Callister - An introduction - 8th edition

Summary • 769
• With these materials, in addition to free electrons, holes (missing electrons in the
valence band) may also participate in the conduction process (Figure 18.11).
• Semiconductors are classified as either intrinsic or extrinsic.
For intrinsic behavior, the electrical properties are inherent in the pure material,
and electron and hole concentrations are equal. The electrical conductivity
may be computed using Equation 18.13 (or Equation 18.15).
Electrical behavior is dictated by impurities for extrinsic semiconductors.
Extrinsic semiconductors may be either n- or p-type depending on whether
electrons or holes, respectively, are the predominant charge carriers.
• Donor impurities introduce excess electrons (Figures 18.12 and 18.13); acceptor
impurities introduce excess holes (Figures 18.14 and 18.15).
• The electrical conductivity on an n-type semiconductor may be calculated using
Equation 18.16; for p-type, Equation 18.17 is employed.
The Temperature Dependence of Carrier Concentration
Factors That Affect Carrier Mobility
• With rising temperature, intrinsic carrier concentration increases dramatically
(Figure 18.16).
• For extrinsic semiconductors, on a plot of majority carrier concentration versus
temperature, carrier concentration is independent of temperature in the “extrinsic
region” (Figure 18.17). The magnitude of carrier concentration in this region
is approximately equal to the impurity level.
• For extrinsic semiconductors, electron and hole mobilities (1) decrease as impurity
content increases (Figure 18.18), and (2) in general, decrease with rising temperature
(Figures 18.19a and 18.19b).
The Hall Effect
• Using a Hall effect experiment, it is possible to determine the charge carrier type
(i.e., electron or hole), as well as carrier concentration and mobility.
Semiconductor Devices
• A number of semiconducting devices employ the unique electrical characteristics
of these materials to perform specific electronic functions.
• The p–n rectifying junction (Figure 18.21) is used to transform alternating current
to direct current.
• Another type of semiconductor device is the transistor, which may be used for amplification
of electrical signals, as well as for switching devices in computer circuitries.
Junction and MOSFET transistors (Figures 18.24, 18.25, and 18.26) are possible.
Electrical Conduction in Ionic Ceramics and Polymers
• Most ionic ceramics and polymers are insulators at room temperature. Electrical
conductivities range between about 10 9 and 10 18 (#
m) 1 ; by way of comparison,
for most metals is on the order of 10 7 (#
m) 1 .
Dielectric Behavior
Capacitance
Field Vectors and Polarization
• A dipole is said to exist when there is a net spatial separation of positively and
negatively charged entities on an atomic or molecular level.
• Polarization is the alignment of electric dipoles with an electric field.
• Dielectric materials are electrical insulators that may be polarized when an electric
field is present.