the field-effect transistor (FET)

Picture a garden tap where, instead of twisting a handle, you bring a charged comb near the spout and the water flow changes all by itself, your hand never touching the water. That is the spirit of a field-effect transistor, or FET. A voltage placed on a control terminal called the gate sets up an electric field, and that field opens or pinches a channel through which current flows between the other two terminals. The gate is insulated or reverse-biased, so it sips essentially no steady current: it commands the flow by its voltage alone.

Compare this with the bipolar junction transistor (BJT), where you must keep pushing a real base current to hold the device on, like having to keep your finger pressed on a button. The FET instead behaves like a voltage-controlled valve: once the gate sits at the right voltage, holding it there costs almost no current at all (nanoamps or less in DC terms). The catch is that the gate is a tiny capacitor, so to CHANGE its voltage quickly you must shove charge in and out, and at high speed that gate current is very real.

FETs come in two families: the JFET, whose gate is a reverse-biased junction, and the far more common MOSFET, whose gate is a metal plate sitting over a thin insulating oxide. Because the gate draws no steady current, FETs make superb switches and very-high-input-resistance amplifiers, and they pack densely on a chip. That is exactly why essentially every microprocessor, memory, and modern power switch is built from FETs rather than BJTs.