Phototransistors are solid state light detectors that possess internal gain and are consequently much more sensitive than photodiodes of comparably sized area. These photodiode-amplifier combinations are put together to overcome the major limitation of photodiodes: unity gain. Many applications demand a greater output than can be generated by a photodiode alone, and even though the signal of a photodiode can be amplified through external circuitry (e.g. an operational amplifier), this is not always cost effective. In such cases, phototransistors provide a lower cost alternative.
Phototransistors can be used to provide either an analog or digital output signal. The phototransistor can be viewed as a photodiode whose output photocurrent is fed into the base of a conventional small signal transistor. While not required for operation of the device as a photodetector, a base connection is often provided allowing the designer the option of using base current to bias the transistor. The typical gain of a phototransistor can range from 100 to over 1500.