4.1. BASIC CONCEPTS 135
Figure 4.2: Wavelength dependence of the absorption coefficient for several semiconductor ma-
terials. (After Ref. [2];©c1979 Academic Press; reprinted with permission.)
As expected,ηbecomes zero whenα=0. On the other hand,ηapproaches 1 if
αW1.
Figure 4.2 shows the wavelength dependence ofαfor several semiconductor ma-
terials commonly used to make photodetectors for lightwave systems. The wavelength
λcat whichαbecomes zero is called the cutoff wavelength, as that material can be
used for a photodetector only forλ<λc. As seen in Fig. 4.2, indirect-bandgap semi-
conductors such as Si and Ge can be used to make photodetectors even though the
absorption edge is not as sharp as for direct-bandgap materials. Large values ofα
(∼ 104 cm−^1 )can be realized for most semiconductors, andηcan approach 100% for
W∼ 10 μm. This feature illustrates the efficiency of semiconductors for the purpose
of photodetection.
4.1.2 Rise Time and Bandwidth
Thebandwidthof a photodetector is determined by the speed with which it responds
to variations in the incident optical power. It is useful to introduce the concept ofrise
time Tr, defined as the time over which the current builds up from 10 to 90% of its final
value when the incident optical power is changed abruptly. Clearly,Trwill depend on