Page 100 - Photonics Essentials an introduction with experiments

Photoconductivity

94 Photonic Devices

Area = Capture
Cross section

Figure 5.15. The volume of a recombination center is given by the capture cross section,
s, times the recombination length, v .

ing the density of recombination centers of the same type will always
reduce the lifetime and reduce the sensitivity. However, if we add re-
combination centers of a different type, it becomes possible to length-
en the lifetime of one carrier while decreasing the lifetime of the oth-
er. Under the right conditions, these new centers deactivate the first
type of centers, causing all the recombination to pass through the sec-
ond type of centers. The result can be a dramatic increase in sensitiv-
ity of the photoconductive material. This is the principle of sensitiza-
tion.
Cadmium sulfide is a semiconductor that is widely used as a photo-
conductive cell in light meters. In its undoped state, the residual im-
purity level is about 10 15 cm –3 and the recombination level lies in the
middle of the band gap as diagrammed in Fig. 5.15. We will call these
recombination centers type 1. The electron and hole lifetimes are
about 10 –7 seconds. To sensitize this material, we will add cadmium
–3
vacancies to the level of 10 16 cm . This new recombination level is
slightly lower in energy than the type 1 level and we will call it type 2.
The capture cross section for electrons by the type 2 center is much
smaller than the capture cross section for electrons by the type 1 cen-
ter. As a result, the electron recombination time is longer, n2 = 10 –2
seconds, and the hole lifetime is shorter, p2 < 10 –8 seconds. Following
the addition of these vacancies, the new energy level diagram is

Table 5.1
Parameter Symbol Value
Density of recombination centers N r 10 16 cm –3
Drift velocity v 10 cm-sec –1
7
Capture cross section s n , s p 10 –20 to 10 –15 cm 2
–3
Photon absorption rate f 10 11 cm -sec –1
Lifetime n , p 10 –6 sec
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