrinsic vs Extrinsic Conduction n-type Extrinsic: (n >> p) no applied electric field Phosphorus atom valence electron Si atom conduction electron hole p-type Extrinsic: (p >> n) no applied electric field Boron atom Adapted from Figs (a) & 18.14(a), Callister 7e.

6 Chapter Allows flow of electrons in one direction only (e.g., useful to convert alternating current to direct current. Processing: diffuse P into one side of a B-doped crystal. Results: --No applied potential: no net current flow. --Forward bias: carrier flow through p-type and n-type regions; holes and electrons recombine at p-n junction; current flows. --Reverse bias: carrier flow away from p-n junction; carrier conc. greatly reduced at junction; little current flow. p-n Rectifying Junction p-typen-type p-typen-type Adapted from Fig , Callister 7e p-type n-type -+

7 Chapter Properties of Rectifying Junction Fig , Callister 7e.Fig , Callister 7e.

8 Chapter 20 - Transistor MOSFET MOSFET (metal oxide semiconductor field effect transistor) Fig , Callister 7e. Fig , Callister 7e.

9 Chapter Integrated Circuit Devices Integrated circuits - state of the art ca. 50 nm line width –> 100,000,000 components on chip –chip formed layer by layer Al is the “wire” Fig , Callister 6e.

10 Chapter Ferroelectric Ceramics Ferroelectric Ceramics are dipolar below Curie T C = 120ºC cooled below T c in strong electric field - make material with strong dipole moment Fig , Callister 7e.

11 Chapter Piezoelectric Materials at rest compression induces voltage applied voltage induces expansion Adapted from Fig , Callister 7e. Piezoelectricity – application of pressure produces current

12 Chapter Electrical conductivity and resistivity are: -- material parameters. -- geometry independent. Electrical resistance is: -- a geometry and material dependent parameter. Conductors, semiconductors, and insulators differ in accessibility of energy states for conductance electrons. For metals, conductivity is increased by -- reducing deformation -- reducing imperfections -- decreasing temperature. For pure semiconductors, conductivity is increased by -- increasing temperature -- doping (e.g., adding B to Si (p-type) or P to Si (n-type). Summary

13 Chapter ISSUES TO ADDRESS... How do we measure magnetic properties ? What are the atomic reasons for magnetism? Materials design for magnetic storage. How are magnetic materials classified? Chapter 20: Magnetic Properties What is the importance of superconducting magnets?

14 Chapter Created by current through a coil: Relation for the applied magnetic field, H: applied magnetic field units = (ampere-turns/m) current Applied Magnetic Field Applied magnetic field H current I N = total number of turns L = length of each turn

15 Chapter Magnetic induction results in the material Magnetic susceptibility,  (dimensionless) Response to a Magnetic Field current I B = Magnetic Induction (tesla) inside the material  measures the material response relative to a vacuum. H B vacuum  = 0  > 0  < 0

16 Chapter Measures the response of electrons to a magnetic field. Electrons produce magnetic moments: Net magnetic moment: --sum of moments from all electrons. Three types of response... Magnetic Susceptibility Adapted from Fig. 20.4, Callister 7e. magnetic moments electron nucleus electron spin

17 Chapter Plot adapted from Fig. 20.6, Callister 7e. Values and materials from Table 20.2 and discussion in Section 20.4, Callister 7e. 3 Types of Magnetism Magnetic induction B (tesla) Strength of applied magnetic field (H) (ampere-turns/m) vacuum (  = 0) -5 diamagnetic (  ~ -10) (1) e.g.,Al 2 O 3, Cu, Au, Si, Ag, Zn ferromagnetic e.g. Fe 3 O 4, NiFe 2 O 4 ferrimagnetic e.g. ferrite(  ), Co, Ni, Gd (3) (  as large as 10 6 !) (2) paramagnetic e.g., Al, Cr, Mo, Na, Ti, Zr (  ~ ) permeability of a vacuum: (1.26 x Henries/m)

18 Chapter Magnetic Moments for 3 Types Adapted from Fig. 20.5(a), Callister 7e. No Applied Magnetic Field (H = 0) Applied Magnetic Field (H) (1) diamagnetic none opposing Adapted from Fig. 20.5(b), Callister 7e. (2) paramagnetic random aligned Adapted from Fig. 20.7, Callister 7e. (3) ferromagnetic ferrimagnetic aligned

19 Chapter As the applied field (H) increases... --the magnetic moment aligns with H. Adapted from Fig , Callister 7e. (Fig adapted from O.H. Wyatt and D. Dew- Hughes, Metals, Ceramics, and Polymers, Cambridge University Press, 1974.) Ferro- & Ferri-Magnetic Materials Applied Magnetic Field (H) Magnetic induction (B) 0 B sat H = 0 H H H H H “Domains” with aligned magnetic moment grow at expense of poorly aligned ones!

20 Chapter Adapted from Fig , Callister 7e. Permanent Magnets Applied Magnetic Field (H) 1. initial (unmagnetized state) B large coercivity --good for perm magnets --add particles/voids to make domain walls hard to move (e.g., tungsten steel: H c = 5900 amp-turn/m) Hard vs Soft Magnets small coercivity--good for elec. motors (e.g., commercial iron Fe) Adapted from Fig , Callister 7e. (Fig from K.M. Ralls, T.H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering, John Wiley and Sons, Inc., 1976.) Applied Magnetic Field (H) B Hard Soft Hard Process: 2. apply H, cause alignment 4 Negative H needed to demagnitize!. Coercivity, H C 3. remove H, alignment stays! => permanent magnet!

21 Chapter Information is stored by magnetizing material. Image of hard drive courtesy Martin Chen. Reprinted with permission from International Business Machines Corporation. Head can apply magnetic field H & align domains (i.e., magnetize the medium). -- detect a change in the magnetization of the medium. Two media types: recording head recording medium Adapted from Fig , Callister 7e. (Fig from J.U. Lemke, MRS Bulletin, Vol. XV, No. 3, p. 31, 1990.) Magnetic Storage -- Particulate: needle-shaped  -Fe 2 O 3. +/- mag. moment along axis. (tape, floppy) Adapted from Fig , Callister 7e. (Fig courtesy P. Rayner and N.L. Head, IBM Corporation.) ~2.5  m Adapted from Fig (a), Callister 7e. (Fig (a) from M.R. Kim, S. Guruswamy, and K.E. Johnson, J. Appl. Phys., Vol. 74 (7), p. 4646, ) ~120 nm --Thin film: CoPtCr or CoCrTa alloy. Domains are ~ nm! (hard drive)

22 Chapter Superconductivity T c = temperature below which material is superconductive = critical temperature Copper (normal) Hg 4.2 K Adapted from Fig , Callister 7e.

23 Chapter Limits of Superconductivity 26 metals + 100’s of alloys & compounds Unfortunately, not this simple: J c = critical current density if J > J c not superconducting H c = critical magnetic field if H > H c not superconducting H c = H o (1- (T/T c ) 2 ) Adapted from Fig , Callister 7e.

24 Chapter Advances in Superconductivity This research area was stagnant for many years. –Everyone assumed T c,max was about 23 K –Many theories said you couldn’t go higher new results published for T c > 30 K –ceramics of form Ba 1-x K x BiO 3-y –Started enormous race. Y Ba 2 Cu 3 O 7-x T c = 90 K Tl 2 Ba 2 Ca 2 Cu 3 O x T c = 122 K tricky to make since oxidation state is quite important Values now stabilized at ca. 120 K

25 Chapter Meissner Effect Superconductors expel magnetic fields This is why a superconductor will float above a magnet normalsuperconductor Adapted from Fig , Callister 7e.

26 Chapter Current Flow in Superconductors Type Icurrent only in outer skin - so amount of current limited Type IIcurrent flows within wire M H Type I Type II complete diamagnetism mixed state H C1 H C2 HCHC normal

27 Chapter A magnetic field can be produced by: -- putting a current through a coil. Magnetic induction: -- occurs when a material is subjected to a magnetic field. -- is a change in magnetic moment from electrons. Types of material response to a field are: -- ferri- or ferro-magnetic (large magnetic induction) -- paramagnetic (poor magnetic induction) -- diamagnetic (opposing magnetic moment) Hard magnets: large coercivity. Soft magnets: small coercivity. Magnetic storage media: -- particulate  -Fe 2 O 3 in polymeric film (tape or floppy) -- thin film CoPtCr or CoCrTa on glass disk (hard drive) Summary

28 Chapter


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