What is a Photo-MOSFET Solid State Relay? A Photo-MOSFET Solid State Relay is similar to a Reed Relay in that it is a switching device that has a control side and an output side, but, unlike a Reed Relay, it is manufactured using semiconductor technology. In a Reed Relay, current through the coil, (the control side), creates a magnetic field that closes a reed switch, (the output side). In a Photo-MOSFET Solid State Relay, current through the IR LED, (the control side), creates IR frequency light. This light energy is gathered by a photo diode array in the PVG generating a voltage sufficient to turn on the MOSFET. The PVG/MOSFET combination is the output side of the Photo-MOSFET Solid State Relay.