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Definition: photocurrent per unit optical power incident on a photodetector
The responsivity of a photodiode or some other kind of photodetector is the ratio of generated photocurrent and incident (or sometimes absorbed) optical power, determined in the linear region of response. In the case of photodiodes, the responsivity is typically highest in a region with photon energies somewhat above the bandgap energy, and declining sharply in the region of the bandgap, where the absorption decreases. It can be calculated according to
where h ν is the photon energy, η is the quantum efficiency, and e the elementary charge. For an avalanche photodiode, there is an additional factor for the internal current multiplication.
The term responsivity should not be confused with sensitivity; the latter is the lowest detectable light level, which is typically determined by detection noise and significantly influenced by the required detection bandwidth.
A photodetector should ideally be operated in a spectral region where its responsivity is not far below the highest possible value, because this leads to the lowest possible detection noise and thus to a high signal-to-noise ratio and high sensitivity.
See also: photodetectors, photodiodes
