ABSTRACT
A phototransistor operates in a manner similar to a conventional small signal transistor except that the base or control current can come from both impinging photons on the depletion region or traditional base current supplied to the base—emitter junction. The phototransistor collector current has a positive temperature coefficient. Most phototransistors are NPN types because this material lends itself to both ease of manufacture and useful electrical parameters. Many applications of the phototransistor are used in a high ambient light condition. The NPN transistor has two junctions, the collector—base and the base—emitter. The phototransistor operated as a switch has two boundary conditions: the “off” position and the “on” position. The switching time of a photodarlington transistor is significantly slower than the transistor counterpart. Different light energy sources with their different spectral content cause a phototransistor to have different outputs. The lens characteristics of the phototransistors are similar to their light-emitting diode counterparts.
