Molecules with just a filled band (which doesn't overlap with an empty band) are insulators and semiconductors. The difference between the two just depends on the band-gap between the filled band and the next empty band:
In the semiconductor above, electrons can be promoted from the filled s band to the empty p band. Both the electrons and the "holes" left behind can act as charge carriers. Since electron promotion increases with temperature, conductivity increases with temperature.
The highest-energy band which contains electrons at T = 0 is called the valence band. The next band up is called the conduction band.
Why do metallic conductors decrease with temperature?
ReplyDeleteI don't know more than the vague explanation of "temperature increases vibrations, vibrations increase collisions with electrons moving through the solid".
DeleteThat is also true in semiconductors, but electrons jumping to the conduction band is a more significant effect.
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