Page 120 - Optical Communications Essentials

Photodiodes and Receivers

110 Chapter Seven

TABLE 7.1. Operating Wavelength Ranges for Several Different Photodetector
Materials
Material Energy gap, eV λ cutoff , nm Wavelength range, nm
Silicon 1.17 1060 400–1060
Germanium 0.775 1600 600–1600
GaAs 1.424 870 650–870
InGaAs 0.73 1700 900–1700
InGaAsP 0.75–1.35 1650–920 800–1650

quaternary materials indium gallium arsenide (InGaAs) and InGaAsP were
developed. Of these, InGaAs is used most commonly for both long-wavelength pin
and avalanche photodiodes. Table 7.1 summarizes the wavelength ranges over
which these materials are sensitive and the corresponding cutoff wavelength.

7.1.3. Quantum efficiency
An important characteristic of a photodetector is its quantum efficiency, which
is designated by the symbol η (eta). The quantum efficiency is the number of
electron-hole pairs that are generated per incident photon of energy hν and is
given by

p
=
number of eletron-hole pairs generated = I / q (7.2)
number of incidentt photons Ph/ν
0
Here q is the electron charge, I p is the average photocurrent generated by a
steady photon stream of average optical power P 0 incident on the photodetector,
and ν c/λ is the light frequency. In a practical photodiode, 100 photons will
create between 30 and 95 electron-hole pairs. This gives a quantum efficiency
ranging from 30 to 95 percent.

7.1.4. Responsivity
The performance of a photodiode may be characterized by its responsivity R.
This is related to the quantum efficiency by

I η q
R = p = (7.3)
P 0 hν

This parameter is quite useful since it specifies the photocurrent generated per
unit optical power. Figure 7.3 shows typical pin photodiode responsivities as a
function of wavelength. Representative values are 0.65A/W for silicon at 900nm
and 0.45A/W for germanium at 1300nm. For InGaAs, typical values are 0.9A/W
at 1300nm and 1.0A/W at 1550nm.

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