In the “real world” (ie, not airsoft) a MOSFET is an electrical component. MOSFET stands for a metal-oxide-semiconductor-field-effect transistor. Which is pretty much useless to know.
It’s a kind of transistor. When you apply voltage at one point, current flows through 2 different points. No current flows through the bit where you originally applied voltage. You can apply a “signal” at one part and allow current to flow through somewhere else.
This is helpful if you want to avoid having high voltages and amps running through a small switch, for instance.
How does this apply to airsoft?
As mosfet is a way of getting electricity to flow directly from the battery to your airsoft gun’s motor, without going through the trigger contacts.
When every airsofter on the field was using the standard NiMH style batteries, Mosfets were pretty rare. But, with the rise in popularity of LiPo batteries, they have become more and more popular.
LiPo Batteries can discharge much faster than older batteries could. The higher discharge rate results in what is known as ‘arcing’. Electricity actually “jumps” the gap between the trigger contacts (when they’re close enough).
The problem is that an airsoft motor will draw more power than the trigger contacts can withstand. When you send power through the trigger contacts, it damages them, especially when the electricity jumps the gap. I’m not going to go into why and how it damages them, just trust me it does!
This is where airsoft mosfets step in.
Wiring a mosfet into an AEG lets the trigger can send a “signal” to the mosfet without letting any energy flow.
All of the dangerous amps, that would otherwise damage the trigger contacts, go straight from the battery to the motor.
This not only protects the trigger contacts but also reduces the overall resistance of the circuit (trigger contacts introduce a lot of resistance). The result is an improved rate of fire and trigger response.
Why are airsoft mosfets so big?
Airsoft mosfets have a bunch of extra components in them. The extra components can do 2 things, they protect the mosfet from overvoltage (and other potential damage), or they can offer extra features, such as battery discharge protection, rate of fire control and active braking.