If in a given material if the doping is not uniform then at one place
large number of charge carriers exist while at other place small number
of charge carriers exist. In a high charge carrier concentration area,
all charge carriers are of similar type, either electrons or holes and
hence start repelling each other. Due to this, charge carriers start
moving from high concentration area, to achieve uniform concentration
all over the material. This process is called diffusion and exists when
there is uniform concentration of charge carriers in the material. In a
p-n junction, on n side there are large number of electrons while on p
side electrons are minority in number. So there is high concentration of
electrons on n side while low concentration of electrons on p side.
Hence diffusion starts and electrons start moving from n side towards p
side.
Similarly the holes from p side diffuse across the junction into the n-region.
The initial diffusion is shown in the Fig. 1.
 |
| Fig. 1 Initial diffusion |
1.1 Formation of Depletion Region
As holes enter the n-region, they find number of donor atoms. The holes
recombine with the donor atoms. As donor atoms accept additional holes,
they become positively charged immobile ions. This happens immediately
when holes cross the junction hence number of positively charged
immobile ions get formed near the junction on n side.
Atoms on p side are acceptor atoms. The electrons diffusing from n side
to p side recombine with the acceptor atoms on p side. As acceptor atom
accept additional electrons, they become negatively charged immobile
ions get formed near the junction p side. The formation of immobile ions
near the junction is shown in the Fig. 2.
 |
| Fig. 2 Formation of immobile ions |
As more number of holes diffuse on n side, large positive charge gets
accumulated on n side near the junction. Eventually the diffusing holes
which are positively charged. get repelled due to accumulated positive
charge on n side. And the diffusion of holes stops.
Similarly due to large negative charge accumulated on p side, the
diffusing electrons get repelled and eventually the diffusion of
electrons also stops.
Thus in thermal equilibrium, in the region near the junction, there
exists a wall of negative immobile charges on p side and a wall of
positive immobile charges on n side. In this region, there are no mobile
charge carriers. Such a region is depleted of the free mobile charge
carriers and hence called depletion region or depletion layer. The
depletion region is also called space-charge region. In equilibrium
condition, the depletion region gets widened upto a point where no
further electrons or holes can cross the junction. The depletion region
acts as the barrier.
The physical distance from one side to other side of the depletion region is called width of the depletion region.
Practically width of the depletion region is very small of the order of few microns where 1 micron = 1 x 10-6 m.
1.2 Barrier Potential
Due to immobile positive charges on n side and negative charges on p
side, there exists an electric field across the junction. This creates
potential difference across the junction which is called barrier,
junction potential, built-in potential or cut-in voltage of p-n
junction.
 |
| Fig. 3 Open circuited p-n junction |
The barrier potential depends on,
1. Types of semiconductor 2. The donor impurity added
3. The acceptor impurity added 4. The temperature 5. Intrinsic concentration
The barrier potential is called height of the depletion region and expressed in volts. Symbolically it is denoted as VJ , Vo or Vγ.
Sponsored links :