Question

Assertion: The dominant mechanism for motion of charge carriers in forward and reverse biased silicon P-N junction are drift in both forward and reverse bias.
Reason: In reverse biasing, no current flow through the junction.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B. Both Assertion and Reason are correct and Reason is not the correct explanation for Assertion
C. Assertion is correct but Reason is incorrect
D. Both Assertion and Reason are incorrect

Answer 1

Semiconductors consist of majority and minority charge carriers. If it’s a P-type semiconductor, then holes are dominant charge carriers and if it’s an N-type semiconductor, then electrons are majority charge carriers. The formation of P-N junction takes place due to two processes: diffusion and drift.

Complete step by step solution:
Semiconductors can be classified in two types: intrinsic and extrinsic .Intrinsic semiconductors are those which are free from any impurities. Extrinsic semiconductors are made in labs by using a very small amount of dopant in order to increase its conductivity. If a trivalent impurity is added to an intrinsic semiconductor, it becomes a P-type semiconductor and if pentavalent impurities are added then it becomes an N-type semiconductor.
A P-N junction is formed when a small amount of pentavalent impurity is added to a P-type silicon chip. Due to the concentration gradient of charge carriers, the electrons in the N-type diffuse into P-type and holes in the P-type diffuse into N-type and hence diffusion current is produced across the junction. As the electrons and holes diffuse from each side, they leave behind ionized donor and ionized acceptor respectively on N-side and P-side resulting in the formation of depletion region.
A circuit is said to be in forward biasing if the positive terminal of the voltage is connected with the P-side of the semiconductor and the negative terminal is connected to the N-side of the semiconductor. When the P-N junction semiconductor is in forward biasing, the depletion region gets reduced and current flows across it due to the diffusion of majority charge carriers.
When the positive terminal of voltage is connected with the N-side of the semiconductor and negative terminal of the voltage connected to the P-side, then the circuit is in reverse bias. As the P-N junction is in reverse bias connection, the depletion region gets broader and hence its resistance increases. But due to drift of the minority charge carriers, there will be a small amount of current across the junction. This current is known as reverse saturation current.
The dominant mechanism of motion of charge carriers in forward bias is diffusion and in reverse bias is drift.
Therefore, option D is correct.

Note:
P-N junctions are widely used to make solar cells, LED, photo-diode etc. A Zener diode made up of P-N junction is used as a voltage regulator. In many circuits, diodes are used as rectifiers. In reverse biased, a sudden increase in current due to rupture of bonds is observed at a certain reverse voltage resulting in a sudden breakdown.