What are Phototransistors?

Both phototransistors are examples of photodiode-amplifier combinations integrated within a single silicon chip. These combinations are put together in order to overcome the major fault of photodiodes: unity gain.

Many applications demand a greater output signal from the photodetector than can be generated by a photodiode alone. While the signal from a photodiode can always be amplified through use of an external op-amp or other circuitry, this approach is often not as practical or as cost effective as the use of phototransistors.

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.

To demonstrate the relative sensitivity of these different types of detectors, compare the output currents that could be expected from a .025" x .025" detector chip exposed to .05 mW/cm₂ of illumination.

The current-voltage characteristics of the phototransistor are similar to NPN   signal transistors, with the major exception that incident light replaces current base drive.

The structure of a phototransistor is very similar to that of a photodiode. In fact, while not optimized for this mode of operation, the collector-base junction of a phototransistor can be used as a photodiode with fairly good results. The major structural difference is that the phototransistor has two junctions compared with one for the photodiode.