What is a FET: Field Effect Transistor Types

- overview of the field effect transistor detailing the basic concepts behind the FET and the different types of FET available..

The field effect transistor, FET has become a very widely used device within the electronics industry.

Although it entered the mainstream electronics scene later than the bipolar transistors, it has very many uses and overall it is more widely used than any other active device.

Many different types of FET are used from discrete small signal devices, through to power electronics. However it is for its use within integrated circuits that the field effect transistor has gained a really dominant place. It is able to use much lower levels of current and as a result it has enabled far higher degrees of integration to be attained than would have been possible by any other means.

FET history

The idea of the FET has been known for many years. It has some of its earliest foundations in a proposal made by Lilienfield in 1926, and to another paper by Heil in 1935.

Then during the 1940s Bell Laboratories set up a semiconductor research group. They investigated a number of areas pertaining to semiconductors and semiconductor technology, one of which was a device that would modulate the current flowing in a semiconductor channel buy placing an electric field close to it.

During these early experiments, the researchers were unable to make the idea work, turning their ideas to another idea and ultimately inventing another form of semiconductor electronics component: the bipolar transistor.

After this much of the semiconductor research was focussed on improving the bipolar transistor, and the idea for a field effect transistor was not fully investigated for some while. Now FETs are very widely used, providing the main active element in many integrated circuits. Without them electronics technology would be very different to what it is now.

What is a Field Effect Transistor?

The FET is based around the concept that charge on a nearby object can attract charges within a semiconductor channel.

The FET consists of a semiconductor channel with electrodes at either end referred to as the drain and the source.

A control electrode called the gate is placed in very close proximity to the channel so that its electric charge is able to affect the channel

In this way, the gate of the FET controls the flow of carriers (electrons or holes) flowing from the source to drain. It does this by controlling the size and shape of the conductive channel.

The semiconductor channel where the current flow occurs may be either P-type or N-type. This gives rise to two types or categories of FET known as P-Channel and N-Channel FETs.

The circuit symbol of a basic junction FET showing the channel with the drain and source and the gate which acts as the control electrode.
Junction FET circuit symbol

The electric field to control the current is applied to a third electrode known as a gate.

As it is only the electric field that controls the current flowing in the channel, the device is said to be voltage operated and it has a high input impedance, usually many megohms. This can be a distinct advantage over the bipolar transistor that is current operated and has a much lower input impedance.

The external field on the gate may serve to deplete the channel of carriers, in which case the FET is known as a depletion mode FET, or it may serve to enhance the carriers in the channel when it is known as an enhancement mode FET.

The conditions where an N channel JFET is working below its saturation.
Junction FET working below saturation

FET circuits

Field effect transistors are widely used in all forms of circuit from those used in circuits with discrete components, to those employed in integrated circuits.

Note on FET circuit design:

FETs can be used in a whole variety of circuits. Like the bipolar transistor, there are basic circuits. These include the common source, common drain and common gate. These form the basis of FET circuits.

Click on the link for further information about FET circuit design

Field Effect Transistor types

There are many ways to define the different types of FET that are available. They may be categorised in a number of ways, but some of the major types of FET can be covered in the tree diagram below.

Diagram showing how the different types of FET are related to each other
Tree diagram of FET types

There are many different types of FET on the market for which there are various names. Some of the major categories are delayed below.

  • Junction FET, JFET:   As the name implies, this form of FET uses a reverse biased diode junction to provide the isolation from the channel. It is the most basic type of FET, and the one that was first developed. However it still provides excellent service in many areas of electronics. Read more about the Junction FET
  • Insulated Gate FET / Metal Oxide Silicon FET MOSFET:   This type of FET uses an insulated later between the gate and the channel. Typically this is formed from a layer of oxide of the semiconductor. The most common type of IGFET. The MOSFET - Metal Oxide Silicon FET. Here the gate is made of a layer of metal set down on the silicon oxide which in turn is on the silicon channel. Read more about the MOSFET
  • Dual Gate MOSFET:   This is a specialised form of MOSFET that has two gates in series along the channel. This enables some considerable performance improvements to be made, especially at RF, when compared to single gate devices. Read more about the Dual Gate MOSFET
  • MESFET:   The MEtal Silicon FET is normally fabricated using gallium Arsenide and is often referred to as a GaAs FET. It provides very high performance, but in view of the gate structure, it is very sensitive to ESD. Read more about the MESFET / GaAsFET
  • HEMT / PHEMT:   The High Electron Mobility Transistor and Pseudomorphic High Electron Mobility Transistor are developments of the basic FET concept, but developed to enable very high frequency operation. Read more about the HEMT / PHEMT
  • FinFET:   FinFET technology is now being used within integrated circuits to enable higher levels of integration to be achieved by allowing smaller feature sizes. Read more about the FinFET
  • VMOS:   VMOS standard for vertical MOS. It is a type of FET that uses a vertical current flow to improve the switching and current carrying performance. VMOS FETs are widely used for power applications. Read more about the VMOS

This list includes many of the major FET types. In addition to these there are many proprietary names of FETs that are used.

By Ian Poole

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