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An introduction to JFET: JunctionField Effect Transistor

Field Effect Transistor

· The PN junction has two dominant feature which are:

1. Injection of minority carriers with forward bias

The first point is used to construct BJT i.e. bipolar junction transistors where there is injection of minority carriers in FB. Thus both type of carriers are present in the functioning of BJT. Hence it’s a bipolar device.


2. Depletion width variation when reverse bias.

JFET depends on the control of depletion region width when reverse baised. Here the flow is of majority carriers. And so it’s a UNIPOLAR device.

· FET is a three terminal device namely gate, drain and source.

· FET is a voltage controlled device. It has high input impedance.

· FETs are classified into JFET(Junction field effect transistor), MESFET(metal semiconductor field effect transistor), MISFET(metal insulator field effect transistor)

In JFET, gate voltage controls the depletion region width. In MESFET, PN junction is replaced by Schottky barrier junction. MISFET, the gate junction is separated from the insulator region with the help of insulating material. MOSFET (metal oxide field effect transistor) comes in this type when insulating material is oxide layer.

· These types of devices are mainly used for switching in between conducting stage and non conducting stage. Used mainly in digital circuits and integrated devices.

Basic Construction:

JFETs are mainly divided into two types: 1. N-channel JFETs where electrons are majority carriers and 2. P-channel JFETs where holes are majority carriers.

We will discuss the construction of N-channel JFET. Here N type of material of silicon is referred to as the channel. P-type silicon is diffused to the two opposite side of the N-type material. This two P-type are connected internally and a common terminal is taken out from the two called the gate. Ohmic contacts are given to the bar on the other two sides, one side is referred to as the source and the other is referred as the drain. Fig 1. shows N-type JFET. N channel JFET are more preferred since mobility of electrons is more than that of holes.

To understand the working, let us study the analogy of water tap. The source of the water can be treated as the applied voltage between the drain and the source of the JFET. This results in the flow of water (electrons). The tap which controls the flow of water is analogous to the gate terminal(controls the flow of charge). This water flow to the drain analogous to the drain in JFET which collects the electrons.

In fig 1. The gate is P region while the source and the drain are N regions. Gate source form one diode and gate-drain forms the other diode. JFET is a silicon device, so barrier potential for forward bias remains 0.7V.

Fig 2. Shows the schematic symbol for JFET. The verticalline can be thought as the channel to which the source and the drain are connected. The arrow direction on the gate indicaes the direcition of gate current. N JFET arrows points inwards and P JFET arrow points outwards.


Operation of JFET

1. To study the working of JFET let us first understand the polarity in which it is connected. In both N and P type JFET the supply between the gate and the drain is such that the gate is reversed biased. This is normal method to connect the JFET. The source terminal is connected to that end of the drain supply which gives the necessary charge carriers. In N channel JFET source is connected to negative terminal of drain voltage supply. And in P channel S is connected to positive end of drain voltage supply. This is shown in figure 2.

2. When no supply is applied to the gate(VGS = 0) and no supply is given to drain wrt source (VDS=0), the depletion region are of equal thickness around both the junctions.

3. When positive voltage is applied to the drain terminal D wrt to the source S without connecting the gate terminal G to supply the electrons flow from terminal S to terminal D whereas conventional drain current ID flows from drain D to source S. Due to the continuous flow of this current, there is voltage drop across the channel resistance.this drops reverse bias of the junction, gate being more negative. Hence depletion region penetrates more into the channel at the points closer to drain than the source thus wedge-shaped depletion region is formed.

4. Now apply the gate voltage. Let us apply the negative voltage to gate as compared to the source while the drain is kept positive because of this the P region is heavily doped as compared to the N region so the depletion region penetrates more into the channel. This depletion region acts as the insulator because of which the channel is narrowed, the resistance is increased and the drain current.

The amount of reverse bias is not the same. when the drain current flow through the channel, there is voltage drop across the length of the channel. The result is that the voltage dropat the drain end is more making the width of the depletion layer more towards the drain than the source end.

5. If the negative voltage to the gate is further increased, depletion layers meet at the center and the drain current is cut-off immediately.and if the negative bias is reduced then the resistance decreases and the drain current increases. The gate to source voltage VGS at which the drain current iscompletely cut –off is called the pinch off voltage.

The device is called Field Effect Transistor because the drain current is controlled by the effect of the extension of the field associated with the depletion region developed by the reverse bias at the gate.

Applications

1. They are used as input amplifiers in oscilloscopes, electronic voltmeters and other measuring and testing devices because of high input impedance.

2. It is used in RF amplifiers in tuning devices for low noise levels.

3. Since it is voltage controlled device it is uesedin Voltage variable resistors and tone controllers.

4. It is used as an analog switch.

5. Since it has small coupling capacitors, it is used in low frequency amplifiers in hearing aids and inductive transducers.


Advantages and disadvantages


1. Its working depends on the flow of majority carriers only so it’s a unipolar device.

2. It’s easy to fabricate, small in size and rugged construction. It has long life and high efficiency.

3. It has high input impedance. So less chance of loading or errors.

4. Its carries very small current and hence acts a vacuum tube.

5. It has high frequency response.

The drawback is that it has relatively small gain bandwidth. It cost is high compared to BJTs.
Jeffrey Arulraj
Jeffrey Arulraj • Jan 30, 2018
When the frequency of operations are clearly specified the cost of using a bjt and get becomes significant.

When u have high frequency operations using a bjt will be costlier than MOSFET. So based on ur needs cost will vary

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