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Chapter 18 - 1 ISSUES TO ADDRESS... How are electrical conductance and resistance characterized ? What are the physical phenomena that distinguish conductors,

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Presentation on theme: "Chapter 18 - 1 ISSUES TO ADDRESS... How are electrical conductance and resistance characterized ? What are the physical phenomena that distinguish conductors,"— Presentation transcript:

1 Chapter 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, temperature, and deformation? For semiconductors, how is conductivity affected by impurities (doping) and temperature? Chapter 18: Electrical Properties

2 Chapter 18 - Ohm’s Law

3 Chapter Electrical Conduction Ohm's Law: V = I R voltage drop (volts = J/C) C = Coulomb resistance (Ohms) current (amps = C/s) Conductivity,  Resistivity,  : -- a material property that is independent of sample size and geometry surface area of current flow current flow path length

4 Chapter Electrical Properties Which will have the greater resistance? Analogous to flow of water in a pipe Resistance depends on sample geometry and size. D 2D2D 2

5 Chapter Definitions Further definitions J =   <= another way to state Ohm’s law J  current density   electric field potential = V/ Electron flux conductivity voltage gradient J =  (V/ )

6 Chapter Room temperature values (Ohm-m) -1 = (  - m) -1 Selected values from Tables 18.1, 18.3, and 18.4, Callister & Rethwisch 8e. Conductivity: Comparison Silver 6.8 x 10 7 Copper 6.0 x 10 7 Iron 1.0 x 10 7 METALS conductors Silicon4 x Germanium2 x 10 0 GaAs10 -6 SEMICONDUCTORS semiconductors Polystyrene < Polyethylene Soda-lime glass 10 Concrete Aluminum oxide < CERAMICS POLYMERS insulators

7 Chapter What is the minimum diameter (D) of the wire so that V < 1.5 V? Example: Conductivity Problem Cu wire I = 2.5 A - + V Solve to get D > 1.87 mm < 1.5 V 2.5 A 6.07 x 10 7 (Ohm-m) m

8 Chapter Electron Energy Band Structures Adapted from Fig. 18.2, Callister & Rethwisch 8e.

9 Chapter Band Structure Representation Adapted from Fig. 18.3, Callister & Rethwisch 8e.

10 Chapter Conduction & Electron Transport Metals (Conductors): -- for metals empty energy states are adjacent to filled states. -- two types of band structures for metals -- thermal energy excites electrons into empty higher energy states. - partially filled band - empty band that overlaps filled band filled band Energy partly filled band empty band GAP filled states Partially filled band Energy filled band filled band empty band filled states Overlapping bands

11 Chapter Energy Band Structures: Insulators & Semiconductors Insulators: -- wide band gap (> 2 eV) -- few electrons excited across band gap Energy filled band filled valence band filled states GAP empty band conduction Semiconductors: -- narrow band gap (< 2 eV) -- more electrons excited across band gap Energy filled band filled valence band filled states GAP ? empty band conduction

12 Chapter 18 -

13 Electron Mobility

14 Chapter Metals: Influence of Temperature and Impurities on Resistivity Presence of imperfections increases resistivity -- grain boundaries -- dislocations -- impurity atoms -- vacancies These act to scatter electrons so that they take a less direct path. Resistivity increases with:  = deformed Cu at%Ni Adapted from Fig. 18.8, Callister & Rethwisch 8e. (Fig adapted from J.O. Linde, Ann. Physik 5, p. 219 (1932); and C.A. Wert and R.M. Thomson, Physics of Solids, 2nd ed., McGraw-Hill Book Company, New York, 1970.) T (ºC) Resistivity,  (10 -8 Ohm-m) 0 Cu at%Ni “Pure” Cu dd -- %CW +  deformation ii -- wt% impurity +  impurity tt -- temperature  thermal Cu at%Ni

15 Chapter Estimating Conductivity Adapted from Fig. 7.16(b), Callister & Rethwisch 8e. Question: -- Estimate the electrical conductivity  of a Cu-Ni alloy that has a yield strength of 125 MPa. Yield strength (MPa) wt% Ni, (Concentration C) wt% Ni Adapted from Fig. 18.9, Callister & Rethwisch 8e. wt% Ni, (Concentration C) Resistivity,  (10 -8 Ohm-m) C Ni = 21 wt% Ni From step 1: 30

16 Chapter Charge Carriers in Insulators and Semiconductors Two types of electronic charge carriers: Free Electron – negative charge – in conduction band Hole – positive charge – vacant electron state in the valence band Adapted from Fig. 18.6(b), Callister & Rethwisch 8e. Move at different speeds - drift velocities

17 Chapter Intrinsic Semiconductors Pure material semiconductors: e.g., silicon & germanium –Group IVA materials Compound semiconductors – III-V compounds Ex: GaAs & InSb – II-VI compounds Ex: CdS & ZnTe – The wider the electronegativity difference between the elements the wider the energy gap.

18 Chapter Intrinsic Semiconduction in Terms of Electron and Hole Migration Adapted from Fig , Callister & Rethwisch 8e. electric field Electrical Conductivity given by: # electrons/m 3 electron mobility # holes/m 3 hole mobility Concept of electrons and holes: + - electron hole pair creation + - no appliedapplied valence electron Si atom applied electron hole pair migration

19 Chapter Number of Charge Carriers Intrinsic Conductivity For GaAsn i = 4.8 x m -3 For Si n i = 1.3 x m -3 Ex: GaAs for intrinsic semiconductor n = p = n i   = n i |e|(  e +  h )

20 Chapter Intrinsic Semiconductors: Conductivity vs T Data for Pure Silicon: --  increases with T -- opposite to metals Adapted from Fig , Callister & Rethwisch 8e. material Si Ge GaP CdS band gap (eV) Selected values from Table 18.3, Callister & Rethwisch 8e.


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