The operation of a phototransistor depends on the intensity of radiation falling at its base region. Here, the symbolic representation is almost similar to a normal BJT but the only variation is the presence of two inward arrows at the base region that shows the incident of light radiation. The figure below represents the symbolic representation of a Phototransistor: Phototransistors are mainly enclosed in a metallic case that consists of the lens at the top in order to gather the incident radiation. This implies they are more capable of changing light energy into electrical energy as compared to homojunction transistors. This is so because these structures exhibit large conversion efficiency. Thereby, providing a heterojunction structure. However, in recent times, these are constructed using materials likes gallium or arsenide. Initially, phototransistors were fabricated from silicon or germanium as their basic material that resultantly provides homojunction structure. Here, as we can see that the light is majorly allowed to incident at the base collector junction. The figure below represents the constructional structure of an NPN Phototransistor: At the time of constructing the phototransistor, the base and collector region is provided with a larger area in comparison to a normal BJT. Content: PhototransistorĪs we have already discussed that a phototransistor is nothing but a normal transistor whose action depends on the incident radiation falling at its base. And the transistor action permits it to perform amplification of the current flowing through it. As it changes light signal incidenting on its surface into its electrical equivalent form. It operates on the principle of Photoelectric effect. This means that as in normal BJT, base current is used to drive the circuit, however, in phototransistor light energy falling on the base region acts as the overall input of the device.Ī phototransistor is said to exhibit the combined operation of the photodiode as well as a normal transistor.
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