GATE 2018 IN Q.25: solution and answer

Official answer: 75Source: Official answer key, GATE 2018 IN (IIT Guwahati)

1. Quantum efficiency is the ratio of the number of electron-hole pairs generated to the number of photons incident: $\eta =\frac{\text{electron-hole pairs generated}}{\text{incident photons}}$.

2. Each absorbed photon creates at most one electron-hole pair, so the $4.5\times 10^6$ pairs created are the most that can be credited to the $6\times 10^6$ incident photons. The word 'maximum' means every created pair is counted, with none lost to recombination.

3. $$\eta =\frac{4.5\times 10^6}{6\times 10^6}=0.75$$

4. In percent, $\eta =0.75\times 100=75\%$.