Question

Which of the following must be zero in an unbiased p-n junction:
A) Current due to majority carriers (Both electrons and holes)
B) Current due to minority carriers (Both electrons and holes)
C) Current due to both minority and majority carriers
D) Current due to majority and minority charge carriers (only holes)

Answer 1

Hint: A p-n junction is said to be in the unbiased state when no external voltage is supplied to the junction. When in an unbiased position there is always a barrier present in the p-n junction.
Complete step by step answer:

Semiconductors are materials whose conductivity lies between the conductivity of metals and the conductivity of insulators. There are mainly two types of semiconductors,
Intrinsic Semiconductors- These are pure semiconductors with no amount of doping present in them.
Extrinsic Semiconductors- These are semiconductors which are doped with trivalent or pentavalent atoms to improve the electrical conductivity of a semiconductor.
There are two kinds of extrinsic semiconductors,
P-type semiconductor- When a pure semiconductor like Silicon or Germanium is doped with trivalent impurities like Boron or Aluminium, we get a p-type semiconductor.
N-type semiconductor- When a pure semiconductor like silicon or germanium is doped with pentavalent impurities like Arsenic or Phosphorus, we get an n-type semiconductor.
When a p-type semiconductor and an n-type semiconductor forms an interface or boundary a p-n junction is formed.
When a p-n junction is formed there will be a barrier potential formed due to the minority charge carriers in the p side and the n side.
These minority carriers were once part of the p or n side, when the junction was formed, these charge carriers diffused from the n side or p side to the opposite side.
The diffusion went on till the majority charge carriers which were diffused, set up an electric field across the junction due to charge difference across the junction, that would eventually counteract the diffusion process and stop it.
This electric field is known as barrier potential. This potential prevents movement of majority carriers.
So holes (majority carriers in p side) cannot go to n side, and electrons (majority carriers in n-side) cannot go to p-side. So the current due to the majority carriers is zero in this position.
But, if minority charge carriers are produced due to thermal agitation in either of the sides, the minority charge carriers will be attracted by the barrier potential to cross the junction. So electrons generated in the p side will be attracted to the positive charges in the n side, and holes generated in the n side will be attracted to the negative charges in the p side. So a minority current is possible in the unbiased condition.
So the answer to the question is option (A)- Current due to majority carriers (Both electron and hole)
Note:
The p-n junction is the basic building block for many electronic devices like diodes, transistors, FET’s etc..
The barrier potential for a silicon diode is 0.3V while the barrier potential for a germanium diode is 0.7V.
When an external potential is applied to the p-n junction, the barrier reduces until it becomes zero when the external voltage is greater than the barrier potential.