Attached is a picture of three different electron configurations in a semiconductor. It seems from the author of my book that it should be obvious that the middle one has a lot lower conductivity than the 2 other. Why is that? I mean it is possible for the electrons to move in the conduction band and holes to move in the valence band for this configuration so it is not that it is nonconducting - it just low compared to the other configuration. How do you quantitatively find an expression for the conductance or what parameters should I at least be looking at other than the electron density which is lowest in the first picture (but this configuration still conducts a lot better!).