What is a Field Effect Transistor and Types of FET?

FET or field-effect transistor is a type of transistor that uses the electric field or voltage to control the current flow. It is unipolar i.e. the current flow only due to the majority of charge carriers that is either electrons or holes. The three terminals of FET are Drain (D), Gate (G), and Source (S). Based on its construction, FET has a channel between its drain and source, and the channel refers to the path for the flow of current. The drain and source are made of the same semiconductor material. However, the drain terminal is the one having more positive voltage. Therefore, the drain and source terminals are interchangeable.

Types of Field Effect Transistors

  • Mosfet Transistor
  • JFET Transistor

Metal oxide semiconductor Field-Effect Transistor (MOSFET)

Metal oxide semiconductor Field Effect Transistors are mainly used to amplify voltages in circuits. It consists of three terminals: Gate, Source, and Drain. Based on their operation modes, the two classes of MOSFETs are –

  • Depletion mode – MOSFET operates owing to the channel that is already present.
  • Enhancement mode – Gate Voltage induces channel.

Junction Field-effect Transistor (JFET)

Junction Field-effect Transistor consists of two configurations: the N-Channel JFET and the P-Channel JFET. The two types of JFETs are classified on the basis of the current flow. Such transistors are used to amplify weak signals

Terminals of FET Transistors

  • Source: The first terminal is the source. The current that enters through the source is denoted by IS.
  • Drain: The current that leaves through the drain is denoted by ID. There is a presence of voltage between the drain and the source which is denoted by VDS.
  • Gate: Gate controls and modulates the conductivity of the channels. It is denoted by G. By applying a voltage at the gate, the current that leaves through the drain can be controlled.

Features of FET

  • Unipolar − It is unipolar as either holes or electrons are responsible for conduction.
  • High input impedance − The input current in a FET flows due to the reverse bias. Hence it has a high input impedance.
  • Voltage-controlled device − As the output voltage of a FET is controlled by the gate input voltage, FET is called the voltage controlled device.
  • Noise is low − There are no junctions present in the conduction path. Hence noise is lower than in BJTs.

Applications of FET

  • FET is used in circuits to reduce the loading effect.
  • Used in many circuits such as Buffer Amplifier, Phase shift Oscillators, and Voltmeters.

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