phototransistor
Any semiconductor device whose electrical characteristics are light-sensitive.
phototransistor: light turns it into a current amplifier
A phototransistor is a semiconductor that acts as both a light detector and an amplifier in a single component. When photons strike its junction, they generate electron-hole pairs that trigger base current; the transistor structure then amplifies this current by a factor of 100 to 1000 or more. This built-in gain makes phototransistors far more sensitive than photodiodes for the same photon flux, and more useful in low-light sensing applications.
The device is typically a bipolar junction transistor (BJT) with the base region exposed or very lightly doped. In the dark, it has near-zero collector current. As light intensity increases, photocurrent in the base multiplies through the transistor's current gain, producing a proportionally larger collector current. The wavelength response depends on the semiconductor material: silicon phototransistors peak around 800 to 950 nanometers in the near infrared; germanium extends response further into the infrared but with higher dark current and lower speed.
Common configurations and limitations
Phototransistors come in two-terminal (emitter and collector) or three-terminal (with accessible base) packages. The three-terminal version allows manual adjustment of sensitivity via base bias, but is less common in production. Response time is typically 10 to 100 microseconds depending on the load resistance and transistor type; this is much slower than a photodiode alone, a real drawback for high-speed switching or modulation. Dark current increases sharply with temperature, making temperature compensation necessary in precision circuits.
The trade-off between sensitivity and speed, coupled with nonlinearity at high light levels, limits use to moderate-speed, low-light detection tasks. Common applications include infrared remote receivers, object counting gates, and ambient light sensors in consumer electronics. For high-speed fiber-optic receivers or precision analog work, engineers often choose a photodiode paired with a separate transimpedance amplifier instead, trading component count for better performance.