How a MOSFET Works - with animation! | Intermediate Electronics

Let’s talk about FETs or, specifically, MOSFETs. We’ve done videos about bipolar junction transistors, or BJTs, and while MOSFETs may also be transistors and share many similar properties superficially, the way they operate is completely different. MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. Right now, that may just seem like random words mashed together but once you understand

how it works, this name will make perfect sense. MOSFETs still act like switches, by varying the voltage on one terminal, the gate, it changes the resistance between the other two terminals, the source and drain. Let’s discuss the makeup of a MOSFET, using an NMOS in enhancement mode as an example. With an NMOS, you have a p-type substrate that you then create two heavily doped n-type

regions. These two regions are called the source and the drain region. With our knowledge of semiconductors, you can see that you’re creating a PN junction between the substrate and these two regions. On top of the substrate, an oxide, which acts as an insulator, is deposited. Then, on top of that, a layer of metal is deposited, which finalizes the gate structure.

Now you can see where the Metal Oxide Semiconductor in MOSFET comes from. But it may seem strange to have your gate being completely electrically isolated from the rest of the circuit. This is where the FET term comes in. Even though there isn’t a direct electrical connection, the voltage on the gate creates a field effect.

As we know from our studies about diodes, at a PN junction, a depletion region is naturally created even when there are no electric fields. This is the natural state when the gate voltage is 0, and the MOSFET is operating in the “cutoff region”. This is an operating region, not a physical region, which can be confusing.