First off, let's talk about the core of a commercially available mosfet, the actual mosfet. Now, initially, that shouldn't make any sense, "There's a mosfet in the mosfet?" Well, sorta. What we call mosfets are actually the whole unit, and can include a great deal of different things.
This is what we often call a mosfet. It's the whole unit, with all the parts and pieces attached to the actual mosfet.
This is what a mosfet actually looks like, and this is the heart and soul of what we airsofters call a mosfet, the actual mosfet. The other pieces and parts attached to the mosfet help to protect it and insulate it from the forces it will experience in an AEG.
Now, the mosfet I have pictured here is an IRF8872. You can use the IRF number on a mosfet to research how powerful the mosfet is; how much voltage it can handle, how much current, heat, etc. For Airsoft, the IRF3034 is just about perfect. It can handle the loads and voltages that 99% of AEGs will require. So, when buying a mosfet unit to install in your gun, make sure that it is based around the IRF3034 fet. If it isn't, it's not worth your money, look elsewhere.
Beyond simple mosfets, you'll find some that have something called a TVS installed. A TVS helps to protect the mosfet by absorbing electrical back-lash, to put it in laymen's terms. When you start to really push your system, and have some crazy builds, you'll start to have some extra electrical bits floating around in your system, and those can damage the mosfet over time, until it eventually dies. A TVS absorbs those electrical bits, turns them into heat, and easily disposes of them, protecting the mosfet.
Even further, in some cases, you'll find mosfet units that have two mosfets. They act to share the load, creating even more reliability and strength of the mosfet unit.
And that leads me into my third point, AB Mosfets. AB mosfets are a little different. Physically, they have two mosfet chips, that each control something a little different. In practice, what they do is when you release your trigger, it sends an opposite charge to the motor, acting as a brake to the motor, slowing it down quickly or stopping it entirely. This often times helps the gun sound crisper, because the motor stops faster, it can help with over spin issues, and can help prevent semi lock up. There are a great many debates on AB Mosfets and Active Braking (What the "AB" stands for) in general. Most agree that it is bad for the motor, and causes them to burn out quicker, though, some experiments say otherwise. Bad or not, one thing everyone agrees on is that it causes your motor to heat up faster than normal, and most people want to avoid warm motors. Another issue with AB is that it is rather .... complex, as is, there are many things that can go wrong. Without a being controlled, they are not nearly as reliable as your non AB mosfets. Reliable AB mosfets are controlled by computers, which are programmed when to cut the current, how to do it, what to do when something goes wrong, etc. It is generally not recommended to get a simple AB mosfet because of what CAN happen, a computer controlled AB mosfet is suggested. There is a reason it's hard to find cheap, quality mosfets that are AB.
Now, to wrap it all up, where can I find these mosfets? Predominately, you find them out of people's garages. They make them in house, and sell them out of their own homes. Generally speaking, you shouldn't pay more than $20-25 for a simple mosfet, and you should generally avoid any mosfets in a hard case, or on a circuit board, ESPECIALLY if they are under $30. Sadly, it's not very common to find quality built mosfets in large retailers. But they do exist, and I will take the time to list them, and go over some short reviews on them.
Clandestine Airsoft
http://clandestineairsoft.com/electrical-parts-c-1_27/
Clandestine is a great place to shop. Besides having a large selection of parts at very reasonable prices, they are one of the few retailers who sell quality mosfets. You'll find them on the page linked above, depending on what's in stock, you'll have different kinds made by different people. At the writing of this article, the APEX mosfets are the only ones in stock. They have a good description, if you care to read through it, but they have two mosfets, as I spoke of earlier, making them extremely heavy duty. While Zardichar's mosfets will handle most anything, these mosfets will handle the 1% that his mosfets can't handle.
Brill Armory
http://www.brillarmory.com/index.php?ma ... vmik870oo1
Brill Armory is like a smaller Clandestine with a slightly different selection of parts, but mostly the same. Similarly to Clandestine, he attempts to find quality mosfets, and then retail them through his store. Currently, everything is out of stock. I do not know when it might be in stock again. That is part of the problem with finding these quality, cheap mosfets.
Zardichar Airsoft
https://www.facebook.com/zardicharairsoft
Zardichar is where I get my mosfets from, and the ones we sell at Escue Airsoft. They are small, simple, cheap, and work really well. You can contact Zardichar themselves, or contact me, and I'll be more than happy to sell you one. I have been running one in my M60 for as long as I have had it. Below you can see that they are small enough that I was able to fit it inside of a gearbox. It's the green part at the rear of the gearbox. And do note that this can't be done to every gun.
BlackTalon Concepts
http://www.btcairsoft.com/
BTC is a fantastic mosfet maker, however, they are significantly more expensive than the previously stated mosfets. However, these are computer controlled, and come with a plethora of customizable options. BTC is responsible for the Chimera mosfet, and the Specter, both act the same way, but have different ways of installing them. You can find out more about that on the website. As for the mosfets themselves, the world of options is immense, able to lock a gun to semi only, turn on precocking, change firing mods of both firing mods (aka, turning semi auto into auto, and turning auto in to semi), you can decrease your rate of fire, turn on burst options, it even has the ability to turn on varying amounts of AB, as well as shutting your gun off if your battery drops too low.
Extreme-Fire
http://extreme-fire.com/
Extreme-fire has been around for years and years and years. They are run by one guy who sells all sorts of mosfets, everything from simple fets to really nice complicated fets. Everything he's got is super strong, and works well, along with a very, very good warranty. As a matter of fact, Extreme-Fire is responsible for our modern mosfet unit designs, as it was first used by Extreme-Fire. Extreme-Fire later went on to develop the SW-Cheetah Mosfet, which the BTC Chimera mosfet was based off of. You could say that Extreme-Fire is the father of the AEG Mosfet. They have similar options as the BTC fets do, such as semi only, or rate of fire control. They also have low voltage detection, so that it'll protect your lipos. The biggest plus about these mosfets is the customer service. The guy who runs Extreme-Fire is very quick to respond, and offers some of the best customer service I have ever come across. Not only that, but he custom builds each mosfet. That basically means, that they are never out of stock, unlike the ever so popular BTC mosfets.
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