Answer to Question #131375 in Electric Circuits for Navya Sharma

Question

What is depletion region in case of a p-n junction? Explain the origin of junction barrier 

potential. Discuss its dependence on the applied voltage, carrier concentration and type 

of semiconductor.

Answer 1

• A depletion region is a natural feature of doping a semiconductor n-type and right next to it p-type. At the p-n junction (the border of the two types), the mobile electrons in the n-type (this is due to the excess electron provided by the n-dopant) diffuse across the junction, into the p-type area. This then leaves the n-type area with a net positive charge, because the electron left.

• Holes (the majority charge-carriers in the p-type, created through doping with an element such as Indium (3 valence electrons)) can be said to 'diffuse' toward the n-type. (This is because of the electrons from the n-type causing the electrons in the p-type to repel away, and thus travel away from the n-type, filling existing holes and thus creating holes on their way, however this is not important)

• In case of forward bias, p-type of the diode is connected to the positive terminal of the battery and n-type is connected to the negative terminal. When load is applied, negative terminal of the battery repels the electrons in the n side of the diode and they start moving towards the p-side.The more you apply voltage, more the width of depletion region is reduced and barrier potential reduces. At the same time, positive terminal also attracts the electrons moving from n side to p side. Once you cross the barrier potential(0.7 V in case of Si), current starts to flow in the opposite direction of the electrons. Here, current flows because of majority charge carriers( electrons in n-side and holes in p-side).