JEE Main 2022 · 25 June · morning · Single correct · +4 / −1
The photodiode is used to detect the optical signals. These diodes are preferably operated in reverse biased mode because :
- (a) fractional change in majority carriers produce higher forward bias current
- (b) fractional change in majority carriers produce higher reverse bias current
- (c) fractional change in minority carriers produce higher forward bias current
- (d) fractional change in minority carriers produce higher reverse bias current
Answer and solution
Answer (d) fractional change in minority carriers produce higher reverse bias current
Solution
Physics sir · 4 steps
Light barely changes majority carriers but multiplies minority ones, and those carry the reverse current.
- What light does. Each photon with energy greater than the band gap creates one electron-hole pair near the junction. It adds the same number of extra electrons and extra holes.
- Compare fractions. Take the n-side with about 10¹⁶ electrons and 10⁴ holes per cm³. Adding 10¹² pairs changes the electrons by only 10⁻⁴, that is 0.01%. The holes rise about 10⁸ times.
- Why reverse bias. Under reverse bias the current is carried by minority carriers. So that large fractional change appears directly in the current, and a small change in light intensity gives a clear, measurable signal. Under forward bias the large majority-carrier current would swamp it.
- So. The fractional change in minority carriers produces a large change in the reverse bias current, which is option (d).
The idea to keep. To detect a small effect, measure the quantity it changes by the largest fraction. Light hardly changes majority carriers but multiplies minority carriers, and minority carriers carry reverse current.
Answer from the NTA final answer key. Solution by PrepWiser.