FET stands for Field-Effect Transistor, which is a type of transistor that uses an electric field to control the flow of current in a semiconductor. It has three terminals: gate, drain, and source. There are two types of FETs: junction-gate FET (JFET) and metal-oxide-semiconductor FET (MOSFET) . FETs can be majority-charge-carrier devices, in which the current is carried predominantly by majority carriers, or minority-charge-carrier devices, in which the current is mainly due to a flow of minority carriers. The device consists of an active channel through which charge carriers, electrons or holes, flow from the source to the drain. FETs can amplify analog or digital signals, switch DC, or function as an oscillator.

On the other hand, FET can also refer to the Federal Excise Tax, which is a tax on the first, initial sale of certain items, including trucks over 26,000 pounds. The FET tax on trucks is 12% of the total sale price that is added on when purchasing a new truck. This tax was established in 1917 to create revenue for World War I and has since increased from the original 3% to a 12% tax. However, this tax is only applicable on a truck that is brand new, and it does not apply after the initial sale of a truck.