chem584.electrical - Electrical Properties ISSUES TO...

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1 ISSUES TO ADDRESS. .. • How are electrical conductance and resistance characterized? • What are the physical phenomena that distinguish conductors, semiconductors, and insulators? • For metals, how is conductivity affected by imperfections, T , and deformation? • For semiconductors, how is conductivity affected by impurities (doping) and T ? Electrical Properties Electrical Conduction Resistivity, and Conductivity, : geometry-independent forms of Ohm's Law E : electric field intensity resistivity (Ohm-m) I/A J : current density Resistivity is a material property & is independent of sample A I L V 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) L conductivity  1 • Resistance: A L A L R
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2 Electrical Properties Which will conduct more electricity? Analogous to flow of water in a pipe So resistance depends on sample geometry, etc. D 2 D I VA RA Definitions Further definitions J = <= another way to state Ohm’s law J current density electric field potential = V / or ( V / ) flux a like area surface current A I Current carriers • electrons in most solids • ions can also carry (particularly in liquid solutions) Electron flux conductivity voltage gradient J = ( V / )
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3 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 10 -4 Germanium 2 x 10 0 GaAs 10 -6 SEMICONDUCTORS semiconductors • Room T values (Ohm-m) -1 = ( -m) -1 Polystyrene <10 -14 Polyethylene 10 -15 -10 -17 Soda-lime glass 10 Concrete 10 -9 Aluminum oxide <10 -13 CERAMICS POLYMERS insulators -10 -10 -11 Electronic Band Structures Adapted from Fig. 18.2, Callister 7e .
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4 Band Structure Valence band – filled – highest occupied energy levels Conduction band – empty – lowest unoccupied energy levels valence band Conduction band Adapted from Fig. 18.3, Callister 7e . Conduction & Electron Transport • Metals ( Conductors ): Thermal energy puts many e - into accessible higher energy states. + - e- filled band Energy partly filled valence band empty band GAP filled states Energy filled band filled valence band empty band
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5 Energy States: Insulators & Semiconductors Insulators: Higher energy states not accessible due to gap (> 2 eV).
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chem584.electrical - Electrical Properties ISSUES TO...

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