Final Prep Answers

Chem 155 Final review topics and questions:
5. By what basic mechanism does the photodiode produce an electric current when
light strikes it
The photodiode is reverse-biased. This means that without light, no current flows. The
PN junction of the photodiode is exposed to light. When light strikes the PN junction it
may create electron-hole pairs. The bias will then sweep apart the electron-hole pairs
and convert the light into an electrical current.
6. What happens at a p-n junction when a reverse-bias is applied Draw a diagram
to support your answer. Why are photodiodes operated in reverse bias
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The photon (red arrow) strikes the photodiode in the depletion region and an
electron-hole pair is generated. The electron and hole (red circles) are swept
apart by an electric field. In reverse bias, the diode produces zero current in the
dark. Low or zero dark current is a good quality in a photo-detector.
7. What is the maximum gain of a photodiode transducer Is this better than a
phototube
Photodiode max gain = 1 = equal to the phototube max gain.