How does a photodiode and phototransistor work?
When light strikes a semiconductor it supplies energy to knock electrons free of their atomic orbits, creating holes and electrons. This light sensitive behavior occurs in the depletion zone of a reverse biased diode. The phototransistor behaves the same way when light impinges upon its reverse biased collector-base junction depletion layer. Of course the transistor amplifies this current so that a much larger current flows in the collector-emitter circuit than in diode. Lenses are used to enhance the light sensitivity. Also, keep in mind that all semiconductors are light sensitive and one function of their package is to shut out light!
