How Does a Phototransistors Works?

A phototransistor is an electronic switching and current amplification component that relies on exposure to light to operate, much like how normal transistors rely on electricity to operate. When light falls on the junction of a phototransistor, reverse current flows in proportion to the luminance of the light. This makes phototransistors excellent at detecting light pulses and converting them into digital electrical signals. Unlike normal transistors, they are operated by light rather than electric current, and provides a large amount of utility for a low cost.

Phototransistors work in a similar manner to photoresistors, but can produce both current and voltage, whereas photoresistors only produce current due to the change in resistance. Phototransistors have their base terminal exposed, and instead of sending current to the base, the photons of striking light activate the transistor. This is because a phototransistor is a bipolar semiconductor with its base region exposed to illumination, which focuses the energy that passes through it. Since they are used in almost every electronic device that depends on light. Phototransistors are frequently used in security systems and punch card readers, encoders, IR photodetectors, computer logic circuitry, lighting control, and relays.

Phototransistors come in several different configurations, including common emitter, common collector, and common base, with common emitter being the most frequently used. Compared to conventional transistors, it has more base and collector areas, and is made from gallium and arsenide for high efficiency. The base is the lead responsible for activating the transistor, and is the gate controller device for the electrical supply. The collector serves as the positive lead, and the emitter is the negative lead and the outlet for the larger electrical supply.

The advantages of phototransistors is that they produce a higher current than photo diodes, are relatively inexpensive to manufacture, simple to use, and small enough to fit several of them onto a single integrated computer chip. They are also very fast and can produce nearly instantaneous output (they operate literally at the speed of light, after all), and they produce a voltage, something that photoresistors cannot do. However, because phototransistors are made from silicon, they cannot handle voltages of over 1,000 volts, are more vulnerable to surges and spikes of electricity, and do not allow electrons to move as freely as other devices do, such as electron tubes.

At ASAP NSN Hub owned and operated by ASAP Semiconductor, we can help you find all the phototransistor systems and parts for the aerospace, civil aviation, and defense industries. For a quick and competitive quote, email us at or call us at 1-920-785-6790.


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