Phototransistors

A phototransistor is a semiconductor device that converts light into an electrical signal. It functions like a regular bipolar transistor, but instead of being controlled by a base current, it is activated by incoming light.

• Operation: Light strikes the base-collector junction, generating electron-hole pairs, which act as base current and allow collector current to flow.
• Types: NPN and PNP; most common is NPN.
• Sensitivity: Higher than photodiodes due to internal gain.
• Applications: Optoisolators, light sensors, object detection, encoders, IR receivers.
• Package: Often has a lens or transparent window; may be in epoxy or metal can.
• Wavelength Range: Typically responsive to visible and near-infrared (e.g., 850–900 nm).

Note: Often used with a load resistor to convert current change into voltage output.