Glossary term
Quantum Efficiency
The ratio of useful charge carriers or photons produced to incident photons or input events.
Definition
metricThe ratio of useful charge carriers or photons produced to incident photons or input events.
Quantum efficiency measures how effectively an optoelectronic device converts incident photons into useful carriers, or injected carriers into emitted photons, depending on device type. It is central to photodiodes, image sensors, solar cells, LEDs, laser diodes, and optical receivers.
Quantum efficiency is a conversion ratio at photon scale. For a photodetector, external quantum efficiency is commonly:
For an emitter, quantum efficiency may compare emitted photons with injected carriers. Internal quantum efficiency excludes optical extraction losses, while external quantum efficiency includes reflection, absorption, collection, and package effects.
Detector interpretation
In photodiodes and image sensors, quantum efficiency depends strongly on wavelength because photon absorption depth and semiconductor bandgap matter. Surface reflection, anti-reflection coating, depletion-region thickness, recombination, fill factor, temperature, and bias all affect the collected signal.
Quantum efficiency is related to responsivity. For a detector, high QE generally increases signal current for a given optical power, improving signal-to-noise ratio if other noise sources are controlled. It does not by itself define bandwidth, dark current, saturation, linearity, or noise figure.
Emitter interpretation
In LEDs and laser diodes, internal quantum efficiency describes radiative recombination efficiency inside the device. External efficiency additionally includes light extraction, optical losses, package geometry, and coupling into a fiber or optical system. Thermal droop, current density, aging, and wavelength shift can reduce useful efficiency in service.
Common mistakes
A common mistake is comparing QE values without wavelength, definition, temperature, bias, and whether the value is internal or external. Another is assuming high QE means a complete receiver is sensitive; amplifier noise, dark current, bandwidth, optical coupling, and quantization can dominate. A good review states spectral range, measurement geometry, calibration source, device bias, temperature, and the rest of the optical/electrical signal chain.