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Difficulty: HardConductors, Insulators, and Semiconductors

An electronics technician observes that inside an uncooled avionics chassis, a pure silicon diode demonstrates a significant decrease in internal resistance as ambient temperature climbs, whereas surrounding standard copper wiring exhibits increased resistance. What is the primary atomic mechanism that causes the electrical conductivity of an intrinsic semiconductor to increase as temperature rises?

  1. A
    Thermal expansion forces the four valence electrons to tightly lock into covalent bonds, preventing atomic vibration and allowing current to pass unimpeded.
  2. Thermal energy breaks covalent bonds within the crystal lattice, exciting valence electrons across the forbidden band gap into the conduction band to generate free electron-hole pairs.Answer
  3. C
    Increased temperature accelerates the drift velocity of preexisting free electrons, eliminating collisions between charge carriers and the stationary nucleus.
  4. D
    The semiconductor atom's outer shell absorbs free electrons from neighboring insulator jackets, increasing the net negative charge and converting the material into an active voltage source.

Answer

Thermal energy excites valence electrons across the forbidden energy gap into the conduction band, creating free electron-hole pairs that increase conductivity.
In intrinsic semiconductors such as pure silicon, the valence band is nearly full and the conduction band is nearly empty at low temperatures, separated by a relatively narrow forbidden band gap (approximately 1.1 eV for silicon). When temperature rises, thermal agitation provides sufficient kinetic energy to break covalent bonds, exciting valence electrons across this gap into the conduction band. This process generates both free conduction electrons and mobile holes, dramatically increasing the carrier density and lowering the material's electrical resistance (negative temperature coefficient of resistance).

Step-by-Step Solution

1
Analyze the valence electron structure of intrinsic semiconductors (Group IV elements like silicon and germanium).
Silicon has four valence electrons that form covalent bonds with four adjacent atoms in a crystalline lattice, leaving virtually zero free electrons at absolute zero.
Understanding the baseline atomic bonding of semiconductors is essential to distinguish them from metallic conductors with abundant free electrons.
2
Evaluate the effect of thermal energy on semiconductor covalent bonds versus metallic conductors.
In metals, heating increases lattice vibrations, causing greater electron scattering and higher resistance (positive temperature coefficient). In semiconductors, heat supplies energy to break covalent bonds, promoting electrons across the narrow forbidden band gap into the conduction band.
This explains the negative temperature coefficient of resistance characteristic of intrinsic semiconductors.
3
Identify the resulting charge carriers and impact on electrical conductivity.
Each liberated electron creates a free negative carrier in the conduction band and leaves behind a positive vacancy (hole) in the valence band, drastically increasing total charge carrier concentration and reducing resistance.
Conductivity is directly proportional to the concentration of mobile charge carriers.

Key Concept

Semiconductor Temperature Coefficient and Electron-Hole Pair Generation
Estimated Time:1m 30s
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Topics

Conductors, Insulators, and Semiconductors

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