In a crystal, the discrete energy levels of the individual atom broaden into energy bands.
When two semiconductors with different band structures are combined, a heterojunction
is formed, a p-n heterojunction is called a diode. Electrons and holes transfer
to the other side, because of different Fermi levels respectively. They recombine with
each other, leaving the p-side with negative charge and n-side with positive charge, this
region is called space charge layer (SCL). A built-in voltage V0 appears because of the
charge transfer and recombination. When there is no applied voltage, the Fermi level
is continuous across the diode EF P = EF N , as indicated in Figure