What Is an FET (Field

A field-effect transistor (FET) is a type of transistor that uses an electric field to control the current flow through a semiconductor channel. FETs are widely used in electronic circuits due to their high input impedance, low output impedance and high gain.

FETs have three terminals: the source (S), the drain (D) and the gate (G). When we apply a voltage to the gate, it creates an electric field that either attracts or repels the charge carriers (electrons or holes) in the channel region. Whether the charge carriers are attracted or repelled depends on the voltage's polarity. The process of applying a voltage to the gate of the FET controls the channel's conductivity and the current flow between the source and drain terminals.

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An FET is a voltage-controlled device. This means that its output current is controlled by the voltage we apply to its gate terminal.

FETs have very high input impedance, which means they do not load down the signal source and can be used as buffer amplifiers. Using FETs as buffer amplifiers can help prevent signal distortion and improve the overall quality of the circuit's output. Additionally, FETs are power-efficient, which makes them an attractive choice for battery-powered devices.

FETs are unipolar devices, which means they use only one type of charge carrier (electrons or holes) to control the current flow. The alternative to a unipolar device is a bipolar device. Unlike a unipolar device like an FET, a bipolar device such as a Bipolar Junction Transistor (BJT) uses both electrons and holes to control the current flow. Bipolar devices have a high current gain and can handle higher power levels, which makes them suitable for power amplification applications.

The source, the drain and the gate are an FET's three terminals. The source and drain are connected to the channel, while the gate controls the flow of current through the channel.

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We can control the conductivity of the channel in an FET by the voltage we apply to the gate. In an n-channel FET, a positive voltage applied to the gate will attract electrons to the channel and increase its conductivity. In a p-channel FET, a negative voltage applied to the gate will attract holes to the channel and increase its conductivity.

In a JFET, the channel consists of a semiconductor material and the channel has two regions at each end. These are known as the source and the drain terminals. The gate is a PN junction that's formed perpendicular to the channel. The gate terminal is biased in reverse. This creates a depletion region that controls the width of the channel. When we apply a voltage to the gate, the depletion region widens, thereby reducing the channel width and the current flowing through it.

Similar to JFETs, in MOSFETs the channel is also formed by a semiconductor material and it has two regions at either end, known as the source and drain terminals. In a MOSFET however, the gate is separated from the channel by a thin insulating layer that typically consists of silicon dioxide. As soon as a voltage is applied to the gate, it creates an electric field that attracts or repels charge carriers in the channel, depending on the voltage's polarity. This process controls the width of the channel and the flow of current between the source and drain terminals.

MOSFETs can be further classified into two subtypes: enhancement-mode and depletion-mode MOSFETs.

In enhancement-mode MOSFETs, the channel is normally off and you must apply a positive voltage to the gate in order to turn it on.

In depletion-mode MOSFETs, the channel is normally on and you must apply a negative voltage to the gate to turn it off.

FETs have several advantages over other types of transistors, which make them popular in a variety of electronic applications.

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Despite their advantages, FETs nevertheless have some disadvantages we should consider when designing electronic circuits.