Transistors

A transistor with gate, drain, and source terminals is known as a field-effect transistor (FET). FETs control the flow of current through a semiconductor by applying a voltage to the gate, which alters the conductivity between the drain and source. Wikipedia autodesk.com

in our case, we have an irf520 transistor and the pinout is like this

1: Gate
2: Drain
3: Source