Microwaves Photonic Links: Components and Circuits

Chapter 4

Photodetectors

4.1. Photodetector definition

Regarding photodetectors, it is possible to speak of current response in units of incident optical power.

In the first English texts concerning optoelectronics, current response as a function of optical power (A/W) of a photodetector was called sensitivity. This significant term was gradually replaced by responsivity, which more specifically describes the sensitivity of a photodetector. This nomenclature will be adopted in this text.

For photodetectors, it is also possible to speak of quantum efficiency, expressed as:

[4.1] 

with:

–n_p= number of electron-hole pairs generated per second;

-φ = number of incident photons per second.

The relation between quantum efficiency and responsivity R_pd in A/W is:

[4.2] 

with:

– q= electron charge = 1.602×10^-19 C;

– h = Planck’s constant = 6.626×10^-34 J/s;

ν= light wave frequency in hertz.

Responsivity takes into account the frequency of the light wave. Consequently, by referring to Table 5.1, it should be noted that for a quantum efficiency of 100%, the responsivity of an optical photodetector at 1.55 µm is 1.25 A/W, whereas to obtain identical quantum efficiency the responsivity of a photodetector at 0.8 µm is only 0.64 A/W.

In the sections below, quantum efficiency notion is used for photodiodes, ...