Summary

• Molecular bonds
• Molecular spectra

• Review molecular spectra
  
  
  
• Band theory of solids
  • energy levels
  • conduction band
  • valence band
  • conductors & insulators
    Example #1
    
  
  
  
• Semiconductors
  • intrinsic
  • n-type
  • p-type
    Example #2
    
  • junctions
  
  
  

gc6 29.q9
What happens to the resistivity of a semiconductor as it is heated?
A. increases due to increased e^- scattering.
B. increases because conduction band energy increases.
C. decreases because e^-'s are thermally boosted to conduction band.
D. decreases because conduction band energy decreases.
Answer







gc6 29.tb19
When a voltage is applied across a p-type semiconductor, the
A. electrons and holes recombine.
B. electrons move toward the negative electrode.
C. holes move toward the positive electrode.
D. holes move toward the negative electrode.
Answer







 



C. decreases because e^-'s are thermally boosted to conduction band.
The small energy gap of semiconductors make it possible for electrons to jump into the conduction band by the addition of random thermal energy. For a transistor to work properly the jumps should only occur when electrical energy, not thermal energy, is added. That is why you have to keep semiconductors (like computer chips) cool.












 



D. holes move toward the negative electrode.
The electrons aren't free to move in a p-type semiconductor; they're stuck in energy levels in the middle of the energy gap. But the holes they leave behind are free to move, and are considered the "majority charge carrier" even though it's true that in order for holes to move, electrons must hop into them.