Question

Compare the features of (i) forward biased and (ii) reverse biased p-n junction. Mention the nature of biasing in solar cell.

Hint:

Remember: LED works in forward bias, while solar cell works in reverse bias.

Answer 1

Step 1: Forward bias.
- External voltage is applied such that $p$-side is connected to positive and $n$-side to negative terminal of the battery.
- Barrier potential decreases, width of depletion layer reduces.
- Large current flows due to movement of majority carriers.
- Resistance is very low.
Step 2: Reverse bias.
- External voltage is applied such that $p$-side is connected to negative and $n$-side to positive.
- Barrier potential increases, width of depletion layer widens.
- Very small current flows due to minority carriers (called leakage current).
- Resistance is very high.
Step 3: Biasing in solar cell.
A solar cell works in reverse bias mode. Light falling on the junction creates electron-hole pairs which are separated by the built-in field, producing emf.
Step 4: Conclusion.
Thus, forward bias allows large current, reverse bias allows negligible current, and solar cells operate under reverse bias.