The special structure of a photodiode allows light to penetrate
into the border layer of the pn junction, where photon energy
creates pairs of charge carriers, that allow electric current to
flow. The intensity of this photo-current is proportional to the
illuminating intensity, so that a photodiode is extremely suitable
for optical measurement tasks. There is no linear relationship to
the voltage generated by the light. Photodiodes are therefore
frequently used as illumination dependent sources of current in
short-circuit mode. When a voltage is applied, the photo-current is
only insignificantly increased.
If the students are to recognize the linear dependence of the
photo-current intensity on the illuminating intensity, then it is
necessary to clarify the connection between the distance of a point
light source from an irradiated surface on the one hand, and the
illuminating intensity on the other hand.
The first experiment shows the proportionality between the
illuminating intensity and the short-circuit current intensity of
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According to which principle does a photodiode work?
Examine the behaviour of a photodiode under illumination.
Measure the short-circuit current intensity and the idling voltage
of the photodiode at various illuminating intensities.