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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 (neglecting noise influences), 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 example, a photodiode with 90% quantum efficiency at a wavelength of 800 nm, the responsivity would be ≈ 0.58 A/W. For avalanche photodiodes and photomultipliers, there is an additional factor for the internal current multiplication, so that values far above 1 A/W are possible. Note that the current multiplication is usually not subsumed in the quantum efficiency.
The responsivity is usually defined for the steady state. The photodiode response typically falls off for signal frequencies above some detection bandwidth.
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.
If some detector has a voltage rather than a current output, one can define its responsivity as the ratio of output voltage and optical power. This leads to units of V/W.
See also: photodetectors, photodiodes, noise-equivalent power
