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Ch.12 - Solids and Modern Materials
All textbooksBrown 14th EditionCh.12 - Solids and Modern MaterialsProblem 115
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Chapter 12, Problem 115

Unlike metals, semiconductors increase their conductivity as you heat them (up to a point). Suggest an explanation.

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Step 1: Understand the basic structure of semiconductors. Semiconductors have a band structure with a valence band filled with electrons and a conduction band that is empty at absolute zero temperature.
Step 2: Recognize the role of thermal energy. As temperature increases, thermal energy excites electrons from the valence band to the conduction band.
Step 3: Explain the increase in charge carriers. The excitation of electrons creates electron-hole pairs, increasing the number of charge carriers available for conduction.
Step 4: Discuss the effect on conductivity. With more charge carriers (electrons and holes), the electrical conductivity of the semiconductor increases.
Step 5: Note the limitations. This increase in conductivity continues up to a certain temperature, beyond which other effects, such as lattice vibrations, may dominate and affect conductivity differently.
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Related Practice
Open Question
The electrical conductivity of aluminum is approximately 109 times greater than that of its neighbor in the periodic table, silicon. Aluminum has a face-centered cubic structure, and silicon has the diamond structure. A classmate of yours tells you that density is the reason aluminum is a metal but silicon is not; therefore, if you were to put silicon under high pressure, it too would act like a metal. Discuss this idea with your classmates, looking up data about Al and Si as needed.
Textbook Question

Silicon carbide, SiC, has the three-dimensional structure shown in the figure.

(b) Would you expect the bonding in SiC to be predominantly ionic, metallic, or covalent?

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Textbook Question

Energy bands are considered continuous due to the large number of closely spaced energy levels. The range of energy levels in a crystal of copper is approximately 1 * 10–19 J. Assuming equal spacing between levels, the spacing between energy levels may be approximated by dividing the range of energies by the number of atoms in the crystal. (b) Determine the average spacing in J between energy levels in the copper metal in part (a).

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Textbook Question

Sodium oxide (Na2O) adopts a cubic structure with Na atoms represented by green spheres and O atoms by red spheres.

(c) The unit cell edge length is 5.550 Å. Determine the density of Na2O.

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Open Question
Teflon is a polymer formed by the polymerization of F2C=CF2. (a) Draw the structure of a section of this polymer. (b) What type of polymerization reaction is required to form Teflon?
Open Question
Hydrogen bonding between polyamide chains plays an important role in determining the properties of a nylon such as nylon 6,6 (Table 12.6). Draw the structural formulas for two adjacent chains of nylon 6,6 and show where hydrogen-bonding interactions could occur between them.