What is a phototransistor?

The phototransistor is a semiconductor device that is able to sense light levels and alter the current flowing between emitter and collector according to the level of light it receives.Although all bipolar transistors themselves are light sensitive, which is why most transistors are encapsulated in metal or plastic cans, phototransistors are a specialised form of bipolar transistor that have been optimised for its light sensitivity, making these electronic components ideal for many light sensing applications.The photo-transistor operates because light striking the semiconductor frees electrons / holes and causes current to flow in the base region.The base of the photo transistor would only be used to bias the transistor so that additional collector current was flowing and this would mask any current flowing as a result of the photo-action. For operation the bias conditions are quite simple.The light enters the base region where it causes hole electron pairs to be generated. This generation mainly occurs in the reverse biased base-collector junction. The hole-electron pairs move under the influence of the electric field and provide the base current, causing electrons to be injected into the emitter. As a result the photodiode current is multiplied by the current gain β of the transistor. Transistor is made positive with respect to the emitter or negative for a PNP transistor.One of the drawbacks of the phototransistor is that is particularly slow and its high frequency response is very poor. There is a small amount of current that flows in the photo-transistor even when no light is present. This is called the dark current, and represents the small number of carriers that are injected into the emitter. Like the photo-generated carriers this is also subject to the amplification by the transistor action.
Phototransistor applications:-
*Encoders where a rotating disc with light and dark stripes rotates – this gives speed and direction or rotation.
*Card readers.
*Security systems
*Infra-red detectors.
*Lighting control.
*Opto-couplers
*Counting systems – a light or IR beam is interrupted for each item counted.
*Lighting control.