Question

The sequence of physical events when forward bias is applied to PN junction: A. Barrier potential reduces B. Majority carriers injected across junction C. Recombination in neutral regions increases D. Diffusion current established across quasi-neutral regions E. Net forward current increases Choose the correct answer from the options given below :

• A. A, E, B, D, C
• B. A, B, D, C, E
• C. A, B, C, D, E
• D. A, C, B, E, D

Hint:

For a forward-biased PN junction: \[ \text{Barrier Reduction} \rightarrow \text{Carrier Injection} \rightarrow \text{Diffusion} \rightarrow \text{Recombination} \rightarrow \text{Forward Current} \] Always remember that diffusion current is caused by carrier concentration gradients created after carrier injection.

Answer 1

The Correct Option is B

Concept: When a PN junction diode is forward biased:
• The applied external voltage opposes the built-in barrier potential.
• The depletion width decreases.
• Majority carriers gain sufficient energy to cross the junction.
• Carrier diffusion takes place.
• Recombination occurs in neutral regions.
• Finally, substantial forward current flows through the diode. The physical events occur in a logical chain and understanding their exact order is important for semiconductor device physics.

Step 1: Analyze the first event occurring under forward bias. When forward bias is applied:
• Positive terminal is connected to p-side.
• Negative terminal is connected to n-side. The applied voltage opposes the internal electric field. As a result: \[ \text{Barrier potential reduces} \] Therefore: \[ A \] is the first event.

Step 2: Determine the next physical process. Because the barrier potential decreases:
• Majority carriers can now cross the junction more easily.
• Holes move from p-region to n-region.
• Electrons move from n-region to p-region. Thus: \[ \text{Majority carriers injected across junction} \] Hence: \[ B \] comes next.

Step 3: Establishment of diffusion current. Once carriers are injected:
• Concentration gradients are created.
• Carrier diffusion begins across quasi-neutral regions. This produces diffusion current. Therefore: \[ D \] comes after carrier injection.

Step 4: Recombination process in neutral regions. Injected carriers entering opposite regions are minority carriers there. These minority carriers recombine with majority carriers in neutral regions. Thus: \[ C \] occurs after diffusion.

Step 5: Final increase in net forward current. Due to all previous processes:
• Carrier transport becomes significant.
• Current through the junction rises rapidly. Hence: \[ E \] is the final consequence.

Step 6: Determine the complete sequence. Therefore, the correct order is: \[ A \rightarrow B \rightarrow D \rightarrow C \rightarrow E \] Hence, the correct option is: \[ \boxed{(B)\ A,B,D,C,E} \]