Slide 1. Slide 2. Slide 3. Chapter 19: Electronic Materials. Learning Objectives. Introduction

Transcription

1 Slide 1 Chapter 19: Electronic Materials 19-1 Slide 2 Learning Objectives 1. Ohm s law and electrical conductivity 2. Band structure of solids 3. Conductivity of metals and alloys 4. Semiconductors 5. Applications of semiconductors 6. General overview of integrated circuit processing 7. Deposition of thin films 8. Conductivity in other materials 9. Insulators and dielectric properties 10. Polarization in dielectrics 11. Electrostriction, piezoelectricity, and ferroelectricity 19-2 Slide 3 Introduction Band structure Doping Superconductor Consists of the array of energy levels that are available to or forbidden for electrons to occupy and determines the electronic behavior of a solid, such as whether it is a conductor, semiconductor, or insulator Deliberate addition of controlled amounts of other elements to increase the number of charge carriers in a semiconductor A material that exhibits zero electrical resistance under certain conditions (which usually includes a very low temperature on the order of 135 K or less) 19-3

2 Slide 4 Ohm s Law and Electrical Conductivity Ohm s law V = IR where V voltage (volts, V) I current (Amperes or amps, A) R resistance (Ohms,Ω) to the current flow where l length (cm) of the resistor A cross-sectional area (cm 2 ) of the resistor ρ electrical resistivity (ohm.cm or Ω.cm) σ reciprocal of ρ, electrical conductivity (ohm -1 cm -1 ) Resistivity varies with the microstructure of a material 19-4 Slide 5 Ohm s Law and Electrical Conductivity Electrical power P = VI = I 2 R Current density, J (A/cm 2 ) Current per unit cross-sectional area J = σe Electric field, E (V/cm) The voltage gradient or volts per unit length 19-5 Slide 6 Figure

3 Slide 7 Table Electrical Conductivity of Selected Materials at T = 300 K * 19-7 Slide 8 Table Some Useful Relationships, Constants, and Units 19-8 Slide 9 Figure

4 Slide 10 Figure Slide 11 Ohm s Law and Electrical Conductivity Band structure of magnesium and other metals Magnesium and other metals in column 2A of the periodic table have two electrons in their outermost s band. These metals have a high conductivity because the p band overlaps the s band at the equilibrium interatomic spacing. In the transition metals, including scandium through nickel, an unfilled 3d band overlaps the 4s band. This overlap provides energy levels into which electrons can be excited Slide 12 Figure

5 Slide 13 Figure Slide 14 Conductivity of Metals and Alloys Mean free path ( e ) The average distance that electrons move without being scattered by other atoms or lattice defects Slide 15 Figure

6 Slide 16 Figure Slide 17 Figure Slide 18 Table

7 Slide 19 Figure Slide 20 Conductivity of Metals and Alloys Conductivity of alloys Alloys typically have higher resistivities than pure metals because of the scattering of electrons due to the alloying additions Slide 21 Table Properties of Commonly Encountered Semiconductors at Room Temperature 19-21

8 Slide 22 Figure Slide 23 Figure Slide 24 Figure

9 Slide 25 Semiconductors Extrinsic semiconductors The behavior of an intrinsic semiconductors cannot be controlled due to the slight variations in temperature significantly changes the conductivity. The conductivity of the extrinsic semiconductor depends primarily on the number of impurity, or dopant, atoms and in a certain temperature range is independent of temperature Slide 26 Figure Slide 27 Figure

10 Slide 28 Figure Slide 29 Semiconductors Direct and indirect bandgap semiconductors Radiative recombination: The emission of light when an electron loses energy and falls from the conduction band to the valence band to occupy a hole; this occurs in direct bandgap materials such as GaAs. Nonradiative recombination: The generation of heat when an electron loses energy and falls from the conduction band to the valence band to occupy a hole; this occurs mainly in indirect bandgap materials such as Si Slide 30 Figure

11 Slide 31 Figure Slide 32 Applications of semiconductors Field effect transistors A second type of transistor, which is almost universally used today for data storage and processing, is the field effect transistor (FET). A metal oxide semiconductor (MOS) field effect transistor (or MOSFET) consists of two highly doped n-type regions (n+) in a p-type substrate or two highly doped p-type regions in an n-type substrate Slide 33 Figure

12 Slide 34 Deposition of Thin Films Thin films Physical vapor deposition (PVD) Chemical Vapor Deposition (CVD) Electrodeposition A coating or layer that is small or thin in one dimension Typical thicknesses range from 10 Å to a few microns depending on the application A thin-film growth process in which a lowpressure vapor supplies the material to be deposited on a substrate A thin-film growth process in which gases undergo a reaction in a heated vacuum chamber to create the desired product on a substrate A method for depositing materials in which a source and workpiece are connected electrically and immersed in an electrolyte Slide 35 Figure Slide 36 Conductivity in Other Materials Conduction in ionic materials Mobility of the charge carriers where D diffusion coefficient k B Boltzmann constant T absolute temperature q electronic charge Z charge on the ion Conductivity σ = n.z.q. The most widely used conductive and transparent oxide is indium tin oxide (ITO), used as a transparent conductive coating on plate glass

13 Slide 37 Figure Slide 38 Insulators and Dielectric Properties Materials used to insulate an electric field from its surroundings are required in a large number of electrical and electronic applications. Electrical insulators obviously must have a very low conductivity, or high resistivity, to prevent the flow of current. Insulators must also be able to withstand intense electric fields. Insulators are produced from ceramic and polymeric materials in which there is a large energy gap between the valence and conduction bands Slide 39 Figure

14 Slide 40 Figure Slide 41 Table Properties of Selected Dielectric Materials Slide 42 Polarization in Dielectrics Linear and nonlinear dielectrics In linear dielectrics, P is linearly related to E and k is constant. The materials in which P and E are not related by a straight line are known as nonlinear dielectrics or ferroelectrics

15 Slide 43 Figure Slide 44 Figure Slide 45 Figure

16 Slide 46 Key Terms Band structure Dopants Superconductors Microstructure-sensitive Current density Electric field Drift velocity Mobility Holes Dielectric constant Fermi energy Valence Conduction Energy gap or bandgap Mean free path Matthiessen s rule Intrinsic semiconductor Extrinsic semiconductor Thermistors Radiative recombination Nonradiative recombination p-n junction Forward bias Slide 47 Reverse bias Rectifier diodes Integrated circuits Capacitors Thin films Physical vapor deposition Sputtering Chemical vapor deposition Electrodeposition Polarization Key Terms Capacitor Dielectric losses Permittivity Dielectric strength Linear dielectrics Nonlinear dielectrics Electrostriction Piezoelectric Ferroelectrics Hysteresis loop Curie temperature 19-47