JFET - Junction Field Effect Transistor

The junction field effect transistor, JFET is a useful active device for many circuits. Consisting of a semiconductor channel with a reverse biased diode in the middle its basic concept is quite straightforward

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The junction field effect transistor or JFET is widely used in electronics circuits. The junction field effect transistor is a reliable and useful circuit component that can be used very easily in a variety of electronic circuits ranging from JFET amplifiers to JFET switch circuits.

The junction field effect transistor is freely available, and JFETs can be bought for very little money. This makes them ideal for use in many circuits where cost and performance are of interest.

JFET basics

Basically a field effect transistor or 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.

N and P channel junction FET circuit symbol
Junction FET circuit symbol

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.

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.

 Junction FET working below saturation
Junction FET working below saturation

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 circuit applications

JFETs are very useful active devices and as a result they are used in many electronic circuit designs. They offer a number of distinct advantages that can be put to use in many circuits.

  • Simple biasing
  • High input impedance
  • Low noise

In view of their characteristics, JFETs are seen in many circuits ranging from amplifiers to oscillators, and logic switches to filters and many more applications.

JFET structure & fabrication

JFETs can be either N-channel of P-channel devices. They can be made in very similar manners, the main exception being that the N and P areas in the structure below are interchanged.

Often devices are made within a larger substrate and the FET itself fabricated as shown in the diagram below.

Typical JFET structure
Typical 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.

Although the JFET is less popular than the MOSFET and fewer JFETs, it is still a very useful component. Offering high input impedance, simple biasing, low noise, and a low cost, it provides a high level of performance which can be used in many situations.

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