Presentation on theme: "Chapter 18: Electrical Properties"— Presentation transcript:
1 Chapter 18: Electrical Properties ISSUES TO ADDRESS...• How are electrical conductance and resistancecharacterized?• What are the physical phenomena that distinguishconductors, semiconductors, and insulators?• For metals, how is conductivity affected byimperfections, temperature, and deformation?• For semiconductors, how is conductivity affectedby impurities (doping) and temperature?
3 Electrical Conduction • Ohm's Law:V = I Rvoltage drop (volts = J/C)C = Coulombresistance (Ohms)current (amps = C/s)• Resistivity, r: a material property that is independent of sample size and geometrysurface areaof current flowcurrent flow path length• Conductivity, s
4 Electrical Properties Which will have the greater resistance?Analogous to flow of water in a pipeResistance depends on sample geometry and size.2D2D
5 Definitions J = <= another way to state Ohm’s law Further definitionsJ = <= another way to state Ohm’s lawJ current density electric field potential = V/Electron flux conductivity voltage gradientJ = (V/ )
6 Conductivity: Comparison • Room temperature values (Ohm-m)-1 = ( - m)-1METALSconductorsPolystyrene <10-14Polyethylene-15-10-17Soda-lime glass 10Concrete-9Aluminum oxide <10-13CERAMICSPOLYMERSinsulators-117Silver6.8 x 107Copper6.0 x 107Iron1.0 x 10Silicon4 x 10-4Germanium2 x 10GaAs10-6SEMICONDUCTORSsemiconductorsSelected values from Tables 18.1, 18.3, and 18.4, Callister & Rethwisch 8e.
7 Example: Conductivity Problem What is the minimum diameter (D) of the wire so that V < 1.5 V?-I = 2.5 A+Cu wireV< 1.5 V2.5 A6.07 x 107 (Ohm-m)-1100 mSolve to get D > 1.87 mm
8 Electron Energy Band Structures So the individual atomic energy levels interact to form molecular energy levelsAdapted from Fig. 18.2, Callister & Rethwisch 8e.
9 Band Structure Representation contains valence electrons from the atomsAdapted from Fig. 18.3, Callister & Rethwisch 8e.
10 Conduction & Electron Transport • Metals (Conductors):-- for metals empty energy states are adjacent to filled states.-- thermal energy excites electrons into empty higher energy states.filledbandEnergypartlyemptyGAPfilled statesPartially filled bandEnergyfilledbandemptyfilled statesOverlapping bands-- two types of band structures for metals- partially filled band- empty band that overlaps filled band
11 Energy Band Structures: Insulators & Semiconductors -- wide band gap (> 2 eV)-- few electrons excited across band gap• Semiconductors:-- narrow band gap (< 2 eV) more electrons excited across band gapEnergyfilledbandvalencefilled statesGAP?emptyconductionEnergyemptybandconductionGAPfilledvalencebandfilled statesfilledband
14 Metals: Influence of Temperature and Impurities on Resistivity • Presence of imperfections increases resistivity-- grain boundaries-- dislocations-- impurity atoms-- vacanciesThese act to scatterelectrons so that theytake a less direct path.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)-200-100123456Resistivity,r(10-8Ohm-m)Cu at%Ni• Resistivityincreases with: =deformed Cu at%Nit-- temperaturethermalCu at%Nii-- wt% impurity+ impurityd-- %CW+ deformation“Pure” Cu
15 Estimating Conductivity • Question:-- Estimate the electrical conductivity of a Cu-Ni alloythat has a yield strength of 125 MPa.Adapted from Fig. 18.9, Callister & Rethwisch 8e.wt% Ni, (Concentration C)Resistivity,r(10-8Ohm-m)1020304050Yield strength (MPa)wt% Ni, (Concentration C)102030405060801001201401601801253021 wt% NiAdapted from Fig. 7.16(b), Callister & Rethwisch 8e.CNi = 21 wt% NiFrom step 1:
16 Charge Carriers in Insulators and Semiconductors Adapted from Fig. 18.6(b), Callister & Rethwisch 8e.Two types of electronic charge carriers:Free Electron– negative charge– in conduction bandHole– positive charge – vacant electron state in the valence bandMove at different speeds - drift velocities
17 Intrinsic Semiconductors Pure material semiconductors: e.g., silicon & germaniumGroup IVA materialsCompound semiconductorsIII-V compoundsEx: GaAs & InSbII-VI compoundsEx: CdS & ZnTeThe wider the electronegativity difference between the elements the wider the energy gap.
18 Intrinsic Semiconduction in Terms of Electron and Hole Migration • Concept of electrons and holes:+-electronholepair creationno appliedappliedvalenceSi atompair migrationelectric fieldelectric fieldelectric field• Electrical Conductivity given by:# electrons/m3electron mobility# holes/m3hole mobilityAdapted from Fig , Callister & Rethwisch 8e.
19 Number of Charge Carriers Intrinsic Conductivityfor intrinsic semiconductor n = p = ni = ni|e|(e + h)For GaAs ni = 4.8 x 1024 m-3For Si ni = 1.3 x 1016 m-3Ex: GaAs
20 Intrinsic Semiconductors: Conductivity vs T • Data for Pure Silicon:-- s increases with T-- opposite to metalsmaterialSiGeGaPCdSband gap (eV)1.110.672.252.40Selected values from Table 18.3, Callister & Rethwisch 8e.Adapted from Fig , Callister & Rethwisch 8e.