Page 211 - Fiber Optic Communications Fund

192 Fiber Optic Communications

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hf E bandgap

+

Figure 5.4 Absorption of photons with energies equal to or greater than the band gap.

Table 5.1 Some common semiconductor materials used in photodetectors
with their E and values.
g co
Semiconductor Bandgap (eV) at 300 K Cutoff wavelengths (μm)
co
Silicon 1.12 (indirect) 1.1
Germanium 0.66 (indirect) 1.85
GaAs 1.42 (direct) 0.87
GaSb 0.73 (direct) 1.7
AlAs 2.16 (direct) 0.57
InAs 0.36 (direct) 3.5
InP 1.35 (direct) 0.92
In 0.14 Ga 0.86 As 1.15 (direct) 1.08
In 0.47 Ga 0.53 As 0.75 (direct) 1.65

Silicon (Si)
• Indirect band-gap material with a small absorption coefficient.
• It has a high ratio of ionization coefficient of holes ( ) to electrons ( ). It is a good choice for avalanche
i i
photodetectors, especially for local area networks (LANs) or short-wavelength applications.
• It is not suitable for the long-haul communications that typically operate at 1.3 or 1.55 μm.

Germanium (Ge)

• Indirect band-gap material with a small absorption coefficient.
• High ∕ ratio. It can be used for avalanche photodetectors for both local area networks and long-distance
i
i
communications due to its long cutoff wavelength.
Gallium arsenide (GaAs)

• Direct band-gap material.
• Easy lattice matched to the InP substrate.