Creating a P-N Junction Diode

A p-n junction is formed when the p-type semiconductor is joined with an n-type semi conductor. The n-type material has free electron introduced by the donor atoms (group V element dopant) while the p-type material has holes introduced by the acceptor (group III dopant element). These holes and electrons are free to move and they are at high concentrations in their respective materials (regions).

Due to high concentration of holes in the p-type region and electrons in the n-type region, a very large density gradient exists between both sides of the junction. Some of the free electrons from the donor impurity atoms in the n-type region begin to diffuse across this newly formed junction to fill up the holes in the P-type material.

However, because the electrons have moved across the junction from the N-type region to the P-type region, they leave behind positively charged donor ions on the negative side and now the holes from the acceptor impurity (p-type region) diffuse across the junction in the opposite direction into the n-type region where there are large numbers of free electrons. As holes diffuse into the n-type region, they leave behind negatively charged acceptor ions in the p-type region.

As a result, the P-type region near  the junction becomes negative while the n-type region near the junction becomes positive. This charge transfer of electrons and holes across the junction is known as diffusion.

As this  process continues,   electrons accumulates in the p-type region while positive chage accumulates in the n-type region and at a large enough electrical charge, they repel or prevent any more diffusion of holes and electrons over the junction. Eventually a state of equilibrium (electrically neutral situation) will occur producing a "potential barrier" zone around the area of the junction as the donor atoms repel the holes and the acceptor atoms repel the electrons.

Since no free charge carriers can rest in a position where there is a potential barrier, the regions on either sides of the junction now become completely depleted of any more free carriers in comparison to the N and P type materials further away from the junction. This area around the junction is now called the Depletion Layer. 

                                    The PN junction

As the N-type material has lost electrons and the P-type has lost holes, the N-type material becomes positive with respect to the P-type. Then the presence of impurity ions on both sides of the junction causes an electric field to be established across this region with the N-side at a positive voltage relative to the P-side. The problem now is that a free charge requires some extra energy to overcome the barrier (the barrier potential difference) that now exists for it to be able to cross the depletion region junction. That is to say energy is required for electric current to flow through the junction diode. This energy can be provided by connecting the ends of the p-n junction to an external voltage source.

If we make electrical connections at the ends of both the N-type and the P-type materials and then connect them to a battery source, an additional energy source now exists to overcome the barrier resulting in free charges being able to cross the depletion region from one side to the other.

The behaviour of the PN junction with regards to the potential barrier width produces an asymmetrical conducting two terminal device, better known as the P-N Junction Diode.