Page 93 - Photonics Essentials an introduction with experiments

Photoconductivity

Photoconductivity 87

Figure 5.9. Engineering of photoconductivity by introduction of shallow trapping cen-
ters. The number of centers is designed to be much larger than the number of free car-
riers in the conduction band.

ulate photo-excited carriers during photoconductivity. We will consid-
er this important engineering tool in more detail in Section 5.6 when
we discuss the principle of sensitization.

5.5 Photographic Film and Photoconductivity
Photoconductivity is the principle that underlies the operation of pho-
tographic film. This is by far the most extensive use of the photocon-
ductive effect. Unlike the examples we have discussed so far, there
are no contacts and no external bias voltage needed to exploit photo-
conductivity in film.
Photographic film consists of a gelatin coating on some kind of plas-
tic or polymer support. For larger view cameras, the gelatin is de-
posited on glass plates. Inside the gelatin are dispersed grains of a
photosensitive material (Fig. 5.10).
Film is a digital medium. The photosensitive grain can respond in
only two ways; either it absorbs a photon, or it does not. If it absorbs a
photon, the exposure and development process will render the entire
grain black. Otherwise, the grain is dissolved and washed out of the
gelatin leaving a transparent region behind. For a fixed level of illu-
mination, the chance that a grain absorbs a photon depends quite ob-
viously on its size. Thus, to make the film more sensitive to light, you
need only make the grain size larger.
The resolution of the image also depends on the grain size. As the
grain size increases, the resolution of the image decreases. Thus there
is a direct trade-off between film sensitivity or speed and the resolu-
tion or graininess of the image. An important part of the film manu-

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