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

PC Fan Pinout

   Black: Ground
   Yellow: +5V, +12V or +24V (depends on fan model, usually 12V for desktops, 5V for laptops)
   Green: Sense. Used to measure RPM
   Blue: PWM control signal at 5V, 25kHz