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Optical Receivers 193

• It is not suitable for high-quality avalanche photodetectors since ≈ .
i
i
• It is not suited for long-distance applications due to its short cutoff wavelength.
Indium gallium arsenide (InGaAs)

• It can have a tunable band-gap energy depending on the ratio of Ga to In.
• It is a very good material for long-haul communications at 1.55 μm.
• It can be lattice matched to the InP substrate.

Indium gallium arsenide phosphide (InGaAsP)
• Suitable for both 1.3- and 1.55-μm applications.
• It can be lattice matched to the InP substrate.
In a semiconductor photodetector, there are two or three key processes depending on the type of photode-
tector.
(i) Absorption and generation. Here, the photons of appropriate energy (that is, the energy of the incom-
ing photon should be at least equal to the active semiconductor material’s band-gap energy) generate free
electron–hole pairs (ehps) through the photoconductive (or internal photoemission) effect when they are
absorbed in the photoresponsive (or active) region of the photodetector. Note that in the photoconductive
effect, the photogenerated carriers remain in the semiconductor material and they result in an increase in
its conductivity. This is in contrast to photoelectric emission, in which the photogenerated electrons escape
from the material and are then free to move outside the material under an applied electric field. Photoelectric
emission is used in photomultiplier tubes (PMTs).
(ii) Transport. The generated ehps drift under the influence of an applied electric field E. This results in a
current that flows in the circuit.
(iii) Amplification. In some photodetectors, when the electric field is sufficiently large, the photogenerated
carriers moving in the applied electric-field can gain sufficient energy to impact ionize. Upon impact
ionization, additional carriers are generated, creating more ehps. In this way, one photogenerated ehp can
result in many more ehps, leading to a photodetector with gain. In more detail, the gain of the photodetector
is defined as the ratio of the number of collected ehps to the number of primary photogenerated pairs. Gain
expresses the sensitivity of the photodetector at the operating wavelength. One popular photodetector with
gain is the avalanche photodiode.

5.2.1 Quantum Efficiency
In a semiconductor photodetector, when a photon of energy E ph ≥ E is absorbed, an ehp is formed. Then,
g
a photocurrent is produced when the photon-generated ephs are separated in an applied electric field, with
electrons moving to the n-region and holes to the p-region (Fig. 5.5). However, the photons of appropriate
wavelength do not always generate ehps, nor are all ehps collected at the respective terminals. Therefore,
quantum efficiency QE (or ) is defined as the probability that a photon incident on the photodetector generates
an ehp (photocarrier) that contributes to the photodetector current and is given by
number of photocarriers that contribute to the photocurrent
= . (5.5)
number of incident photons
Note that 0 < ≤ 1, that is, the maximum value of in a photodetector without gain is 1 or 100%, which
means that each incident photon generates an ehp. The QE depends on the photon wavelength, type of semi-
conductor, and structure of the photodetector.