1、1The Science and Engineering of Materials,4th edDonald R.Askeland Pradeep P.PhulChapter 18 Electronic Materials2Objectives of Chapter 18o To study electronic materials insulators,dielectrics,conductors,semiconductors,and superconductors.o To study conductivity in electronic materials.3o18.1 Ohms Law
2、 and Electrical Conductivityo18.2 Band Structures of Solidso18.3 Conductivity of Metals and Alloyso18.4 Superconductivityo18.5 Conductivity in Other Materialso18.6 Semiconductorso18.7 Applications of Semiconductorso18.8 Insulators and Dielectric Propertieso18.9 Polarization in Dielectricso18.10 Elec
3、trostriction,Piezoelectricity,Pyroelectricity,and FerroelectricityChapter Outline42003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.1 Classification of technologically useful electronic materials.5oCurrent density-The current flow
4、ing through per unit cross-sectional area.oElectric field-The voltage gradient or volts per unit length.oDrift velocity-The average rate at which electrons or other charge carriers move through a material under the influence of an electric or magnetic field.oMobility-The ease with which a charge car
5、rier moves through a material.oDielectric constant-The ratio of the permittivity of a material to the permittivity of a vacuum,thus describing the relative ability of a material to polarize and store a charge;the same as relative permittivity.Section 18.1 Ohms Law and Electrical Conductivity62003 Br
6、ooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.2(a)Charge carriers,such as electrons,are deflected by atoms or defects and take an irregular path through a conductor.The average rate at which the carriers move is the drift velocity v.
7、(b)Valence electrons in the metallic bond move easily.(c)Covalent bonds must be broken in semiconductors and insulators for an electron to be able to move.(d)Entire ions must diffuse to carry charge in many ionically bonded materials.78TABLE 18-1 Electrical conductivity of selected materials at TF30
8、0 K*(Continued)910Design an electrical transmission line 1500 m long that will carry a current of 50 A with no more than 5 105 W loss in power.The electrical conductivity of several materials is included in Table 18-1.Example 18.1 SOLUTIONElectrical power is given by the product of the voltage and c
9、urrent or:Example 18.1 Design of a Transmission LineFrom Equation 18-2:11Example 18.1 SOLUTION(Continued)Lets consider three metalsaluminum,copper,and silverthat have excellent electrical conductivity.The table below includes appropriate data and some characteristics of the transmission line for eac
10、h metal.Any of the three metals will work,but cost is a factor as well.Aluminum will likely be the most economical choice,even though the wire has the largest diameter.However,other factors,such as whether the wire can support itself between transmission poles,also contribute to the final choice.12S
11、ome manufacturers of automotives are considering using a 42-volt battery system instead of the standard 12/14-volt battery system.Why do you think this change is being considered?Example 18.2 SOLUTIONThere is a need for increased overall power as newer cars come equipped with many electronic feature
12、s such as power windows,power doors,power liftgates,and advanced lighting,braking,and steering systems.Increasing the voltage means less current is needed(V=IR),so the total weight of the copper wires can be reduced.Less weight in both the wiring and the motors contributes to fuel efficiency.Example
13、 18.2 A 42-volt Battery System for Cars13Example 18.3 Drift Velocity of Electrons in CopperAssuming that all of the valence electrons contribute to current flow,(a)calculate the mobility of an electron in copper and(b)calculate the average drift velocity for electrons in a 100-cm copper wire when 10
14、 V are applied.Example 18.3 SOLUTION1.The valence of copper is one:therefore,the number of valence electrons equals the number of copper atoms in the material.The lattice parameter of copper is 3.6151 10-8 cm and,since copper is FCC,there are 4 atoms/unit cell.From Table 18-114Example 18.3 SOLUTION(
15、Continued)2.The electric field is:15oValence band-The energy levels filled by electrons in their lowest energy states.oConduction band-The unfilled energy levels into which electrons can be excited to provide conductivity.oHoles-Unfilled energy levels in the valence band.Because electrons move to fi
16、ll these holes,the holes move and produce a current.oHybridization-When valence and conduction bands are separated by an energy gap,leading to the semiconductive behavior of silicon and germanium.oEnergy gap(Bandgap)-The energy between the top of the valence band and the bottom of the conduction ban
17、d that a charge carrier must obtain before it can transfer a charge.Section 18.2 Band Structures of Solids162003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.3 The energy levels broaden into bands as the number of electrons groupe
18、d together increases.172003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.4 The simplified band structure for sodium.The energy levels broaden into bands.The 3s band,which is only half filled with electrons,is responsible for condu
19、ction in sodium.182003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.5 (a)At absolute zero,all of the electrons in the outer energy level have the lowest possible energy.(b)When the temperature is increased,some electrons are excit
20、ed into unfilled levels.Note that the Fermi energy is unchanged.192003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.6 The band structure of carbon in the diamond form.The 2s and 2p levels combine to form two hybrid bands separated
21、 by an energy gap,Eg.202003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.7 Schematic of band structures for(a)metals,(b)semiconductors,and(c)dielectrics or insulators.(Temperature is 0 K.)21oMean free path-The average distance tha
22、t electrons can move without being scattered by other atoms.oTemperature Effect-When the temperature of a metal increases,thermal energy causes the atoms to vibrateoEffect of Atomic Level Defects-Imperfections in crystal structures scatter electrons,reducing the mobility and conductivity of the meta
23、loMatthiessens rule-The resistivity of a metallic material is given by the addition of a base resistivity that accounts for the effect of temperature(T),and a temperature independent term that reflects the effect of atomic level defects,including impurities forming solid solutions(d).oEffect of Proc
24、essing and StrengtheningSection 18.3 Conductivity of Metals and Alloys222003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.8 Movement of an electron through(a)a perfect crystal,(b)a crystal heated to a high temperature,and(c)a crys
25、tal containing atomic level defects.Scattering of the electrons reduces the mobility and conductivity.232003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.9 The effect of temperature on the electrical resistivity of a metal with a
26、perfect crystal structure.The slope of the curve is the temperature resistivity coefficient.2425Calculate the electrical conductivity of pure copper at(a)400oC and(b)100oC.Example 18.4 Resistivity of Pure Copper2627Example 18.4 SOLUTIONSince the conductivity of pure copper is 5.98 105-1.cm-1 the res
27、istivity of copper at room temperature is 1.67 10-6 ohm.cm.The temperature resistivity coefficient is 0.0068 ohm.cm/oC.282003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.10 The electrical resistivity of a metal is composed of a c
28、onstant defect contribution d and a variable temperature contribution T.292003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.11 (a)the effect of solid-solution strengthening and cold working on the electrical conductivity of copper
29、,and(b)the effect of addition of selected elements on the electrical conductivity of copper.3031oSuperconductivity-Flow of current through a material that has no resistance to that flow.oApplications of Superconductors-Electronic circuits have also been built using superconductors and powerful super
30、conducting electromagnets are used in magnetic resonance imaging(MRI).Also,very low electrical-loss components,known as filters,based on ceramic superconductors have been developed for wireless communications.Section 18.4 Superconductivity322003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson
31、 Learning is a trademark used herein under license.Figure 18.12 The electrical resistivity of a superconductor becomes zero below some critical temperature Tc.33Figure 18.13(a)The effect of a magnetic field on the temperature below which superconductivity occurs for a Type I superconductor.(b)The cr
32、itical surface of Nb3Sn,a Type II superconductor.The superconducting envelope or surface showing the combined effects of temperature,magnetic field,and current density on a Nb3Sn superconductor.Conditions within the envelope produce superconductivity.34Figure 18.13(c)The effect of a magnetic field o
33、n the temperature below which superconductivity occurs for a Type II superconductor.(d)Critical-current density as a function of magnetic-flux density(in Tesla)created by the applied magnetic fields(for different superconductors).3536Figure 18.14 Transmission electron micrographs of carbon nano-tube
34、s(CNT).The micrographs shows two carbon nanotubes that are bent considerably(shows they are highly elastic).The ends of these nanotubes were welded using a nanomanipulator in the transmission electron microscope.These materials are promising for microelectronics and other applications.(Micrograph Co
35、urtesy of Professor Vinayak Dravid,Northwestern University).37Design the limiting magnetic field that will permit niobium to serve as a superconductor at liquid helium temperatures.Assume H0=1970 oersteds.Example 18.5 Design of a Superconductor System3839Example 18.5 SOLUTIONFrom Table 18-5,Tc=9.25
36、K.We are given the value of and H0=1970 oersted.Since the operating temperature will be 4 K,we need to determine the maximum permissible magnetic field,using Equation 18-9:Therefore,the magnetic field(H)must remain below 1602 oersted in order for the niobium to remain superconductive.40oConduction i
37、n Ionic Materials-Conduction in ionic materials often occurs by movement of entire ions,since the energy gap is too large for electrons to enter the conduction band.Therefore,most ionic materials behave as insulators.oConduction in Polymers-Because their valence electrons are involved in covalent bo
38、nding,polymers have a band structure with a large energy gap,leading to low-electrical conductivity.Section 18.5 Conductivity in Other Materials41Suppose that the electrical conductivity of MgO is determined primarily by the diffusion of the Mg2+ions.Estimate the mobility of the Mg2+ions and calcula
39、te the electrical conductivity of MgO at 1800oC.Example 18.6 SOLUTIONFrom Figure(5.18),the diffusion coefficient of Mg2+ions in MgO at 1800oC is 10-10 cm2/s.For MgO,Z=2/ion,q=1.6 10-19 C,kB=1.38 10-23 J/K,and T=2073 K:Example 18.6 Ionic Conduction in MgO42Example 18.6 SOLUTION(Continued)Since the ch
40、arge in coulomb is equivalent to Ampere.second,and Joules is equivalent to Amp.sec.volts:MgO has the NaCl structure,with four magnesium ions per unit cell.The lattice parameter is 3.96 10-8 cm,so the number of Mg2+ions per cubic centimeter is:432003 Brooks/Cole,a division of Thomson Learning,Inc.Tho
41、mson Learning is a trademark used herein under license.Figure 18.15 Effect of carbon fibers on the electrical resistivity of nylon.44oIntrinsic semiconductor-A semiconductor in which properties are controlled by the element or compound that makes the semiconductor and not by dopants or impurities.oE
42、xtrinsic semiconductor-A semiconductor prepared by adding dopants,which determine the number and type of charge carriers.oDoping-Deliberate addition of controlled amounts of other elements to increase the number of charge carriers in a semiconductor.oThermistor-A semiconductor device that is particu
43、larly sensitive to changes in temperature,permitting it to serve as an accurate measure of temperature.oRadiative recombination-Recombination of holes and electrons that leads to emission of light;this occurs in direct bandgap materials.Section 18.6 Semiconductors45462003 Brooks/Cole,a division of T
44、homson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.16 When a voltage is applied to a semiconductor,the electrons move through the conduction band,which the electron holes move through the valence band in the opposite direction.472003 Brooks/Cole,a division of Tho
45、mson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.17 the distribution of electrons and holes in the valence and conduction bands(a)at absolute zero and(b)at an elevated temperature.482003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a tradema
46、rk used herein under license.Figure 18.18 The electrical conductivity versus temperature for intrinsic semiconductors compared with metals.Note the break in y-axis scale.49For germanium at 25oC,estimate(a)the number of charge carriers,(b)the fraction of the total electrons in the valence band that a
47、re excited into the conduction band,and(c)the constant n0.Example 18.7 Carrier Concentrations in Intrinsic Ge5051Example 18.7 SOLUTION1.From Equation 18-12:522.The lattice parameter of diamond cubic germanium is 5.6575 10-8 cm.The total number of electrons in the valence band of germanium is:3.From
48、Equation 18-14(a):532003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.19 When a dopant atom with a valence greater than four is added to silicon,an extra electron is introduced and a donor energy state is created.Now electrons are
49、 more easily excited into the conduction band.54552003 Brooks/Cole,a division of Thomson Learning,Inc.Thomson Learning is a trademark used herein under license.Figure 18.20 The effect of temperature on the carrier concentration of an n-type semiconductor.At low temperatures,the donor or acceptor ato
50、ms are not ionized.As temperature increases,the ionization process is complete and the carrier concentration increases to a level that is dictated by the level of doping.The conductivity then essentially remains unchanged,until the temperature becomes too high and the thermally generated carriers be
