In photo detector quantum efficiency is defined as the number of electron hall pair generated per incident photon of energy hv and is given as ȵ
ȵ = Number of electron hall pair generated / number of incident photons
= IP / q / Pin / hv
IP = Average photo current
Pin = Average optical power incident on photo detector
Absorption coefficient of material determines the quantum efficiency. Quantum efficiency ȵ < 1 as all photons incident will not generate e-h pair. It is normally expressed in percentage.
For example,
On an GaAs photo detector
Number of photon emitted = 6 × 106
Average e-h pair generated = 5.4 × 106
ȵ = 5.4 × 106 / 6 × 106
= 0.9
= 90%