Phototransistor is a transistor which functions to move electric current from one side to the other using light. It is used in various sensors that detect the presence of light. This phototransistor combines a diode and a transistor together to be able to produce more output.
A junction transistor, has different functions, some function as a light absorber (collector) and emit light (emitter lead) or some function as a light carrier (base lead); when this light-carrying transistor is exposed to light through a small lens, the collector will absorb more as the light intensity increases, as a result of the strengthening of the current carried by the transistor structure.
Like diodes, all transistors are sensitive to light. The phototransistor is thus specially designed to take advantage of this fact. The most common variant is the NPN bipolar transistor with an open base region. Here, the light illuminating the base, replaces what the mains voltage would normally provide - so a phototransistor amplifies the variation in light intensity. Please note again that the phototransistor may not have a light carrier function (base lead). If so, this light carrier can allow you to bias the response of the light with this phototransistor.
In fact, diodes can also provide the same function, although with a much lower gain, meaning that the diode's function to drain a voltage is lower than that of a phototransistor.
How the phototransistor works.
Bipolar phototransistors resemble bipolar transistors in that they have extra large p-type silicon and are very open to receiving light. When photons from a light source collide with electrons in the p-type semiconductor, they will get enough energy to jump across the pn-junction carrying the energy from the photon. By transferring electrons from the p-area to the lower n-area, holes are created in the p-type semiconductor. When electrons are added to the lower n-area, they are attracted towards the positive terminal of the battery, while the electrons that are on the negative side of the battery are attracted into the n-type semiconductor through the np-junction, where they join the holes. -the hole, then it will make electrons flow from the emitter to the collector. As shown in the image below:
This is a brief description of this phototransistor. Hopefully this description of the phototransistor can be of use to those of you who read it.