Page 97 - Photonics Essentials an introduction with experiments

Photoconductivity

Photoconductivity 91

ver–bromine bond. The bonding electron contributed by the silver
atom is promoted to the conduction band of the silver bromide crys-
tal. This electron is highly mobile and is attracted to the silver sul-
fide site, which has a lower energy level for electrons (see Fig. 5.8).
The lower energy level acts like the positive terminal of a battery.
This movement of charge constitutes photoconductivity. In less than
a microsecond, the electron will be trapped on a silver sulfide site,
which now has a net negative charge. The ionized silver atom has a
net positive charge and is also mobile, but much less so than the
electron. It will diffuse through the crystal looking for a region with
a net negative charge. It is attracted to the silver sulfide site. The
bromine atom stays put because of its larger size and its neutral
charge (Fig. 5.12).
In the context of photography, the flux of photons from an object
that you are trying to image is quite high. A silver bromide crystal
6
that is exposed will receive 10 to 10 photons. The photoconductivity
8
process does not have to be 100% efficient in order to produce an im-
age.
In the final step (Fig. 5.13), the liberated silver atom diffuses
through the lattice and finds the silver sulfide site, neutralizing its
negative charge. This motion of silver atoms is also part of the photo-
conductive process. This means that the site can capture another elec-
tron and subsequently another silver atom. This “regeneration” of the
silver sulfide center means that each such center may eventually at-
tract many silver atoms. The agglomeration of silver atoms forms a
latent image. The image cannot yet be visualized, but it is physically
present in the film.
Development of the photographic film fixes the latent image and
renders it visible. The development consists of three steps: amplifica-
tion, desensitization to light, and stabilization. The initial step of am-
plification consists of a chemical reaction that causes all the silver
atoms in an exposed silver bromide crystal to be separated from the
bromine atoms and attached to the silver sulfide sites, thus amplify-
ing the exposure to light. In the next step, the bromine atoms and the
silver bromide molecules are dissolved, leaving behind only the silver
atoms in the exposed grains. At this point, there are no more silver
bromide atoms. The film is no longer light sensitive, so it can be
viewed, and the image is now apparent. Finally, the film is stabilized
or fixed.

5.6 Sensitization

In Section 5.4, we showed that the introduction of shallow trapping
centers can change the response time of a photoconductive detector so
that it is different from the carrier lifetime. This change occurs be-

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