How does an "opto-fet" work?


Joined Apr 20, 2004
Just like an IGFET, except that the impinging photons cause a local build up of electrons in the gate area, thus causing an electric field that opens the conduction channel from source to drain. Photons don't carry as much oomph as a voltage source, so a photo-fet will not have as low an on resistance as a "regular" fet.

Oddly enough, an old germanium 2N404 will also act as a phototransistor if you grind off the top of the TO-5 can. But, then they're so leaky it's hard to tell gate-emitter polarity.