Photodiodes and Phototransistors
Photodiodes and phototransistors are often compared to one another, both being capable of transforming light energy into an electrical current. While providing similar methods of operation, both components differ from each other in their current gain and internal components. In order to better understand the distinct roles that the photodiode and phototransistor serve, one must first familiarize themself with the internal makeup and processes of each.
In their most basic form, photodiodes are a semiconductor diode device that feature two terminals. When exposed to light, the terminals are capable of producing an electrical current through the photoelectric effect. As the intensity of the radiation increases, so too does the current that passes through the device. Through the formation of an electron-hole pair with the action of light, reverse saturation current may flow through the photodiode. By increasing radiation, the temperature and resultantly current both increase as well. If a current is produced when there is no presence of light, it is known as dark current.
A Phototransistor, meanwhile, is fairly similar to a conventional junction transistor. In the base-collector region, however, base current is not used as an input. Rather, exposure to light serves as the trigger for the transistor, resulting in the generation of electric current due to the effects of the photoelectric effect. Depending upon the presence of a base terminal, a phototransistor may come in the form of a two or three terminal device. Through light, electron-hole pairs are produced, transitioning from the emitter to the collector as the barrier potential is surpassed. This is what creates the electrical current, and the amount of light exposure will directly affect the amount of current that is generated.
While the photodiode utilizes a semiconductor diode for its standard set of operations, phototransistors take advantage of a junction transistor to achieve the same effects. Despite these similarities, the phototransistor is typically known to exhibit increased efficiency over the photodiode. Phototransistors are also immune to noise, while photodiodes can be subject to such issues. Nevertheless, photodiodes operate better at high-frequency ranges, allowing for a more effective conversion of light energy into electrical energy. Photodiodes are also much quicker in their ability to generate an output current, adding to their benefits.
Depending on the application in question, both devices can prove to be an optimal fit. With photodiodes, applications such as optical fiber communication, digital and logic circuits, alarm and counter circuits, and high computing devices all may take advantage of the various benefits that the photodiode has to offer. Phototransistors, on the other hand, are highly efficient and may be used as a light detector for printers, remotes, and other such devices. Beyond such examples, phototransistors also are commonly found in relays, punch card devices, counting systems, and communication systems for their encoder circuitry.
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