What are Phototransistors?

Phototransistors are semiconductor devices that are either tri-terminal or bi-terminal in nature, featuring base regions that are sensitive to light. When implemented within an assembly, phototransistors act as electronic switching and current amplification devices that utilize light pulses to create digital electrical signals. With their capabilities and low price-point, such components are often found in relays, computer logic circuitry, security systems, punch-card readers, and more.

Phototransistors are quite comparable to standard transistor components, albeit relying on light intensity instead of current for operations. Nevertheless, they can still provide for switching and amplification. During standard operations, the light falling on the junction will cause reverse current flows, those of which are proportional to luminance. Phototransistors share many operational qualities to photoresistors, but differ in their ability to produce current and voltage.

For their construction, phototransistors are quite similar to bi-polar transistors with the exception of their exposed base region. While the common emitter configuration is the most widely used design for phototransistors, such components may come in P-N-P and N-P-N types. Phototransistors can even be designed with an open base, making them fairly flexible. For early variations, the most common materials were silicon and germanium, though such materials have since been superseded by gallium and arsenide in terms of efficiency levels.

As the base serves as the lead that activates the resistor, it acts as a gate controller device. The collector, meanwhile, is the positive lead of the assembly and acts as a bigger electrical supply. Lastly, the emitter operates as the negative lead, allowing it to be the outlet for a larger electrical supply. When there is no light falling on the phototransistor, only a small current flow will occur as hole-electronic pairs and the output voltage are less than the supply value. Once light reaches the collector-base junction, however, current flow will quickly increase. The intensity of light that falls upon the junction is paramount for how the device operates, and the gain of the transistor will amplify the base current from incident photons so that a large current may be generated.

A typical transistor is composed of an emitter, base, and collector terminal assembly, the collector terminal is positively biased while the BE junction is reversed biased. With a phototransistor, activation ensues when light collides with the base terminal, causing a configuration of hole-electron pairs as current begins to flow to either the emitter or collector. As current begins to increase, it will concentrate and transform into voltage.

Phototransistors may come in a few common forms, the two primary types being BJT and FET components. BJT phototransistors permit collector leakage, and they are capable of reaching 50mA when exposed to light. With a FET phototransistor, two terminals are connected through the emitter and collector, and current flowing along terminals can be controlled by the base terminal when the device is present in light.

When implemented within a circuit, a phototransistor may act as a switch or amplifier, depending on the needs of an application. Due to their functionality in relation to light, phototransistors can act as switches where there is no flow of current until light exposure occurs. As exposure increases, so too does the supply of current. As current gain is often determined by the intensity of light, phototransistors can act as amplification devices. Phototransistors are often compared to similar devices such as photodiodes, differing in the fact that they are more sensitive yet perform with a decreased amount of noise.

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