Semiconductors
In a semiconductor bands that are filled are also electrically inert. But now the fermi energy falls
in between two bands in a gap. These two bands are called valence band and conduction band. It
is only a semiconductor if the gap is less than 3 eV. The value of 3 eV comes through the fact that
for low temperatures some electrons are excited thermally above the gap from the valence band into
the conduction band and we measure some kind of conductivity. If the gap is bigger there are zero
electrons excited into the conduction band at low temperatures. In fig. 46 we see the fermi energy
dropping in the gap leaving a few holes in the valence band and a few electrons in the conduction
band.
Figure 46: Bands in a semiconductor.