What is a Photoconductive Cell?
Semiconductor light detectors can be divided into two major
categories: junction and bulk effect devices. Junction devices, when
operated in the photoconductive mode, utilize the reverse
characteristic of a PN junction. Under reverse bias, the PN junction
acts as a light controlled current source. Output is proportional to
incident illumination and is relatively independent of implied voltage as
shown in Figure 1. Silicon photodiodes are examples of this type
detector.
Figure 2
Bulk Effect Photoconductor (Photocell)
In contrast, bulk effect photoconductors have no junction. As shown in
Figure 2, the bulk resistivity decreases with increasing illumination,
allowing more photocurrent to flow. This resistive characteristic gives
bulk effect photoconductors a unique quality: signal current from the
detector can be varied over a wide range by adjusting the applied
voltage. To clearly make this distinction, PerkinElmer Optoelectronics
refers to it’s bulk effect photoconductors as photoconductive cells or
simply photocells.
Photocells are thin film devices made by depositing a layer of a
photoconductive material on a ceramic substrate. Metal contacts are
evaporated over the surface of the photoconductor and external
electrical connection is made to these contacts. These thin films of
photoconductive material have a high sheet resistance. Therefore, the
space between the two contacts is made narrow and interdigitated for
low cell resistance at moderate light levels. This construction is shown
in Figure 3.
Figure 1
Junction Photoconductor (Photodiode)
Figure 3
Typical Construction of a Plastic Coated Photocell
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