Question

1.pn junction when forward bias is applied
Draw the energy band diagram.
Explain all the quasi fermi levels in the depletion region and why they are so drawn.

2.pn junction when reverse bias is applied
Draw the energy band diagram.
Explain all the quasi fermi levels in the depletion region and why they are so drawn.

Answer 1

1. To forward bias the p-n junction, the p side is made more positive, so that it is "downhill" for electron motion across the junction. An electron can move across the junction and fill a vacancy or "hole" near the junction. It can then move from vacancy to vacancy leftward toward the positive terminal, which could be described as the hole moving right. The conduction direction for electrons in the diagram is right to left, and the upward direction represents increasing electron energy.

2. To reverse-bias the p-n junction, the p side is made more negative, making it "uphill" for electrons moving across the junction. The conduction direction for electrons in the diagram is right to left, and the upward direction represents increasing electron energy.

In forward bias, the separation Efc – Efv > 0, which means that in the junction, now np>ni 2 . When will we have Efc – Efv < 0? If we reverse bias. When we reverse bias, the energy band on the n side moves down with respect to the p side, so Efc – Efv is now negative. Obviously, when you reverse bias, the depletion region further widens and carriers are pulled back, and you have much less carriers in the depletion region.