VT83CT datasheet(7/76 Pages) PERKINELMER

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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 1

Junction Photoconductor (Photodiode)

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 3

Typical Construction of a Plastic Coated Photocell

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