A phototransistor is a device that is capable of converting light energy into electric energy. Phototransistors work in a similar way to photoresistors but are able to produce both current and voltage while photoresistors are only capable of producing current. This is because a phototransistor is made of a bipolar semiconductor and focuses the energy that is passed through it. Phototransistors are activated by photons (particles of light) and are used in virtually all electronic devices that depend on light in some way.
How A Phototransistor Works
The actual operation of a phototransistor depends on the biasing arrangement and light frequency. For instance, if a PN junction is forward biased, the increased current through the junctions due to incident light will be relatively insignificant. On the other hand, if the same junction is reverse biased, the increase in current flow will be considerable and is a function of the light intensity. Therefore, reverse bias is the normal mode of operation.
Now, if the PN junction is the collector-base diode of a bipolar transistor, the light-induced current effectively replaces the base current. The physical base lead of the transistor can be left as an open terminal, or it can be used to bias up to a steady state level. It is the nature of transistors that a change in base current can cause a significant change (increase) in collor current. Thus, light stimulation causes a change in base current, which in turn causes a bigger increase in collector current and, considering the current gain (hfe), a rather large increase at that.
Applications
Phototransistors are used for a wide variety of applications. In fact, phototransistors can be used in any electronic device that must sense light around it. For example, phototransistors are often used in smoke detectors, infrared receivers, and CD players. Phototransistors can also be used for industrial purposes such as astronomy, night vision, and laser rangefinding.
Advantages
Phototransistors have several important advantages that separate them from other optical sensors. For example, phototransistors produce a higher current than photodiodes while also being able to produce a voltage, something that photoresistors cannot do. Phototransistors are very fast and are capable of providing nearly instantaneous output. Phototransistors are relatively inexpensive, simple, and small enough to fit several of them onto a single integrated computer chip.
Disadvantages
While phototransistors can be advantageous, they also have several disadvantages. For example, phototransistors made of silicon are not capable of handling voltages over 1,000 Volts. Phototransistors also do not allow electrons to move as freely as other devices do, such as electron tubes. Phototransistors are also more vulnerable to surges and spikes of electricity as well as electromagnetic energy.
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