A field-effect transistor (FET) is a type of transistor commonly utilized for weak-signal amplification. The device may amplify analog or digital signals. It may also switch DC or function as an oscillator. In the FET, current pass along a semiconductor path called the channel. At one end of the channel, there is an electrode called as the source. At the other end of the channel, there is electrode known as the drain. The physical diameter of the channel is fixed, however its effective electrical diameter may be varied by the application of a voltage to a control electrode called the gate. The conductivity of the FET based, at any given instant in time, on the electrical diameter of the channel. A small change in gate voltage may cause a large variation in the current from the source to the drain. It is how the FET amplifies signals. Field-effect transistors present in two major classifications. These are called as the junction FET (JFET) and the metal-oxide- semiconductor FET (MOSFET).
The junction FET contain a channel consisting of N-type semiconductor (N-channel) or P-type semiconductor (P-channel) material; the gate is built of the opposite semiconductor type. In P-type material, mostly electric charges are carried in the form of electron deficiencies called holes. In N-type material, the charge carriers are primarily electrons. In a JFET, the junction is the boundary among the channel and the gate. In general, this P-N junction is reverse-biased (a DC voltage is applied to it) so that no current flows between the channel and the gate. Though, under some conditions there is a small current through the junction during part of the input signal cycle.
In the MOSFET, the channel may be either N-type or P-type semiconductor. The gate electrode is piece of metal whose surface is oxidized. The oxide layer electrically insulates the gate from the channel. For this purpose, the MOSFET was originally called the insulated-gate FET (IGFET), but this term is now rarely utilized. Because the oxide layer acts as a dielectric, there is essentially never any current among the gate and the channel during any part of the signal cycle. It gives the MOSFET very large input impedance. Since the oxide layer is tremendously thin, the MOSFET is susceptible to destruction by electrostatic charges. Special precautions are essential when handling or transporting MOS devices.
The FET has some advantages and some of the disadvantages associative to the bipolar transistor. For weak-signal work, field-effect transistors are preferred for instance in wireless communications and broadcast receivers. They are also preferred in systems and circuits acquiring high impedance. The FET is not, generally, utilized for high-power amplification, such as is needed in large wireless communications and broadcast transmitters.
Field-effect transistors are fabricated onto silicon integrated circuit (IC) chips. A single IC may contain several thousands of FETs, along other components such like capacitors, resistors and diodes.