Basic Structure of PN Junction

Basic Structure of P‒N Junction

• Fig. 2.1.1 shows the p‒n junction at x = 0, with uniform doping in each region.

• Both types of charge carriers, majority as well as minority exist in each region, assuming thermal equilibrium.

• The interface between the two regions at x = 0 is called metallurgical junction.

• As the two regions are at thermal equilibrium, we can write,

On P side :

P_p = Majority charge carrier concentration

 = N_A

n_p = Minority charge carrier concentration

= n_i² / N_A

• As holes are in majority and electrons in minority, n_p << P_p'

On n side:

n_n = Majority charge carrier concentration

 = N_D

P_n = Minority charge carrier concentration

= n_i² / N_D

• As electrons are in majority and holes in minority, P_n <<n_n.

• At the junction at x = 0, the large changes in the carrier concentrations occur. As the junction is crossed from p side to n side, at the junction there is sudden decrease in the hole concentration from P_p to P_n.

• Similarly there is sudden increase in the electron concentration from n_p to n_n.

Fig. 2.1.2 Step change in concentrations at x = 0

• These changes in the carrier concentrations at the junction are shown in Fig. 2.1.2

• It can be seen that at x = 0 i.e. at the junction, the changes in the concentrations of carriers are sudden.

• These sudden changes appear as the step type changes at the junction.

• For the holes, there is a step change from Pp to Pn while for the electrons there is a step change from n_p to n_n, at x = 0.

• Due to this, the p‒n junction is called step graded p‒n junction or abrupt p‒n junction.

Key Point: As the atomic characteristics of trivalent and pentavalent impurity atoms differ from each other, the valence and conduction bands on n side are at little bit lower energy levels than on P side.

Review Questions

1. What is a p‒n junction?

2. Explain the basic structure of p‒n junction.