In our previous tutorials, we’ve discussed the basics of semiconductors, band gap, current
carriers, and its classification.
We learned that in order for a semiconductor material to be useful, it needs to be doped
with an impurity atom and it could be a pentavalent atom to produce an n-type material or a trivalent
atom to come up with a p-type material.
Now, if we take a small piece of semiconductor material and dope half of it with a trivalent
atom and the other half with a pentavalent atom, that would result in a semiconductor
device with p-type and n-type portions, with a pn junction where they meet.
In this video, we are going to talk about this pn junction, as it is the basis for diodes.
After doping an intrinsic semiconductor material, usually silicon, so that part of it is p-type
and the other part is n-type, a pn junction is formed at the boundary between the p and
n regions.
In the p region, the majority carriers are holes while in the n region, the majority
carriers are free electrons.
When the pn junction is formed, the free electrons in the n region begin to diffuse across the
junction and combine with the holes in the p region near the junction.
This makes the n region lose free electrons and creates a layer of positive charges near
the junction.
As the free electrons combine with the holes in the p region, the p region also loses holes
and creates a layer of negative charges near the junction.
These two layers of positive and negative charges create a region where charge carriers
are depleted.
This region is called, fittingly, the depletion region.
In reality, the depletion region is formed in an incredibly short amount of time and
its width is very thin compared to the p and n regions.
The depletion region will continue to expand until the total negative charge in the depletion
region repels any further diffusion of electrons into the p region.
This time, the depletion region acts as a barrier to the movement of electrons across
the junction.
In the depletion region, there are many positive and negative charges on opposite sides, and
as described by Coulomb’s law, if there’s a positive charge and a negative charge near
each other, there is a force acting on these charges.
The forces between the positive and negative charges in the depletion region create an
electric field which blocks the free electrons from the n region to flow across the junction.
In order for the electrons to move through this electric field, a certain amount of voltage
equal to the barrier potential of the electric field must be applied across the pn junction
with the proper polarity.
Now, let’s try to analyze the formation of the pn junction and the depletion region
through an energy diagram.
The energy levels of the valence and conduction bands in an n-type material are lower than
the valence and conduction bands in a p-type material.
This is because, trivalent impurities exert lower forces on the outer-shell electrons
and lower forces mean that the electron orbits are slightly larger and have greater energy.
If we try to examine the energy diagram for a pn junction at the instant of formation,
we can see that the valence and conduction bands in the n region are at a noticeably
lower energy level compared to the p region valence and conduction bands.
But, we can also see that the upper part of the conduction band in the n region and the
lower part of the conduction band in the p region overlap.
In this case, the free electrons in the upper part of the conduction in the region can easily
diffuse across the pn junction and temporarily become free electrons in the lower part of
the p region conduction band.
Temporary because, after crossing the pn junction, these free electrons rapidly lose energy and
fall into the holes in the valence band of the p region.
As the free electrons from the upper part of the n region conduction band cross the
junction, the depletion region starts to form and expand.
But, the energy level of the n region conduction band also starts to decrease.
This process will continue until there will be no electrons left in the n region conduction
band with enough energy to cross the junction.
At that time, the junction is at equilibrium and the depletion region is complete because
the diffusion is done.
As you can see in the energy diagram, the top of the n region conduction band and the
bottom of the p region conduction band is now aligned and this time, there’s now an
energy “hill” that the electron from the n-region must climb in order to get to the
p region.
Another thing to observe here is that the energy level of the valence band in the n
region also decreased.
So the energy gap between the valence band and the conduction band in the n region remains
the same.
In this video, we discussed about the pn junction, its formation and the formation of the depletion
region.
We also discussed the barrier potential and the energy diagrams of the pn junction and
the depletion region.
If you have any questions, leave it in the comments below and if you’ve found this
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