How FET works

How a field-effect transistor (FET) works

All transistors work by controlling the movement of electrons, but not all of them do it the same way. Like a junction transistor, a FET (field effect transistor) has three different terminals—but they have the names source (analagous to the emitter), drain (analagous to the collector), and gate (analagous to the base). In a FET, the layers of n-type and p-type silicon are arranged in a slightly different way and coated with layers of metal and oxide. That gives us a device called a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).


Although there are extra electrons in the n-type source and drain, they cannot flow from one to the other because of the holes in the p-type gate in between them. However, if we attach a positive voltage to the gate, an electric field is created there that allows electrons to flow in a thin channel from the source to the drain. This "field effect" allows a current to flow and switches the transistor on:



For the sake of completeness, we could note that a MOSFET is a unipolar transistor because only one kind ("polarity") of electric charge is involved in making it work.
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