P - N Junction

What is a P-N Junction?

A P-N Junction is a boundary between p-type semiconductors and n-type semiconductors built inside a single crystal semiconductor.

Most of the electronic appliances such as diodes, transistors and integrated circuits (IC s) work on the action of the P-N junction.
It is impossible to make a P-N junction just by joining a P-type semiconductor and a N-type semiconductor.

A sample P-N junction will be as follows:

The right half of the above junction has been doped with an element in the 15^th group of the periodic table so that it makes a n type semiconductor. For example As (Arsenic) can be used to dope these n type semiconductors. The donor cations shown in the above diagram represent As⁺ ions.

In contrast, the left half of the above junction has been doped with an element in the 13th period of the periodic table so that it makes a p type semiconductor. For example B (Boron) can be used to dope these p type semiconductors. The receiver anions shown in the above diagram represent B^- ions.

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Both these anions and the cations are firmly bonded to the crystal lattice structure.

The small circles in the left side represent the holes created by the receiver atoms.
For simplicity let us ignore the atoms belonging to the semiconductor in the above diagram.

The boundary where the p - type semiconductor (or the left side in the above figure) and the n-type semiconductor (or the right side in the above figure) meet is called the p-n junction.

When such a p-n junction is made different carriers inside the p-n semiconductor move in arbitrary directions. However, the concentration of holes in the p region is greater than that in the n region. Furthermore, when two holes approach each other mutual repulsion occurs.

(Photo credit: electrapk.com)


Due to these two reasons, holes close to the junction diffuse from the p side to the n side. For a similar reason, electrons close to the junction diffuse from the n side to the p side.

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Furthermore due to the mutual attraction between the holes and the electrons, both these carriers will not further remain as 'free electrons'.

Thus, at some instant after initiating these movements the region close to the junction becomes void of 'free carriers'.

(Photo credit : electronics.stackexchange.com)

Meanwhile due to this diffusion process, the p part close the junction, which was then a neutral region, now becomes a negatively charged region and the n part close to the junction becomes a positively charged region.

These two newly formed layers of positive and negative charges are similar to a parallel plate capacitor.

(Photo credit: Boundless.com)

Thus a potential difference and an electric field is formed in this region.

However the direction of the electric field is such that it is against the diffusion of the major carriers.

Although the magnitude of the electric field increases as soon as the junction was created, when the diffusion of the carriers decreases, its magnitude decreases and gains a state of equilibrium.

Since, after gaining equilibrium, a small area close to the junction is void of carriers, this region where there is a large resistance layer. Its thickness is about 1 μm.

(Photo credit: topperchoice.com)

The potential difference that is created in this layer depends upon the nature of the p-n junction. For example, the potential difference for silicon is about 0.7 V and that for Germanium is about 0.3 V.

This is known as the potential barrier over the p-n junction.

All these data can be summarized in a graph as follows:

(Photo credit: Wikipedia)