phototransistors

Phototransistors are light-sensitive transistors in which illumination controls the transistor’s base region, thereby regulating the collector current. When photons strike the base region, electron-hole pairs are generated, increasing base current and producing a larger collector current. This makes phototransistors useful for converting optical signals into electrical signals with inherent current gain proportional to light intensity.

Construction and operation: Phototransistors are typically silicon devices and come in NPN or PNP varieties. They are designed so that the base is illuminated rather than driven by an external base lead. In many packages the base lead is not accessible and the device is used with the collector and emitter only; light incident on the body provides the base drive. They can be operated in common-emitter or common-collector configurations, with a bias resistor to convert the collector current into a usable voltage signal.

Characteristics: The light-driven current gain can be substantial, but the gain and response depend on illumination, temperature, and biasing. Phototransistors generally offer higher sensitivity than photodiodes but slower response, with rise and fall times ranging from microseconds to milliseconds depending on construction and load. They are usually matched to specific spectral ranges, with silicon devices sensitive to visible and near-infrared light; infrared variants may use different materials or filters. Environmental factors such as ambient light and temperature influence performance.

Applications: Phototransistors are used in optical switches, light sensors, and optocouplers, where electrical isolation between input and output is advantageous. They also appear in simple light detectors, proximity sensing, and remote-control receivers, providing amplification of a light signal to drive subsequent circuitry.

Comparison: Compared with photodiodes, phototransistors provide higher current gain and simpler circuitry but at the cost of slower response and potential saturation. They are chosen when light levels are low and amplification is beneficial, rather than when high-speed detection is required.