Field-effect transistors (FET) are electronic devices which use an electric field to control the flow of current. Application of a voltage to the gate terminal alters the conductivity between the drain and source terminals. FETs are also known as unipolar transistors since they involve single-carrier-type operation. That is, FETs use electrons or holes as charge carriers in their operation, but not both. Field effect transistors generally display very high input impedance at low frequencies.