Page 101 - Photonics Essentials an introduction with experiments

Photoconductivity

Photoconductivity 95

Figure 5.16. To sensitize the photoconductor, a second recombination level is intro-
duced. To be effective, its concentration of centers must be larger than the density of
centers of the type 1 level.

shown in Fig. 5.16. In the absence of illumination, the Fermi level lies
at the class 1 level. All the class 2 centers are filled, and some of the
class 1 centers are empty. Under illumination by photons having an
energy greater than the band gap, electrons are promoted to the con-
duction band. These recombine preferentially on the type 1 sites,
which have the higher capture cross section. The type 1 centers are
less numerous than the type 2 centers and are saturated by electrons.
On the type 2 sites, the shorter lifetime for holes relative to lifetime
for holes on the type 1 centers means that most of the holes will be
drawn to the type 2 sites. As a result, electrons will fill the type 1 sites
and holes will mostly be attracted to type 2 sites. This situation is dia-
grammed in Fig. 5.17.
We can calculate how the addition of type 2 centers affects the life-
time of electrons and holes and the sensitivity using the parameters
given in Table 5.2.

Figure 5.17. Energy level diagram of the sensitized photoconductor under illumination.
In effect, the electrons are preferentially attracted to type 1 sites and the holes to type 2
sites.

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