What is a FET: Field Effect Transistor Types & Technology
The FET or Field Effect Transistor is a three terminal device that uses an electric field to control the current flowing through the device - it also has a high input impedance which is useful in many circuits.
The field effect transistor, FET is a key active device for the electronics industry.
The FET used in many circuits constructed from discrete components in areas from RF technology to power control and switching to general amplification.
However the major use for the field effect transistor, FET is within integrated circuits. In this application FET circuits are able to consume only very small levels of power, and this enables the huge 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 accommodate within a single integrated circuit.
Field Effect Transistor, FET history
Before the first FETs were introduced into the market, the concept had been known for a number of years. There had been many difficulties in realising a 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 them electronics technology would be very different to what it is now.
Field Effect Transistor – the basics
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.
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 Field Effect Transistor Circuit Design:
The field transistor transistors can be used in many types of circuits although the three basic configurations are common source, common drain (source follower) and common gate. The circuit design itself if fairly straightforward and can be undertaken quite easily.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.
- Junction FET, JFET: The junction FET, or JFET uses a reverse biased diode junction to provide the gate connection. Essentially a small diode is fabricated onto the channel semiconductor. In operation this is reverse biased and this means that it is effectively isolated from the channel - only the diode reverse current can flow between the two. The JFET 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.
- Insulated Gate FET / Metal Oxide Silicon FET MOSFET: This type of FET uses an insulated layer between the gate and the channel. Typically this is formed from a layer of oxide of the semiconductor. The name IGFET refers to any type of FET that has an insulated gate. The most common form of IGFET is the silicon 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.
- 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. The second gate provides additional isolation between the input and output, and in addition to this it can be used in applications like mixing / multiplication.
- MESFET: The MEtal Silicon FET is normally fabricated using Gallium Arsenide and is often referred to as a GaAs FET. Often GaAsFETs are used for RF applications where they can provide high gain low noise performance. One of the drawbacks of GaAsFET technology results from the very small gate structure, and this makes its very sensitive to damage from static, ESD. Great care must be taken when handling these devices. 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. Although expensive, they enable very high frequencies and high levels of performance to be achieved.
- FinFET: FinFET technology is now being used within integrated circuits to enable higher levels of integration to be achieved by allowing smaller feature sizes. As higher density levels are needed and it becomes increasingly difficult to realise ever smaller feature sizes, FinFET technology is being used more widely.
- 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.
FET specifications
Apart from selecting a particular type of FET for any given circuit, 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 specifcations 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.
