Direct Energy, 2018a

6 PHOTOVOLTAICS 115
acts as a conductor, and electrons ow. Photodiodes and solar cells are
made from semiconductors. If enough energy is supplied to a photodiode,
for example from an optical beam, the valence electrons will ow. More
specically, the photons ofthe external optical beam must have more energy
than the energy gap ofthe semiconductor for the valence electrons to ow.
The term semimetal is used to describe conductors with low electron
concentration. Similar to conductors, in a semimetal at T =0K, there
is no energy gap because the conduction band is partially lled with electrons,
and there are plenty ofavailable energy states. The concentration of
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electrons for semimetals, however, is in the range n