Junction FET, JFET Tutorial

- the JFET, or junction field effect transistor is one of the easier forms of FET to manufacture and as such it was the first. However it is still widely used and is able to provide the performance levels that are needed in many of today's circuits.

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.

JFET circuit symbol

The JFET symbol indicates the channel as the main line between the drain and source. Sometimes the drain and source will be marked with the letters to indicate which connection is which.

The JFET symbol shows the gate as the connection from the channel. IT has an arrow on it. This arrow points towards the channel for an N-channel JFET, and away from it for a P channel JFET.

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 structure

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.

JFET fabrication

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.

JFET operation

The Junction FET is a voltage controlled device. In other words, voltages appearing on the gate, control the operation of the device.

Both N-channel and P-channel devices operate in similar ways, although the charge carriers are inverted, i.e. electrons in one and holes in the other. The case for the N-channel device will be described as this is the more commonly type used.

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.

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

JFET circuits

JFETs are widely used in many types of circuit as FETs offer advantages over bipolar transistors in many applications:

  • High input impedance
  • Simple biasing
  • Low noise

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


Summary

Although the JFET is less popular than the MOSFET and fewer JFETs are used, it nevertheless fulfils a vital role in many applications. It is a relatively simple semiconductor electronics component simple to fabricate, and in addition to this 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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