The fact that traditional transistors (these days called “Bipolar Junction Transistors” or BJTs) are photosensitive has been known ever since they were invented. This is one of the reasons that they are usually sealed in light tight cans - because unwanted light is a source of noise. Phototransistors are very similar to ordinary BJTs except they are designed for use as detectors. Transistors are amplifiers and in a phototransistor the amplifier gain is controlled by the amount of light striking the device.

Light arriving in the device is absorbed and creates charge carrier pairs in many different places. The action is different depending on where the charge carrier pairs are created. However, in an intentional phototransistor, absorbtion is intended to take place in the depletion zone between the emitter and the base. Creation of charge carriers here causes a current to flow in the E-B circuit which is then amplified by the transistor action of the device. These are much lower in noise and have a higher output than APDs, but are significantly less responsive than either APDs or p-i-n diodes. The major problem with phototransistors is materials. We would like to use silicon or gallium arsenide but these have too great a bandgap energy and are limited to detecting wavelengths shorter than about 1 micron. Germanium is usable in the 1300 nm band but while it is easily possible to build transistors with smaller bandgap materials (such as InP) there is no established technology to do so. This takes away much of the potential cost advantage in the 1550 region. The major use of phototransistors is in non-communications applications using visible (or near visible) light. Alarm systems (light beam detection) and remote controls for TV sets and automobiles are among the most common uses. Phototransistors are occasionally built as part of an integrated circuit. In this configuration they are referred to as Integrated Preamplifier Detectors (IPDs).