May 29 2014
Quantum efficiency (QE) may be defined as the fraction of incident photons that are recorded by a photo detector. The quantum efficiency of a photo-sensitive device is measured in terms of electrons per photon, or amps per watt. The value of quantum efficiency is a measure of the device’s electrical sensitivity to light.
In the case of a photo-sensitive device, the quantum efficiency is the tunnel magneto resistance of the magnetic tunnel junction; and for a charge coupled device, QE is the percentage of photons that hit the photo reactive surface and produce charge carriers.
Measurement of QE is mostly done over a range of wavelengths; the measured QE value sheds light on the device’s efficiency at various photon levels. In the case of an ideal photo detector, the QE is said to be 100%; this, however, is not possible in a practical scenario.
QE measurements are used to quantify the spectral response of a device. QE is calculated by the following formula:
QE (λ) = Number of electrons collected/Number of incident photons
Depending on the method of measurement, QE can be either external or internal.
External quantum efficiency is calculated as the ratio of the number of charge carriers collected by the photo cell to the number of photons incident from outside. External QE factors in the effect of external elements, like reflections and absorptions.
Internal QE, on the other hand, considers only the incidence of photons on the photo sensitive junction. Internal QE is derived by the ratio of the number of charge carriers collected by the photo cell to the number of incident photons from outside and absorbed by the cell.
Sources and Further Reading