Biophotonics_Concepts_to_Applications

kcðlmÞ¼

hc

Eg

¼

1 : 240

EgðeVÞ

ð 5 : 4 Þ

The cutoff wavelength is about 1.06μm for Si and 1.6μm for Ge. For longer
wavelengths the photon energy is not sufficient to excite an electron from the
valence to the conduction band in these two materials. Thus other semiconductor
alloys have been developed for long wavelength operation.

Example 5.3Consider a GaAs photodiode that has a bandgap energy of

1.43 eV at 300 K. What is the cutoff wavelength of this device?

Solution: From Eq. (5.2) the long-wavelength cutoff is

kc¼

hc

Eg

¼

ðÞ 6 : 625  10 ^34 JsðÞ 3  108 m=s

ð 1 :43 eVÞðÞ 1 : 6  10 ^19 J=eV

¼869 nm

This GaAs photodiode will not respond to photons that have a wavelength

greater than 869 nm.

At the lower-wavelength end, the photon response cuts off as a result of the very
large values ofαsat the shorter wavelengths. In this case, the photons are absorbed
very close to the photodetector surface, where the recombination time of the

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Wavelength (μm)

Light absorption coefficient (cm

-1)

102

101

103

104

Fig. 5.3 Optical absorption 105
coefficient as a function of
wavelength of several
different photodetector
materials

5.1 ThepinPhotodetector 123