For a Pn-Junction Formation, a pure semiconductor such as silicon is taken. It is then doped
with impurity in such a way that its one-half becomes p-type semiconductor and the other half
becomes n-type semiconductor as shown.
Now, a silicon atom bonded to impurity atom in p-type
material, shown in red color, has seven electrons in its valence shell and
therefore, this silicon atom can radily accept one electron to complete its
octet.
Similarly, Arsenic atom in n-type material, shown in
black color, has nine elctrons in its valence shell and therefore, it can
readily donate one electron to restore its octet.
Therefore, near the pn-junction, arsenic
atoms give away electrons and silicon atoms bonded to impurity in p-type
material, accept electrons.
Due to this migration of electrons, donor arsenic
atoms are converted to +ve ions and acceptor silicon atoms in p-region are
converted to –ve ions.
This creates an electric field near the junction from
n-type to p-type.
Any hole near the junction is pushed by the electric field
into the p-type.
Similarly, any conduction electron near the junction is pushed
by electric field into the n-type. Thus, no charge carrier can remain in the
small region near the junction. This region is called the depletion region.
So now, n-region is relatively at a higher potential
than p-region. This potential difference developed across the junction, due to
drifting of electrons from n-type to p-type, is called potential barrier.
IMPORTANT :-
The depth of diffusion of pn-junction into the n-type and the p-type depends on
relative doping concentration of the regions. The junction diffuses more into
the lightly doped region and less into heavily doped region.
