Definition
A solid-state electronic component that combines a light-sensitive element with a field-effect transistor (FET), allowing a small light signal to control a larger electrical current. The light striking the device causes it to switch on or vary its output, providing electrical isolation between the light source side and the controlled circuit side.
Plain English
A small electronic part that uses light shining on it to turn an electrical circuit on or off, with no physical wire connecting the two sides.
Context Anchor
Seen in aircraft electrical and electronic system descriptions, especially where a circuit needs to sense light without using a mechanical switch.
Derivation
The name combines 'photo,' from the Greek 'phos' meaning light, with 'FET,' the abbreviation for field-effect transistor. So a photofet is literally a 'light-controlled FET' — a transistor that responds to light instead of an electrical signal at its input.
Why Pilots Care
Pilots don't interact with photofets directly, but knowing the term helps when reading maintenance manuals or troubleshooting avionics where light-triggered isolation is used to keep sensitive circuits clean and protected.
Analogy
A photofet is like an automatic night light sensor: when the light level changes, the electrical behavior changes too.
Intuition Check
A photofet is not a camera part or a photograph. It is an electronic component that uses light as the signal that changes current flow.
Example Sentence 1
The avionics technician traced the fault to a failed photofet in the autopilot's signal isolation circuit.
Example Sentence 2
A failed photofet caused the automatic dimming feature in the instrument panel to stop working.