Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-12 GTBL042-Callister-v2 August 13, 2007 18:22

Learning Objectives

After careful study of this chapter you should be able to do the following:

1.Describe the four possible electron band

structures for solid materials.

2.Briefly describe electron excitation events that

produce free electrons/holes in (a) metals,

(b) semiconductors (intrinsic and extrinsic), and

(c) insulators.

3.Calculate the electrical conductivities of metals,

semiconductors (intrinsic and extrinsic), and

insulators given their charge carrier density(s)

and mobility(s).

4.Distinguish betweenintrinsicandextrinsic

semiconducting materials.

5. (a)On a plot of logarithm of carrier (electron,
   hole) concentration versus absolute
   temperature, draw schematic curves for
   both intrinsic and extrinsic semiconducting
   materials.

(b)On the extrinsic curve note freeze-out,

extrinsic, and intrinsic regions.

6.For ap–njunction, explain the rectification

process in terms of electron and hole motions.

7. Calculate the capacitance of a parallel-plate
   capacitor.
   8.Define dielectric constant in terms of
   permittivities.
   9.Briefly explain how the charge storing capacity
   of a capacitor may be increased by the insertion
   and polarization of a dielectric material
   between its plates.
   10.Name and describe the three types of
   polarization.
   11.Briefly describe the phenomena of
   ferroelectricityandpiezoelectricity.

12.1 INTRODUCTION

The prime objective of this chapter is to explore the electrical properties of materials,

that is, their responses to an applied electric field. We begin with the phenomenon of

electrical conduction: the parameters by which it is expressed, the mechanism of con-

duction by electrons, and how the electron energy band structure of a material influ-

ences its ability to conduct. These principles are extended to metals, semiconductors,

and insulators. Particular attention is given to the characteristics of semiconductors

and then to semiconducting devices. Also treated are the dielectric characteristics

of insulating materials. The final sections are devoted to the peculiar phenomena of

ferroelectricity and piezoelectricity.

Electrical Conduction

12.2 OHM’S LAW

One of the most important electrical characteristics of a solid material is the ease

Ohm’s law with which it transmits an electric current.Ohm’s lawrelates the currentI—or time

rate of charge passage—to the applied voltageVas follows:

V=IR (12.1)

Ohm’s law

expression

whereRis the resistance of the material through which the current is passing. The

units forV,I, andRare, respectively, volts (J/C), amperes (C/s), and ohms (V/A).

The value ofRis influenced by specimen configuration, and for many materials is

independent of current. Theresistivityρis independent of specimen geometry but

resistivity

related toRthrough the expression

ρ=

RA

l

(12.2)

Electrical

resistivity—

dependence on

resistance, specimen

cross-sectional area,

and distance between

measuring points

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