Home Page Textbooks Physics: Principles and Problems Page 783: Section Review

Physics: Principles and Problems

9th Edition

Elliott, Haase, Harper, Herzog, Margaret Zorn, Nelson, Schuler, Zitzewitz

ISBN: 9780078458132

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Page 783: Section Review

Exercise 16

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Since the number of free electrons is directly proportional to the conductance the insulators will most likely keep electrons bound to their atoms.

Result

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Insulators.

Exercise 17

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Intrinsic semiconductors’ conductivity will depend on the temperature because the number of the free electrons in it is proportional to the temperature. On the other hand, doped semiconductors have free electrons that originate from the dopant material.

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The intrinsic semiconductors.

Exercise 18

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Since the insulators are defined as the materials in which the energy gap is $E>5$ eV the value of the gap ($E=9$ eV) suggests that silicon dioxide is used as the insulator.

Result

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Insulator.

Exercise 19

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Since the insulators are defined as the materials in which the energy gap is $E>5$ eV the value of the gap ($E=9$ eV) suggests that mgnsium oxide is used as the insulator.

Result

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Insulator.

Exercise 20

Solution 1

Solution 2

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Do not dope the region. Leave it as an intrinsic semiconductor since the region already has good insulation.

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**Doped semiconductors**

When we dope a pure semiconductor, we are adding elements with one more or one less valent electron. We do this to increase the conductivity of the semiconductor.

The doping element with one more electron creates more conducting electrons. This is called the $mathrm{bold{n}-type}$ semiconductor.

The doping element with one less electron creates more conducting “holes”. This is called the $mathrm{bold{p}-type}$ semiconductor.

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Both the $mathrm{bold{n}-type}$ and the $mathrm{bold{p}-type}$ are more conducting than an intrinsic (or pure) semiconductor, since there are more charge carriers (electrons and holes).

So, if we want to have a good insulating semiconductor, we want it to have low conductivity. Since the doped semiconductors have more conductivity than intrinsic semiconductors, this means that the doped semiconductors are less insulating than the intrinsic ones.

Thus, we choose the intrinsic (non-doped) semiconductor.

Exercise 21

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The explanation of this, we could say first-sight paradox is that the silicon has a way fewer free electrons to start with ($approx10^8$) and the rate of change would have to be much higher for silicon to reach the germanium number of free electrons.

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