Question

In an unbiased PN junction,
A. The junction current at equilibrium is zero as charges do not cross the junction.
B. The junction current reduces with rise in temperature.
C. The junction current at equilibrium is zero as equal but opposite carriers are crossing the junction
D. The junction current is due to minority carries only.

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. When n-type and p-type semiconductors are combined by some unique process, a p-n junction is formed. p-n junctions are basic building blocks of ICs, diodes, and transistors.

Complete 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. Therefore, when the PN junction is unbiased the junction current at equilibrium is zero as equal but opposite carriers are crossing the junction

Hence, option (C) is correct.

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 0.7 V. 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.