Phototransistors are either three-terminal (emitter, base and collector) or bi-terminal (emitter and collector) semiconductor devices whose base region is sensitive to light. Although all transistors exhibit a light-sensitive nature, these are specifically designed and optimized for photo applications. They are made by diffusion or ion implantation and have much larger collector and base regions than conventional transistors. These devices can be either homojunction structured or heterojunction structured, as shown in Figure 1a and 1b, respectively.
In the case of homojunction phototransistors, the entire device will be made of the same type of material. Either silicon or germanium. However, to increase their efficiency, phototransistors can be made of dissimilar materials (group III-V materials such as GaAs) on both sides of the pn junction, leading to heterojunction devices. However, homojunction devices are more frequently used than heterojunction devices because they are economical.
The circuit symbol for NPN phototransistors shown in Figure 2 is nothing more than a transistor (with or without a base lead) with two arrows pointing towards the base to indicate light sensitivity. do A similar symbolic representation holds well in the case of pnp phototransistors with the only change being that the current points towards the emitter instead of outward.
Phototransistors behave like normal transistors except for the fact that here the effect induced by the base voltage will be felt by the incident light. This can be further clarified by analyzing the following points.
- Phototransistors have properties similar to normal transistors except that they have a base current that changes with the intensity of light. This means that these devices also have three operating regions i.e. Cutoff, Active and Saturation. This further indicates that phototransistors can be used either for switching (depending on cutoff and saturation mode) applications or for amplification (active mode operation) like normal transistors.
- Phototransistors can be configured in two different configurations, such as common collector and common emitter, depending on the terminal that is shared between the input and output terminals, similar to normal transistors.
- A small reverse saturation current, called the dark current, flows through the phototransistor even in the absence of light and its value increases with increasing temperature, which is similar to that exhibited by ordinary transistors. Similar to the trait.
- Phototransistors suffer permanent damage due to breakdown if the voltage applied to the collector-emitter junction exceeds its breakdown voltage, just as in the case of ordinary transistors.
Generally, in the case of phototransistor circuits, the collector terminal will be connected to the supply voltage and the output will be obtained at the emitter terminal while the base terminal, if present, will be left unconnected. In this case, if light is made to fall on the base region of the phototransistor, it creates electron-hole pairs that give rise to the base current, which is nothing but the photocurrent, applied.The Under is to the influence of an electric field. . This results in the flow of emitter current through the device, resulting in an amplification process. This is because, here, the magnitude of the photocurrent produced will be proportional to the light and will be amplified by the gain of the transistor leading to a larger collector current.
The output of a phototransistor depends on various factors e.g
- The wavelength of the incident light
- Collector-base junction area exposed to light
- DC current gain of a transistor.