Fototransistors

Phototransistors are semiconductor devices that detect light and amplify the resulting signal. They operate as transistors in which light incident on the base region generates carriers, allowing light to modulate the collector-emitter current without external base drive. In practice, the device is biased so that light-induced carriers alter the transistor current, producing an output that is roughly proportional to the incident optical power. Phototransistors commonly come in NPN or PNP configurations and can be used in common-emitter or common-collector arrangements. Many packages expose a base lead, but most light-sensing applications use the base tied to the emitter, making a two-terminal device with a photocurrent input.

Because light provides the drive signal, phototransistors offer higher current gain and simpler interfacing than photodiodes, but they are slower and more temperature dependent. Their speed depends on device design and packaging, with typical response times ranging from microseconds to milliseconds.

The spectral response is determined by the semiconductor material: silicon devices detect visible light and near-infrared, while gallium-arsenide versions are usable at higher frequencies.

Typical applications include optocouplers (providing electrical isolation between input and output), ambient light sensing, and object detection in consumer electronics and instrumentation. They require a bias resistor to convert the output current to a usable voltage.

In summary, phototransistors are light-controlled transistors that provide amplified electrical signals from optical input, trading speed for convenience and integration.