Phototransistor/Photodiode

Phototransistors and photodiodes are semiconductor optoelectronic components that detect light intensity and convert it into proportional electrical current. In optocoupler applications, they serve as the receiver side, isolated from the light-emitting diode (LED) transmitter by a transparent dielectric barrier, enabling signal transmission without electrical contact. Phototransistors provide current amplification through internal gain, while photodiodes offer faster response times and linearity.

Operates on the photoelectric effect where incident photons generate electron-hole pairs in the semiconductor depletion region. In photodiodes, this creates a photocurrent proportional to light intensity. Phototransistors amplify this current through transistor action, where the photogenerated base current controls a larger collector-emitter current. In optocouplers, an LED emits light across an isolation gap, which is detected by the photodetector to recreate the electrical signal.