Question 1

What is the depletion region in a p-n junction?

Accepted Answer

It is the thin strip on both sides of the junction that is emptied of mobile charge carriers. Electrons and holes diffuse across the junction and recombine there, leaving behind only the immobile ionised dopant atoms — negative acceptor ions on the p side and positive donor ions on the n side. These fixed charges create the built-in electric field and the barrier potential.

Question 2

Is an n-type semiconductor negatively charged?

Accepted Answer

No. Both n-type and p-type semiconductors are electrically neutral. Doping only changes which carrier is in the majority — electrons in n-type, holes in p-type. Every donor electron added comes with an equally charged fixed positive donor ion, so the net charge of the crystal remains zero.

Question 3

What is barrier potential in a p-n junction?

Accepted Answer

Barrier potential is the potential difference that develops across the depletion region due to the fixed ionised dopant charges. It opposes further net diffusion of majority carriers. Its approximate value is 0.7 V for silicon and 0.3 V for germanium at room temperature, and it equals the knee voltage of the diode.

Question 4

Does diffusion stop at equilibrium in a p-n junction?

Accepted Answer

No. At equilibrium both diffusion and drift currents keep flowing continuously — they are simply equal in magnitude and opposite in direction, so the net current is zero. The built-in field does not stop diffusion; it balances it.

Question 5

What happens to a p-n junction in forward and reverse bias?

Accepted Answer

In forward bias (p to +, n to −) the applied field opposes the built-in field, so the barrier and depletion width decrease and a large current flows due to majority carriers once the knee voltage is crossed. In reverse bias the applied field reinforces the built-in field, the barrier and depletion width grow, and only a tiny leakage current carried by minority carriers flows — until breakdown, when avalanche multiplication makes the current surge and the resistance collapse.

Question 6

What causes the reverse saturation current in a diode?

Accepted Answer

It is caused by minority carriers — electrons on the p side and holes on the n side — which are swept across the junction by the built-in field. Because these minorities are generated thermally, the reverse saturation current depends almost entirely on temperature and is nearly independent of the applied reverse voltage.

The p–n Junction: Where the Depletion Layer Comes From

Practice

Learned it? Now prove it under the clock.