Semiconductor physics

What is the mass introduced so that the motion of an electron in a crystal can be treated like that of a free electron in response to external forces?

  1. ARest mass
  2. BAtomic mass
  3. CEffective mass
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Answer: C. Effective mass

Electrons in a crystal are affected by the periodic potential of the atoms and move differently from electrons in vacuum. The effective mass lumps these effects together by treating the electron as if it had a different mass. Smaller effective mass tends to give higher mobility.

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