Silicon Photonics: Fundamentals and Devices

8.3 Important Characteristics of Photodetectors

Depending on its application, specific performance measures of the photodetector are emphasized. For example, for optical communication receivers, some of the key performance characteristics are quantum efficiency, speed, dynamic range, and minimum detectable power. In this section, we discuss some of the important characteristics of (silicon) photodetectors.

8.3.1 Quantum Efficiency

Quantum efficiency (QE) (sometimes called internal QE) is the ratio of the number of generated electron–hole pairs per incident photon that are collected at the contacts. QE is an important measure of high-sensitivity photodetectors since it should produce the maximum electrical signal per unit optical power.

External quantum efficiency (EQE or η), the more practical measure, compares the collected photocurrent to the number of incident photons in a real photodetector. It is less than the internal quantum efficiency because of effects such as surface reflections decreasing the amount of light on the absorption region and surface and nonradiative recombinations of carriers (holes and electrons) before they are collected. The external quantum efficiency is a function of wavelength (λ) of the incident light because both the absorption coefficient and penetration depth of the light vary with λ.

In a photodetector, its quantum efficiency η is given by

(8.4)

where I_PC is primary photocurrent (without internal gain), P_I is the incident optical power, is the ...