Chapter 18- Electrical Properties.ppt

1、1,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 conducti

2、vity affectedby impurities (doping) and temperature?,Chapter 18: Electrical Properties,2, Scanning electron micrographs of an IC:,Fig. (d) from Fig. 12.27(a), Callister & Rethwisch 3e. (Fig. 12.27 is courtesy Nick Gonzales, National Semiconductor Corp., West Jordan, UT.),View of an Integrated Circui

3、t,3,Electrical Conduction, Ohms Law:,V = I R,voltage drop (volts = J/C)C = Coulomb,resistance (Ohms),current (amps = C/s),4,Electrical Properties,Which will have the greater resistance?Analogous to flow of water in a pipe Resistance depends on sample geometry and size.,D,2D,2,5,Definitions,Further d

4、efinitionsJ = = another way to state Ohms lawJ current density electric field potential = V/,6, 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

5、,conductors,7,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,8,Electron Energy Band Structures,Adapted from Fig. 18.2, Callister & Rethwisch 8e.,9,Band Structure Representation,Adapted from Fig. 18.3, Callist

6、er & Rethwisch 8e.,10,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

7、 filled band,11,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,12,Metals: Influence of Temperature and Impurities on Resistivity, Presence of imperfections increas

8、es resistivity- grain boundaries- dislocations- impurity atoms- vacancies,These act to scatter electrons so that they take a less direct path.,13,Estimating Conductivity,Adapted from Fig. 7.16(b), Callister & Rethwisch 8e., Question:,- Estimate the electrical conductivity of a Cu-Ni alloythat has a

9、yield strength of 125 MPa.,14,Charge Carriers in Insulators and Semiconductors,Two types of electronic charge carriers:Free Electron negative charge in conduction bandHole positive charge vacant electron state in the valence band,Adapted from Fig. 18.6(b), Callister & Rethwisch 8e.,Move at different

10、 speeds - drift velocities,15,Intrinsic Semiconductors,Pure material semiconductors: e.g., silicon & germanium Group IVA materials,Compound semiconductors III-V compoundsEx: GaAs & InSbII-VI compoundsEx: CdS & ZnTeThe wider the electronegativity difference between the elements the wider the energy g

11、ap.,16,Intrinsic Semiconduction in Terms of Electron and Hole Migration,Adapted from Fig. 18.11, Callister & Rethwisch 8e.,electric field,electric field,electric field,17,Number of Charge Carriers,Intrinsic Conductivity,18,Intrinsic Semiconductors: Conductivity vs T, Data for Pure Silicon:- s increa

12、ses with T- opposite to metals,Adapted from Fig. 18.16, Callister & Rethwisch 8e.,materialSiGeGaPCdS,band gap (eV)1.110.672.252.40,Selected values from Table 18.3, Callister & Rethwisch 8e.,19, Intrinsic:- case for pure Si- # electrons = # holes (n = p), Extrinsic:- electrical behavior is determined

13、 by presence of impurities that introduce excess electrons or holes- n p,Intrinsic vs Extrinsic Conduction,20,Extrinsic Semiconductors: Conductivity vs. Temperature, Data for Doped Silicon:- s increases doping- reason: imperfection siteslower the activation energy toproduce mobile electrons., Compar

14、ison: intrinsic vsextrinsic conduction.- extrinsic doping level:1021/m3 of a n-type donorimpurity (such as P).- for T 450 K: “intrinsic“,21, Allows flow of electrons in one direction only (e.g., usefulto convert alternating current to direct current). Processing: diffuse P into one side of a B-doped

15、 crystal.,- No applied potential:no net current flow.,- Forward bias: carriersflow through p-type andn-type regions; holes andelectrons recombine atp-n junction; current flows.,- Reverse bias: carriersflow away from p-n junction;junction region depleted of carriers; little current flow.,p-n Rectifyi

16、ng Junction,p-type,n-type,Adapted from Fig. 18.21 Callister & Rethwisch 8e.,22,Properties of Rectifying Junction,Fig. 18.22, Callister & Rethwisch 8e.,Fig. 18.23, Callister & Rethwisch 8e.,23,Junction Transistor,Fig. 18.24, Callister & Rethwisch 8e.,24,MOSFET Transistor Integrated Circuit Device,Int

17、egrated circuits - state of the art ca. 50 nm line width 1,000,000,000 components on chip chips formed one layer at a time,Fig. 18.26, Callister & Rethwisch 8e.,MOSFET (metal oxide semiconductor field effect transistor),25,Ferroelectric Ceramics,Experience spontaneous polarization,Fig. 18.35, Callis

18、ter & Rethwisch 8e.,BaTiO3 - ferroelectric below its Curie temperature (120C),26,Piezoelectric Materials,Adapted from Fig. 18.36, Callister & Rethwisch 8e. (Fig. 18.36 from Van Vlack, Lawrence H., Elements of Materials Science and Engineering, 1989, p.482, Adapted by permission of Pearson Education,

19、 Inc., Upper Saddle River, New Jersey.),Piezoelectricity application of stress induces voltage application of voltage induces dimensional change,27, Electrical conductivity and resistivity are:- material parameters- geometry independent Conductors, semiconductors, and insulators.- differ in range of

20、 conductivity values- differ in availability of electron excitation states For metals, resistivity is increased by- increasing temperature- addition of imperfections- plastic deformation For pure semiconductors, conductivity is increased by- increasing temperature- doping e.g., adding B to Si (p-type) or P to Si (n-type) Other electrical characteristics- ferroelectricity- piezoelectricity,Summary,28,Core Problems:,Self-help Problems:,ANNOUNCEMENTS,Reading:,