What is p-n junction?

What is p-n junction?

The p-n junctions are elementary building blocks of semiconductor electronic devices. As the name suggest it is a junction between two types of semiconductor materials the p-type and the n-type. The p-type is the positive side of junction and n-type is the negative side. The p-type consists of excess holes (missing electrons) and n-type consists of excess electrons. The junction is the boundary or interface between these 2 types inside a single crystal of semiconductor. When a p-n junction is formed i.e when it is fabricated there are excess electrons on n side and holes on p sides so electrons and holes combine at the junction. Departure of electrons from n side to p side leaves with a positive donor ion on n side and likewise hole leaves negative acceptor ion on p side. The uncompensated ions are positive on n side and negative on p side which creates an electric field which provides a force opposing the continued exchange of charge carriers.

When the electric field is sufficient to stop further transfer of charge carriers the depletion layers reaches an equilibrium. When a positive voltage is applied to the p-side with respect to the n-side, a large current will flow through the junction. However, when a negative voltage is applied, virtually no current flows. This “rectifying” behaviour is the most important characteristic of p-n junction. This is a very short description of p-n junction and how it works.

The p-n junction was invented by Russell Ohl an American physicist of Bell Laboratories in 1939.  The p-n junction is created by doping (adding impurities) by various methods on any one type of base semiconductor material. Various methods like ion implantation, diffusion, etc. Semiconductors for instance silicon, is doped with boron, phosphorus, or arsenic by ion implantation method. In diffusion method the impurity moves into the semiconductor crystal by solid-state diffusion. In the ion-implantation method, the intended impurity is introduced into the semiconductor by accelerating the impurity ions to a high-energy level and then implanting the ions in the semiconductor.

The p-n junction can be used in electronics circuits in two different configurations, forward bias and reverse bias.

In forward bias, the p-type is connected with the positive terminal and the n-type is connected with the negative terminal. With a battery connected this way, the holes in the p-type region and the electrons in the n-type region are pushed toward the junction and start to neutralize the depletion zone, reducing its width. And thus current flows through the junction.Connecting p-type region to the negative terminal of the battery and the n-type region to the positive terminal corresponds to reverse bias. Very little current will flow until the diode breaks down in reverse bias configuration.

The forward-bias and the reverse-bias properties of the p–n junction imply that it can be used as a diode. A p–n junction diode allows electric charges to flow in one direction, but not in the opposite direction; negative charges (electrons) can easily flow through the junction from n to p but not from p to n, and the reverse is true for holes. When the p–n junction is forward-biased, electric charge flows freely due to reduced resistance of the p–n junction. When the p–n junction is reverse-biased, however, the junction barrier (and therefore resistance) becomes greater and charge flow is minimal. p-n junction is used in many semiconductor devices such as diodes, transistors, solar cells, LED’s, integrated circuits, etc.

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