Overview: This article provides an overview of field effect transistors, explaining the structure, principle of operation, advantages, types, and applications. It also compares FETs with bipolar junction transistors.

The continuous reduction in transistor size has led to remarkable advances in microchip capabilities. One of the most significant inventions of the past century is the field effect transistor (FET). Because of their superior electrical characteristics, FETs have been extensively researched.

What is a Field Effect Transistor?

A FET is a three-terminal semiconductor device that uses an electric field to control current flow. The three terminals, as shown in Fig. 1, are:

  • Gate (G)
  • Source (S)
  • Drain (D)

Fig. 1 Product image and internal schematics of a field effect transistor. Source: Rakesh Kumar, Ph.D.

Structure and Working Principle

The source and drain terminals connect to form a semiconductor channel, while the gate terminal is the third control electrode that modulates current flow. The fundamental operating principle is that voltage applied between the gate and source terminals creates an electric field that controls the current flow between the source and drain. This makes the FET a voltage-controlled device, which is one of its primary advantages. They are fundamental building blocks of modern integrated circuits and computers. 

FET Vs. BJT

FETs and bipolar junction transistors (BJT) operate on fundamentally different principles, which are listed below

  • BJTs function as current-controlled devices, with the input base current controlling the output collector current, in contrast to FETs, which are voltage-controlled devices.
  • FETs have lower power consumption than BJTs because of minimal gate current requirements during operation. In contrast, BJTs require continuous base current to maintain conduction and increase power consumption.
  • Due to their insulated gate structure, FETs feature very high input impedance (in the megaohm range). BJTs, however, exhibit relatively low input impedance (in kiloohm range) due to their current-controlled nature.
  • FETs are unipolar devices, utilizing only one type of charge carrier—either electrons or holes—for current conduction. However, BJTs are bipolar devices and rely on both electrons and holes.
  • FETs have compact physical dimensions compared to BJTs and faster switching speed, making them particularly advantageous for integrated circuit applications.
  • There is no production or recombination noise in FET devices because minority carriers are not part of the conduction process.
  • FETs are more radiation-resistant and less temperature-dependent compared to BJT.

Types of FETs

The two primary types of FETs, as shown in Fig. 2, are

Fig. 2 Classification of field effect transistors. Source: Rakesh Kumar, Ph.D.

Junctional Field Effect Transistor

A junction field-effect transistor (JFET) is a three-terminal semiconductor device that controls current flow using an electric field. The JFET utilizes a reverse-biased p-n junction to create a depletion region that controls current flow through the semiconductor channel. It is a voltage-controlled, unipolar device that operates using a majority of charge carriers. There are two types of JFETs based on their channel composition, as shown in Fig. 3.

  • N-channel JFET
  • P-channel JFET

Fig. 3 Illustration of junctional field effect transistor. Source: Rakesh Kumar, Ph.D.

A JFET To Consider: JFE150

The JFE150 N-channel JFET, as shown in Fig. 4, is a significant advancement in JFET technology developed by Texas Instruments using a high-performance analog bipolar process. The N-Channel JFE150 has improved noise performance with ultra-low voltage noise levels of 0.8 nV/√Hz at 1 kHz.

Fig. 4 JFE150DBVT by Texas Instruments. Source: oemsecrets

The device features several technical characteristics, including an extremely low gate current (maximum 10 pA), low input capacitance of 24 pF, and a high breakdown voltage of -40 V for gate-to-drain and gate-to-source connections. Its high transconductance of 68 mS enables precise signal handling across various applications.

The JFET is designed for diverse applications, including professional audio mixers, guitar amplifiers, condition monitoring sensors, and high-impedance transducer interfaces. It's available in compact packaging (small SC70 and SOT-23 formats).

Insulated Gate Field Effect Transistor

Insulated gate field effect transistors (IGFET) employ a different approach to gate control. Instead of a p-n junction, they use a metal gate electrode separated from the semiconductor channel by a thin insulating oxide layer, typically silicon dioxide. This structure allows for more precise control over the channel current while providing excellent input isolation. The most popular type of IGFET is the MOSFET (metal oxide semiconductor field effect transistor), which is more commonly employed. 

The electric field created at the gate terminal either increases or decreases the quantity of charge carriers when voltage is supplied. Based on the channel type involved and operating mode, it is classified, as shown in Fig. 5, as 

  • N-Channel Enhancement MOSFET
  • N-Channel Depletion MOSFET
  • P-Channel Enhancement MOSFET
  • P-Channel Depletion MOSFET

Fig. 5 Diagrammatic illustration of different types of MOSFETs. Source: MDPI

Enhancement Type

They don't have a built-in conducting channel, so they are usually off in normal conditions. When the gate voltage is applied, current flows between the source and drain terminals. Because of this, enhancement-type MOSFETs are the best choice for digital circuits and new integrated circuits and are extensively employed due to polycrystalline silicon gate materials.

Depletion Type

When no gate voltage is applied, these devices have a built-in path for electricity to flow between the source and drain contacts. A gate voltage can remove charge carriers from the channel, which lowers its conductivity.

A MOSFET To Consider: SPW47N60C3

The SPW47N60C3, as shown in Fig. 6, is the most effective power MOSFET technology with its high voltage capabilities, featuring a 650 V drain-source voltage rating and an exceptionally low on-resistance of 0.07 Ω. It has remarkable power handling capabilities, managing continuous drain currents up to 47 A and delivering outstanding performance across various electrical and thermal parameters.

Fig. 6 SPW47N60C3 by Infineon Technologies Source: oemsecrets

These include ultra-low gate charge characteristics and extreme dV/dt rating. It has the ability to maintain consistent performance across wide temperature ranges (-55 °C to +150 °C) with RoHS compliance and lead-free packaging. They are particularly suitable for industrial, automotive, and renewable energy applications.

Applications

  • Their low power consumption and efficient operation make FETs advantageous in high-power applications, switch-mode power supplies, motor controllers, etc.
  • Their low noise level and improved signal quality are important in wireless communications equipment, including mobile phones, radar systems, etc.
  • Their relatively high input impedance enables them to be utilized in oscilloscopes, voltmeters, and measuring devices.
  • Due to their resistance to radiation, FETs are utilized in amplifiers needed for GHz frequency range satellite transmission.
  • FETs are fundamental to modern integrated circuits because of their compact size and high packing density capability.
  • FETs excel as analog switches due to their fast switching speeds and low on resistance

Summarizing the Key Points

  • FETs are three-terminal devices that control current flow using an electric field, making them essential in modern electronics for efficient operation and low power consumption.
  • FETs differ from bipolar junction transistors as they are voltage-controlled, unipolar devices and exhibit significantly lower power consumption during operation.
  • The two main types of FETs are MOSFETs and JFETs, each with unique characteristics and applications, such as MOSFETs being ideal for digital circuits and JFETs for high-performance analog applications.
  • FETs are widely used in various applications, including switch-mode power supplies, motor controllers, and amplifiers, due to their compact size, high packing density, and fast switching capabilities.

Reference

Lin, J. (2022). Advancement and Challenges of Field Effect Transistors based on Multi-gate Transistor. Journal of Physics Conference Series, 2370(1), 012004.
https://doi.org/10.1088/1742-6596/2370/1/012004 

Jiya, I. N., & Gouws, R. (2020). Overview of Power Electronic Switches: A Summary of the Past, State-of-the-Art and Illumination of the Future. Micromachines, 11(12), 1116.
https://doi.org/10.3390/mi11121116

Guran, I., Florescu, A., & Perișoară, L. A. (2022). A novel ON-State resistance modeling technique for MOSFET power switches. Mathematics, 11(1), 72.
https://doi.org/10.3390/math11010072

ALL ABOUT ELECTRONICS. (2019a, January 18). What is Field Effect Transistor (FET)? || Differences between BJT and FET || Types of FET [Video]. YouTube.
https://www.youtube.com/watch?v=cOICDYuY-gA

Oemsecrets. (n.d.). JFE150 - Compare Distributor Prices & Inventory | oemsecrets.com. oemsecrets.com. https://www.oemsecrets.com/compare/JFE150

Oemsecrets. (n.d.). SPW47N60C3 - Compare Distributor Prices & Inventory | oemsecrets.com. oemsecrets.com.
https://www.oemsecrets.com/compare/SPW47N60C3