The field-effect transistor (FET) is a three-terminal device used for a variety of applications that match, to a large extent. Although there are important differences between the two types of devices, there are also many similarities.The term field-effect in the chosen name deserves some explanation. For the FET, an electric field is established by the charges present that will control the conduction path of the output circuit without the need for direct contact between the controlling and controlled quantities.