Photodetector Responsivity: Definition and Parameters

This page covers the definition of responsivity of a photodetector. Responsivity is used to measure the input output gain of the detector in a fiber optic system. In the case of a photodetector, it’s the measure of electrical output per optical input.

Most photodetectors exhibit a linear relationship with the input incident power. The responsivity of a photodetector is typically expressed in units of amperes per watt (A/W) or volts per watt (V/W) of radiant power.

The responsivity of a photodetector depends on the following parameters:

• Wavelength of incident radiation
• Sensor properties, e.g., bandgap of the material

Mathematical formula

Responsivity of photodiodes and similar devices can be expressed as follows.

Where, Iph = Photo current in amperes, Pin = incident optical power in watts

Alternaively, it can also be expressed in terms of quantum efficiency
𝜂 and Photon energy ‘hv’ as follows.

Key points for responsivity

• It is defined in linear operating region of photodetector.
• It can vary with temperature and hence affects the measurement accuracy.
• In photodiode, application of reverse bias increases depletion region and consecutively increases responsivity.
• Different photo-detectors such as photodiodes, photoconductors will have varying responsivity characteristics