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What Is an FET (Field-Effect Transistor)?

Posted:10:42 AM February 23, 2024 writer: iotbyhvm


Field-Effect Transistors (FETs) are extensively used as the main active elements in numerous integrated circuits. They play a big role in modern electronics. These transistors are very important in influencing how innovation is happening right now. Without them, our technology would be very different from what it is today. Julius Edgar Lilienfeld was the first person to come up with the idea of a special electronic device called a field-effect transistor in 1926. However, this design had some problems like bad connections between parts and instability which made it not very useful. Later in 1934, another person named Oskar Heil also had a similar idea and applied for a patent. But his version had different issues, like being harder to make and control.

In the current age where technology is at the forefront of our lives, electronic devices play an important role. They help make everything accessible with just a click. Throughout the evolution of technology, we have transitioned from desktop computers to powerful supercomputers. Among the various types of transistors, the Field-Effect Transistor (FET) stands out as an important component. It offers unique characteristics and functionalities. In this article, we will delve into what an FET (Field-Effect Transistor) is, exploring its design, types, specifications, and applications in electronic circuits.

FET (Field-Effect Transistor) is a three-terminal semiconductor device that controls the flow of current based on the electric field applied to one of its terminals. Unlike the Bipolar Junction Transistor (BJT), the Field-Effect Transistor (FET) controls the flow of electric charge using an electric field instead of an electric current. The FET consists of three main components: the source, the drain, and the gate. The source is responsible for supplying the current, while the drain acts as the output terminal. The gate, on the other hand, controls the flow of current between the source and the drain by modifying the electric field. There are three primary types of FETs: the Junction Field-Effect Transistor (JFET), the Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), and the Insulated-Gate Bipolar Transistor (IGBT). The FET's ability to provide high input impedance, low noise, and low power consumption makes it an ideal choice for various applications, including amplifiers, oscillators, voltage regulators, and digital logic circuits.

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Field Effect Transistor

A Field-Effect Transistor, commonly known as FET, is a three-terminal semiconductor device that utilizes an electric field to control the flow of current within the circuit. It operates based on the modulation of the conductivity between its source and drain terminals, induced by an electric field applied to the gate terminal. FETs are important parts of electronic circuits. They do essential jobs like making signals louder, turning things on and off, and controlling voltage.

Unlike bipolar junction transistors (BJTs), which rely on the movement of charge carriers (electrons and holes), FETs operate on the principle of controlling the flow of current through an electric field. This fundamental difference makes FETs suitable for certain applications where BJTs may not be as efficient. The path through which the electric current flows in the semiconductor can be either P-type or N-type. This distinction leads to two categories of FETs: P-Channel and N-Channel FETs.

Junction FET circuit symbol Junction FET circuit symbol

Because it's the electric field that governs the current flow in the channel, the Field-Effect Transistor (FET) is considered voltage-operated. This characteristic grants it a high input impedance, often in the range of many megohms. This high input impedance is advantageous compared to the bipolar transistor, which is current-operated and typically has a much lower input impedance. The FET's voltage-operated nature and high input impedance make it well-suited for various electronic applications, offering benefits in terms of signal amplification, sensitivity, and overall circuit performance.

Junction Field Effect Transistor, JFET working below saturation Junction Field Effect Transistor, JFET working below saturation

FET Circuits Design

When designing FET circuits, it's important to choose the right type of FET for your specific needs. There are two main types: the Junction Field-Effect Transistor (JFET) and the Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). A crucial aspect of FET circuit design involves biasing. Biasing refers to the process of applying a DC voltage to the FET to achieve the desired operating point. Proper biasing ensures that the FET operates within its linear region, allowing for optimal performance and minimizing distortion. Furthermore, FET circuits often require coupling and decoupling capacitors to block unwanted DC signals and ensure proper AC signal transmission.

Despite this variance, electronic circuit design with FETs remains relatively straightforward, differing only slightly from designs involving bipolar transistors. With FETs, it is possible to create circuits such as voltage amplifiers, buffers, current followers, oscillators, filters, and more. Remarkably, the circuit designs for FETs share similarities with those for bipolar transistors and even with thermionic valves or vacuum tubes, although bias arrangements differ. Notably, valves/tubes are also voltage-operated devices, resulting in comparable circuit designs, including bias arrangements.

When designing FET circuits, several factors must be taken into account:

  • Biasing: Proper biasing is crucial to ensure the FET operates in its desired region. The article mentioned earlier explains the different biasing techniques, such as self-bias and voltage divider bias.
  • Load Line Analysis: Analyzing the FET's load line helps determine the appropriate operating point and the maximum signal swing.
  • Input and Output Impedance: Understanding the FET's input and output impedance is essential for matching with other circuit components and optimizing signal transfer.
  • Protection Circuits: Implementing protection circuits, such as overvoltage and overcurrent protection, safeguards the FET and the entire circuit from potential damage.

Before finalizing an FET circuit design, it is advisable to simulate and test its performance using specialized software or hardware tools. This enables designers to verify the circuit's functionality, analyze its behavior, and make necessary adjustments.

Field Effect Transistor types

There are several types of Field-Effect Transistors (FETs), and they can be broadly classified into three main categories: Junction Field-Effect Transistor (JFET), Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), and Insulated Gate Bipolar Transistor (IGBT). Here's a brief overview of each type:

Junction Field-Effect Transistor (JFET)

  • N-Channel JFET: In the N-Channel JFET, the semiconductor material forming the channel is of N-type.
  • P-Channel JFET: In the P-Channel JFET, the semiconductor material forming the channel is of P-type.
  • Operation: JFETs operate by controlling the current flow through a semiconductor channel using an electric field applied across a PN junction.

Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)

  • N-Channel MOSFET: In the N-Channel MOSFET, the channel is formed in an N-type semiconductor material.
  • P-Channel MOSFET: In the P-Channel MOSFET, the channel is formed in a P-type semiconductor material.
  • Operation: MOSFETs operate by applying a voltage to a metal gate, creating an electric field that controls the flow of charge carriers (electrons or holes) between the source and drain terminals.

Insulated Gate Bipolar Transistor (IGBT)

  • Structure: IGBTs combine features of both MOSFETs and bipolar junction transistors (BJTs). They have a gate like a MOSFET and conductivity modulation similar to a BJT.
  • Operation: IGBTs are used for high-power applications and are often employed in electronic devices requiring high current and voltage capabilities.

High Electron Mobility Transistor (HEMT)

  • Characteristics: Known for high electron mobility and low noise characteristics.
  • Applications: Commonly used in high-frequency applications such as satellite communications, radar systems, and cellular networks.

Gallium Nitride Field Effect Transistor (GaN FET)

  • Characteristics: Known for high power density and efficiency.
  • Applications: Used in power electronics applications such as electric vehicle charging, renewable energy systems, and wireless power transfer.

Each type of FET(Field Effect Transistor types) has its own characteristics and is suitable for specific applications. FET(Field Effect Transistor) Circuit designers choose the suitable type based on factors such as the required voltage, current, speed, and application-specific requirements.

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FET Specifications

FET specifications are important for circuit designers to ensure that the FET operates within the desired parameters for a given application. The datasheet provides detailed information to help designers make informed decisions about FET selection and usage in electronic circuits. Field Effect Transistors (FETs) are important electronic components used in a wide range of applications. To fully understand their performance, it is essential to familiarize ourselves with their specifications and parameters.

  • Drain-Source Breakdown Voltage (BVDS) : This parameter indicates the maximum voltage that can be applied between the drain and source terminals without causing breakdown. It is crucial to select a FET with a BVDS rating higher than the expected operating voltage to ensure reliable operation.
  • Gate-Source Voltage (VGS) Range : VGS represents the voltage applied between the gate and source terminals to control the FET's conductivity. The specified range determines the voltage levels required for proper operation.
  • Drain Current (ID) : ID is the maximum current that can flow through the drain terminal when the FET is fully turned on. It is essential to choose a FET with an ID rating that exceeds the expected current requirements to prevent overheating and potential damage.
  • On-Resistance (RDS(on)) : RDS(on) is a measure of the FET's resistance when it is fully turned on. Lower values indicate better conductivity and efficiency. It is important to select a FET with a low RDS(on) to minimize power losses.
  • Gate Threshold Voltage (VGS(th)) : VGS(th) is the minimum voltage required between the gate and source terminals to turn the FET on. It is crucial to ensure that the controlling circuit can provide a sufficient voltage to meet this threshold.
  • Capacitances : FETs have various capacitances that affect their high-frequency performance. These include input capacitance (Ciss), output capacitance (Coss), and transfer capacitance (Crss). Understanding these capacitances helps optimize the FET's performance in high-frequency applications.

By considering these specifications and parameters, engineers and designers can select the most suitable FET for their specific application, ensuring optimal performance and reliability.


In conclusion, Field-Effect Transistors (FETs) stand as integral components in the world of electronics, providing unique advantages such as high input impedance, low power consumption, and rapid switching capabilities. Their applications span from signal amplification in audio systems to high-frequency switching in digital circuits. Designing circuits with FETs necessitates a deep understanding of their characteristics, types, and specifications. As technology advances, FETs are poised to play an increasingly significant role in shaping the future of electronic devices and systems. Engineers and designers must stay abreast of the latest developments in FET technology to harness their full potential and contribute to the ever-evolving landscape of electronic innovation.

Whether in consumer electronics, telecommunications, or power systems, FETs will undoubtedly continue to be key players in enabling progress and efficiency in electronic designs. Field-Effect Transistors (FETs) are semiconductor devices that leverage an electric field to control the flow of current. With their high input impedance, they are ideal for applications requiring signal amplification and control. FETs find use in amplifiers, switches, oscillators, sensors, and power supplies, among others. Their versatility and unique characteristics have solidified their position as integral components in modern electronic systems.

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