The responsivity of a photodiode can be defined as the ratio of the photocopic current produced to the power of light that occurs at a certain wavelength. Responsivity (or radiation sensitivity) of a photodiode or other photodetector type is the strength ratio of the photo produced at this time and the occurrence (or sometimes absorbed) of optical power (ignoring the effect of noise), determined in the linear region of the response.
The term receptivity should not be confused with sensitivity. the latter is the lowest detectable light level, which is usually determined by the detection noise and significantly influenced by the required detection bandwidth.
Photodetectors should ideally be used in spectral areas where reactivity is not much lower than the highest possible value, because this results in the lowest possible noise detection and hence a high signal-to-noise ratio and sensitivity.
A photon that is absorbed by the semiconductor material will produce a pair of electron holes, which in turn will begin to move into the material under the influence of the electric field and thus produce a current. This limited flow length is known as the spread of transit time and can be evaluated using the Ramo theorem. It can also be shown by this theorem that the total charge produced in an external circuit is and not as expected by the presence of the two carriers. Indeed, the integration of currents due to electrons and holes from time to time must be equal to e.
The resistance and capacity of the photodiode and the external circuit give rise to another response time called RC constant time. This R & C combination integrates photo responses from time to time and thus extends the impulse response of the photodiode. When used in optical communication systems, response time determines the bandwidth available for signal modulation and hence data transmission.
If a detector has a voltage rather than a current output, its response can be defined as the ratio of the output voltage to the optical power. This leads to V / W units (volts per watt). If a photodiode is combined with a particular electron of the detector that generates an output voltage, the output voltage is the photocurrent time or the transimpedance of the electronics. In the simplest case, a shunt resistor is used and the transimpedance is then its resistance.
