Phototransistors are solid state light detectors that possess internal gain. This makes them much more sensitive than photodiodes of comparably sized area. These devices can be used to provide either an analog or digital output signal. A phototransistor is operated with a floating base but with the transistor operating point defined by prebiasing the base, typically by injecting the base current through a prebias emitter in the collector region outside of the depletion layer. A phototransistor is similar to a bipolar transistor in that it has an emitter, base, and collector. While the base current of a bipolar transistor is provided through an electrical contact, the base current of a phototransistor, as its name suggests, is provided through photons. The phototransistor has a signal to noise ratio comparable to those of optimized avalanche photodiodes but operates at a significantly lower voltage and without need for temperature compensation. The phototransistor is especially well-suited for optical communications at high data rates. Since phototransistors are generally more sensitive than photodiodes to changes of illumination, they are preferred as switching devices in some applications where fairly small changes in the level of illumination are expected. Photodetectors that use silicon crystals to replace group III-V compound semiconductors have the advantages of low costs, high yields, and high integratability in post circuits in the fiber communications market.