Question

Explain with the help of a diagram, how depletion region and potential barrier are formed in a junction diode.

State briefly the processes involved in the formation of p-n junction explaining clearly how the depletion region is formed.

Answer 1

We know that in an n-type semiconductor, the concentration of electrons is more compared to the concentration of holes. Similarly, in a p-type semiconductor, the concentration of holes is more than the concentration of electrons

During the formation of a p–n junction and because of the concentration gradient across the p and n sides, holes diffuse from the p-side to the n-side (p → n) and electrons diffuse from the n-side to the p-side (n → p). This motion of charge gives rise to a diffusion current across the junction. When an electron diffuses from n → p, it leaves behind an ionised donor on the n-side. This ionised donor (positive charge) is immobile as it is bonded to the surrounding atoms. As the electrons continue to diffuse from n → p, a layer of positive charge (or positive space–charge region) on nside of the junction is developed. Similarly, when a hole diffuses from p → n due to the concentration gradient, it leaves behind an ionised acceptor (negative charge) which is immobile. As the holes continue to diffuse, a layer of negative charge (or negative space–charge region) on the p-side of the junction is developed. This space–charge region on either side of the junction together is known as the
depletion region.

Because of the positive space–charge region on the n-side of the junction and negative space charge region on the p-side of the junction, an electric field directed from the positive charge towards the negative charge develops. Due to this field, an electron on the p-side of the junction moves to the n-side and a hole on the n-side of the junction moves to the p-side. The loss of electrons from the n-region and the gain of electrons by the p-region cause a difference of potential across the junction of the two regions. This is how the barrier potential is formed.

Answer 2

As we know that n-type semi-conductor has more concentration of electrons than that of a hole and p-type semi-conductor has more concentration of holes than an electron. Due to the difference in concentration of charge carriers in the two regions of p-n junction, the holes diffuse from p-side to n-side and electrons diffuse from n-side to p-side.

When an electron diffuses from n to p, it leaves behind it an ionised donor on n-side. The ionised donor (+ ve charge) is immobile as it is bound by the surrounding atoms. Therefore, a layer of positive charge is developed on the n-side of the junction. Similarly, a layer of negative charge is developed on the p-side

Hence, a space-charge region is formed on either side of the junction, which has immobile ions and is devoid of any charge carrier, called as depletion layer or depletion region