Junction FET, JFET Tutorial

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

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.

The circuit symbols for both N and P channel junction FETs showing the channel with the drain and source and the gate with the arrow on the gate terminal showing the channel characteristics
Junction FET circuit symbol


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.

Structure of a JFET device

Structure of a JFET device

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.

Structure of a JFET device

Structure of a JFET device


Summary

Although the 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.

By Ian Poole


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