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Formation of P-N Junction | What is P-N Junction

Formation of P-N Junction | What is P-N Junction

What is P-N Junction?

A P-N junction is formed by P-type and N-type semiconductor materials. The junction formed at P-type region and N-type region is called a P-N junction. 

Several different techniques are used for producing the P-N junctions such as grown junction, alloyed junction, diffused junction, ion implantation etc.
Formation of P-N Junction | What is P-N Junction

In P-type semiconductor uniformly distributed holes are majority carriers and in N-type semiconductor uniformly distributed electrons are majority carriers. When P-N junction is formed, holes and electrons come close together at the junction. 

Some free electrons from the N-side cross the junction and combine with the holes on the P-side. We say, that they diffuse across the junction; i.e., flow from a region of high carrier concentration to one of lower concentration. 

When the carriers migrate from one region to another they leave behind ion of opposite polarity. 
For example, when an electron migrates across the junction from n region to p region, it leaves behind an atom having deficiency of one electron and becomes a positive ion.

Similarly a hole migrates from P-region to N-region, an atom in the P-region becomes a negative ion. Such created ions are immobile in nature.

Before the charge carriers diffused across the junction, both the N-type and the P-type material were electrically neutral. However, as negative ions are created on the p-side of the junction the region of the P-side close to the junction acquires a negative charge. 
Formation of P-N Junction | What is P-N Junction
Similarly, the positive ions created on the N-side give the N-side a positive charge. The accumulated negative charge on the P-side tends to repel electrons that are crossing from the N-side, and the accumulated positive charge on the N-side tends to repel holes crossing from the P-side. Thus, it becomes difficult for more charge carriers to diffuse across the junction. 

The final result is that a barrier potential is created across the junction, negative on the P-side and positive on the N-side. The electric field produced by the barrier potential is large enough to prevent any further movement of electrons and holes across the junction.

By considering, doping concentrations, electronic charge and temperature, it is possible to calculate the magnitude of the barrier potential. Typical barrier potentials at room temperature are 0.2 V for germanium and 0.7 V for silicon.

The movement of charge carriers across the junction leaves a layer on either side which is depleted of charge carriers. This is the depletion region.

On the N-side, the depletion region consists of donor impurity atoms which have lost the free electrons associated with them, and have thus become positively charged. On the P-side, the region is made up of acceptor impurity atoms which are negatively charged because of losing the holes associated with them.

Formation of P-N Junction | What is P-N Junction

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