Junction FET, JFET Tutorial

- overview or tutorial about the basics and essential details of the Junction FET or JFET, its structure, operation, circuits and circuit design, and its applications.

Field effect transistor, FET technology includes:

The Junction FET or JFET is one of the more widely used electronics components. It was the first form of field effect transistor to be developed and it is used in many applications within electronics design.

The junction FET or JFET is an electronics component that used both in the form of discrete electronics components, and also within integrated circuits. However, the younger relation of the JFET, the MOSFET is the one that is most widely used in integrated circuits as a result of the fact that it can be used in ultra-low power circuits - an essential parameter for any large scale integrated circuit design.

Junction FET, JFET 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.

FET basics

The JFET uses the same basic concepts of operation that are used by any filed effect transistor. Basically a FET consists of a section of silicon whose conductance is controlled by an electric field. The section of silicon through which the current flows is called the channel, and it consists of one type of silicon, either n-type or p-type. The connections at either end of the device are known as the source and drain. 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.

Junction FET

In order to understand how a FET operates it is helpful to look at its construction. Here an n channel JFET can be seen. This type is shown because it is more common than the alternative p channel JFET. However the same principles apply, the only changes that need to be made are that n-type material is replaced by p-type and so forth, and holes are used as the majority carriers instead of electrons.

In the n-channel FET the channel itself is formed within a p-type substrate as shown, and a further p-type area acts as the gate. The junction between the channel and p-type gate has a depletion layer. The thickness of this layer varies in accordance with the magnitude of the reverse bias on the junction. In other words when there is a small reverse bias the depletion layer only extends a small way into the channel and there is a large area to conduct current. When a large negative bias is placed on the gate, the depletion layer increases, extending further into the channel, reducing there area over which current can be conducted. With increasing bias the depletion layer will eventually increase to the degree that it extends right across the channel, and the channel is said to be cut off.

When a current flows in the channel the situation becomes slightly different. With no gate voltage electrons in the channel (assuming an n-type channel) will be attracted by the positive potential on the drain, and will flow towards it enabling a current to flow within the device, and hence within the external circuit. The magnitude of the current is dependent upon a number of factors and included the cross sectional area of the channel, its length and conductivity (i.e. the number of free electrons in the material) and the voltage applied.

From this it can be seen that the channel acts as a resistor, and there will be a voltage drop along its length. As a result of this it means that the p-n junction becomes progressively more reverse biased as the drain is approached. Consequently the depletion layer takes becomes thicker nearer the drain as shown. As the reverse bias on the gate is increased a point is reached where the channel is almost closed off by the depletion layer. However the channel never completely closes. The reason for this is that the electrostatic forces between the electrons cause them to spread out, giving a counter effect to the increase in thickness of the depletion layer. After a certain point the field around the electrons flowing in the channel successfully opposes any further increase in the depletion layer. The voltage at which the depletion layer reaches its maximum is called the pinch off voltage.

JFET structure

There are a number of ways in which FETs can be fabricated. For silicon devices a heavily doped substrate normally acts as a second gate. The active n-type region may then be grown using epitaxy, or it may be formed by diffusing the impurities into the substrate or by ion implantation. Where gallium arsenide is used the substrate is formed from a semi-insulating intrinsic layer. This reduces the levels of any stray capacitances and enables good high frequency performance to be obtained. Whatever the material used for the FET, the distance between the drain and source is important and should be kept to a minimum. This reduces the transit times where high frequency performance is required, and gives a low on resistance that is vital when the device is to be used for power or switching applications.

Summary

Althought he JFET is less popular than the MOSFET and fewer JFEts are used, it nevertheless fulfills a vital role in many applications. It is a relatively simple semiconductor electronics component simple to fabricate, and in addition tot his it is robust. As a result it is sometimes used as a power transistor. However junctions FETs are widely used as simple cheap general purpose FETs for use in many circuits and applications.

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