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CHAPTER 18: ELECTRICAL PROPERTIES

CHAPTER 18: ELECTRICAL PROPERTIES. • Electrical Conductance and Resistance Classification of Materials based on Conductivity Conductors Insulators Semiconductors Metals: Effect of temperature Effect of crystal imperfections Effect of plastic deformation Semiconductors:

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CHAPTER 18: ELECTRICAL PROPERTIES

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  1. CHAPTER 18:ELECTRICAL PROPERTIES • • Electrical Conductance and Resistance • Classification of Materials based on Conductivity • Conductors • Insulators • Semiconductors • Metals: • Effect of temperature • Effect of crystal imperfections • Effect of plastic deformation • Semiconductors: • Intrinsic vs. Extrinsic semiconductors • Effect of temperature • Effect of impurities

  2. ELECTRICAL CONDUCTION • Ohm's Law: V = I R voltage drop (volts) resistance (Ohms) current (amps) • Resistivity, r and Conductivity, s: --geometry-independent forms of Ohm's Law E: electric field intensity resistivity (Ohm-m) J: current density conductivity • Resistance:

  3. CONDUCTIVITY: COMPARISON -1 • Room T values (Ohm-m)

  4. Example: Find the minimum diameter of a 100-m long copper wire needed so that a voltage of less 1.5 V develops when 2.5-A current flow through. What is the minimum diameter (D) of the wire so that V < 1.5V? 100m < 1.5V 2.5A 7 -1 6.07 x 10 (Ohm-m) Solve to get D > 1.88 mm

  5. Energy Band Structures in Solids electrons are perturbed by electrons and nuclei of adjacent atoms – results in split of atomic state into a series of closely spaced electron states Result: Electron energy band.

  6. Energy Band Structures • May not have bands for subshells near nucleus • Band gap – energies not available for electrons to occupy • Valence band – band that contains the valence electrons • Conduction band – next higher energy band

  7. Four Band Structures: a) Valence band only partially filled. (e.g., Cu) b) Full valence band, but overlaps the conduction band (e.g., Mg) c) Full valence band, wide energy band gap (insulators) d) Full valence band, narrow energy band gap (semiconductors)

  8. CONDUCTION & ELECTRON TRANSPORT • Metals: -- Thermal energy puts many electrons into a higher energy state. • Energy States: -- the cases below for metals show that nearby energy states are accessible by thermal fluctuations.

  9. ENERGY STATES: INSULATORS AND SEMICONDUCTORS • Semiconductors: --Higher energy states separated by a smaller gap. • Insulators: --Higher energy states not accessible due to gap.

  10. ELECTRICAL CONDUCTIVITY: Metals • Free electrons: • electrons that participate in conduction • have energies above the Fermi energy. • Electron Mobility: • d = eE •  = n |e| e n = number of electrons per unit volume e = electron mobility in (m2/V-s), (an indication of frequency of scattering events) Electrical Resistivity of Metals: increases linearly with temperature (increased scattering events) increases with impurity concentration increases with plastic deformation

  11. METALS: RESISTIVITY VS T, IMPURITIES • Imperfections increase resistivity --grain boundaries --dislocations --impurity atoms --vacancies These act to scatter electrons so that they take a less direct path. • Resistivity increases with: --temperature --wt% impurity --%CW

  12. PURE SEMICONDUCTORS: CONDUCTIVITY VS T • Data for Pure Silicon: --s increases with T --opposite to metals electrons can cross gap at higher T material Si Ge GaP CdS band gap (eV) 1.11 0.67 2.25 2.40

  13. CONDUCTION IN TERMS OF ELECTRON AND HOLE MIGRATION • Concept of electrons and holes: • Electrical Conductivity given by: # holes/m 3 hole mobility 3 electron mobility # electrons/m Intrinsic: n ≈ p

  14. INTRINSIC VS EXTRINSIC CONDUCTION • 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) • N-type Extrinsic: (n >> p) • P-type Extrinsic: (p >> n)

  15. P-N RECTIFYING JUNCTION • 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.

  16. SUMMARY • Electrical conductivity and resistivity are: --material parameters. --geometry independent. • Electrical resistance is: --a geometry and material dependent parameter. • Conductors, semiconductors, and insulators... --different in whether there are accessible 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).

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