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Bertha Leder

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FETs, Field Effect Transistors Includes: FET basics     FET specifications     JFET     MOSFET     Dual gate MOSFET     Power MOSFET     MESFET / GaAs FET     HEMT & PHEMT     FinFET technology     IGBT     Silicon carbide, SiC MOSFET     GaN FET / HEMT

The field effect transistor, FET is a key electronic component using within many areas of the electronics industry.

The FET used in many circuits constructed from discrete electronic components in areas from RF technology to power control and electronic switching to general amplification.

However the major use for the field effect transistor, FET is within integrated circuits. In this application FET circuits consume much lower levels of power than ICs using bipolar transistor technology. This enables the very large scale integrated circuits to operate. If bipolar technology was used the power consumption would be orders of magnitude greater and the power generated far too large to dissipate from the integrated circuit.

Apart from being used in integrated circuits, discrete versions of these semiconductor devices are available both as leaded electronic components and also as surface mount devices.

Before the first FETs were introduced into the electronic components market, the concept of these semiconductor devices had been known for a number of years. There had been many difficulties in realising this type of device and making it work.

Some of the early concepts for the field effect transistor were outlined in a paper by Lilienfield in 1926, and in another paper by Heil in 1935.

The next foundations were set in place during the 1940s at Bell Laboratories where the semiconductor research group was set up. This group 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 these electronic components electronics technology would be very different to what it is now.

The concept of the field effect transistor is based around the concept that charge on a nearby object can attract charges within a semiconductor channel. It essentially operates using an electric field effect - hence the name.

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.

In addition to this, there are two further categories. Increasing the voltage on the gate can either deplete or enhance the number of charge carriers available in the channel. As a result there are enhancement mode FET and depletion mode FETs.

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.

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

As the field effect transistor is a voltage operated semiconductor device rather than a current device like the bipolar transistor, this means that some aspects of the circuit are very different: the bias arrangements in particular. However electronic circuit design with FETs is relatively easy - it is just a bit different to that using bipolar transistors.

Using FETs, circuits like voltage amplifiers, buffers or current followers, oscillators, filters and many more can all be designed, and the circuit designs are very similar to those for bipolar transistors and even thermionic valves / vacuum tubes, although the bias arrangements are different. Interestingly valves / tubes are also voltage operated devices, and therefore their circuit designs are very similar, even in terms of the bias arrangements.

There are many ways to define the different types of FET that are available. The different types mean that during the electronic circuit design, there is a choice of the right electronic component for the circuit. By selecting the right device it is possible to obtain the best performance for the given circuit.

FETs may be categorised in a number of ways, but some of the major types of FET can be covered in the tree diagram below.

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

Although there are some other types of field effect transistor that may be seen in the literature, often these types are trade names for a particular technology and they are variants of some of the FET types listed above.

Apart from selecting a particular type of field effect transistor for any given circuit design, it is also necessary to understand the different specifications. In this way it is possible to ensure that the FET will operate to the required performance parameters.

FET specifications include everything from the maximum voltages and currents permissible to the capacitance levels and the transconductance. These all play a part in determining whether any particular FET is suitable for a given circuit or application.

Field affect transistor technology can be used in a number of areas where bipolar transistors are not as suitable: each of these semiconductor devices has its own advantages and disadvantages, and can be used to great effect in many circuits. The field effect transistor has a very high input impedance and is a voltage driven device and this opens it up to being used in many areas.

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