Field Effect Transistors
Field effect transistors are transistors which are made up of 3 regions, a gate, a source, and a drain. Unlike bipolar transistors, FETs are voltage-controlled devices. A voltage placed at the gate controls current flow from the source to the drain of the transistor.
Field effect transistors have very high input impedance, from several ohms of resistance to much larger values. This high input impedance prevents them from generating high current to run through. That’s why FETs both draw very little current from a circuit's power source, nut it is ideal because they don't disturb the original circuit's power elements to which they are connected to. Also, they won't cause the power source to be loaded down. The drawback of FETs is that they won't provide the same amplification that could provide by bipolar transistors.
Field effect transistors can be divided into 2 main types: JFETs and MOSFETs. JFETs and MOSFETs are very similar but MOSFETs have even higher input impedance values than JFETs. This causes even less loading in a circuit.