Field-effect transistors (FETs)

Field-effect transistors (FETs) are a type of semiconductor device that is widely used in electronic devices. They have a number of advantages, including high input impedance, low noise, and low power consumption. FETs work by using an electric field to control the flow of current.

Structure

A FET has three main parts: a gate, a drain, and a source. The gate and drain are metal electrodes, and the source can be either metal or semiconductor material.

• Gate: The gate is located at the bottom of the FET and is connected to a metal electrode. When the gate is connected to a control voltage, it creates an electric field that controls the flow of current through the channel.
• Drain: The drain is also located at the bottom of the FET and is connected to a metal electrode. The drain collects the current from the channel and outputs it to the external circuit.
• Source: The source can be either metal or semiconductor material. When the source is biased positive, it attracts electrons from the channel; when the source is biased negative, it repels holes from the channel.

Operation

• **When the gate is not connected to a control voltage, there is no electric field in the channel. In this case, the channel is depleted of charge carriers, and the FET is in cutoff mode, with almost no current flow.
• **When the gate is connected to a control voltage, it creates an electric field in the channel. This field causes the charge carriers in the channel to drift. If the gate voltage is positive and large enough, the electrons in the channel are attracted to the gate, forming a conducting channel; if the gate voltage is negative and large enough, the holes in the channel are repelled to the gate, forming a conducting channel. In this way, the FET switches from cutoff mode to conduction mode.
• **When the gate voltage is reduced to a certain point, the electric field in the channel is reduced below a critical value. At this point, the concentration of charge carriers in the channel no longer changes with the gate voltage, but remains constant. This means that the FET has entered saturation mode. In saturation mode, the concentration of charge carriers in the channel is high, but the current density in the channel is low. Therefore, the conduction characteristics of the FET are primarily determined by the control capability of the gate.
• **When the gate voltage is further reduced to a value below a certain threshold, the electric field in the channel is further reduced to zero. At this point, the concentration of charge carriers in the channel also becomes zero. This means that the FET is completely cutoff.

Types and characteristics

Based on their structure and operating principles, FETs can be divided into two main categories: junction field-effect transistors (JFETs) and metal-oxide semiconductor field-effect transistors (MOSFETs).

• JFETs: JFETs have the advantages of high input impedance, low noise, and high temperature stability. They have a simple structure and are suitable for low-frequency amplifiers and switching circuits. However, their high input impedance leads to poor current amplification.
• MOSFETs: MOSFETs have the advantages of low input impedance, low noise, and high current amplification. They have a complex structure and are suitable for high-frequency amplifiers, switching circuits, and power amplifiers. In addition, MOSFETs also have good thermal stability and reliability.

Applications

FETs are widely used in a variety of electronic devices, such as computers, communication devices, and household appliances. Some common applications include:

• Switching circuits: FETs can be used to switch digital signals and are widely used in digital circuits in computers and communication devices. For example, the clock signal in a CPU is controlled by a FET switch.
• Amplifier circuits: FETs can be used to amplify weak signals, such as in audio amplifiers and video amplifiers. For example, televisions and audio equipment use a large number of FETs for signal amplification.
• Power management: FETs can be used to precisely regulate the power voltage and are widely used in power management circuits such as chargers and battery management systems. For example, the charging control in a smartphone charger is implemented by a FET.
• MOS drive: Metal-oxide semiconductor FETs have high current amplification and can be used as switching elements to drive other devices. For example, pixel drive in liquid crystal displays is implemented by MOS drive.