Fototransistor

Fototransistor, also known as phototransistor, is a light-sensitive transistor that converts light into an electrical current. In most devices the main active terminals are the collector and emitter, and a light-induced current in the base region acts as the control signal, providing current gain to the transistor action. Many phototransistors have no external base lead, with illumination forming the base current internally.

Operation and materials: Photons create electron-hole pairs in the base or depletion region, generating a base current that is amplified by the transistor. The collector current is approximately proportional to the light intensity for a given bias. The spectral response depends on the semiconductor and typically peaks in the visible range for silicon devices; other materials extend sensitivity to near-infrared or ultraviolet.

Characteristics: Phototransistors offer higher sensitivity than photodiodes but slower response times. They exhibit dark current and leakage that set a baseline current in darkness. Devices are usually operated with a bias (NPN or PNP) and configured as common-emitter or common-collector; rise and fall times range from microseconds to milliseconds depending on design and packaging.

Variants and packaging: Phototransistors are available as discrete devices or as the detector element in optocouplers (photocouplers) where an LED provides the light source. They may be packaged in common semiconductor forms such as TO-92, SOT-23, or DIP, and sometimes include an external base lead for direct control.

Applications: They are used as light detectors in optocouplers to provide electrical isolation, in photointerrupters and encoders, line sensors, ambient light sensors in displays, and other light-level sensing circuits.