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Junction Field Effect Transistor

#FET stands for Field Effect Transistor. It is a voltage-controlled device. It is one of the simplest Field-Effect Transistors. Furthermore, it can be used as an #amplifier or switch. FETs require virtually no input current. The most important advantage of FET over #BJT is its high input impedance. It is a three-terminal device. The three terminals are Source, Drain, and #Gate. The terminals Source, #Drain, and Gate are similar to the #emitter, #collector, and counterpart of the #base of BJT respectively. Drain and #Source are connected by a channel. FETs are unipolar devices i.e., the operation of FET depends only on the majority of charge carriers. FETs are less noisy than BJT. Fabrication of FET is easier and is suitable for ICs.


FETs are classified as MOSFET and JFET. These are further classified as N-channel and P-channel devices.


Let us understand about JFET.


JFET is a Junction Field Effect Transistor. #JFET is classified as n channel and p channel based on their structure. The channel of N-channel JFET is of n-type and the channel of P-channel JFET is of p-type.




Schematic symbols of N-Channel and P-Channel JFETs are given below:

N - Channel FET
N - Channel FET
P - Channel FET
P - Channel FET














N-channel JFET:


The channel in n-channel JFET is of n-type. The symbol of N-channel JFET is shown below.


Construction and operation of n channel JFET:


Symbol for n - channel FET
Symbol for n - channel FET














If the gate is n-type material then the channel must be p-type material.

Channel is a piece of n-type material and two similar pieces of p-type material are attached to its sides. The ends of the channel are termed as drain and source and the gate terminal is formed by connecting the two p-type materials together. Here, the channel is of the n-type bar, hence it is known as N-channel JFET.


Basic operation:


The operation of JFET is mainly based on the varying width of the channel to control the drain current.


A PN junction is formed when two p-type materials are attached to the two sides of the n-type material. When a positive potential is applied at the drain, a negative potential is applied at the source, and the gate terminal is not connected, then the drain current (Id) flows.


When the gate terminal is biased negative with respect to the source terminal, the PN junction is reversed biased and a depletion region is formed. The channel is more lightly doped than p-type regions so that the depletion regions are deeply penetrated into the channel, and it behaves as an insulator. The channel gets narrower and the resistance of the channel increases and thereby the drain current is reduced. Further increase in negative bias voltage at the gate makes the depletion region meets the center of the channel and the drain current is cut off completely.


The channel width of the channel can be varied

  1. By varying the voltage Vgs

  2. By varying the voltage Vds keeping Vgs constant

1. By varying the voltage Vgs:


By varying the channel width the drain current can be varied. In the operation of FET, the PN junction is always reverse biased. As the reverse bias is increased the depletion region also increases thereby decreasing the effective width of the channel. Thus, the width of the channel can be varied by changing Vgs.


2. By varying the voltage Vds keeping Vgs constant:


When the gate to source voltage Vgs is zero and Vds is applied between the source and drain the flow of electrons takes place and thus the drain current (Id) flows through the channel.


When Vgs=0 for Id=0, the channel between the gate junction is completely open. When a small voltage Vds is applied the entire bar behaves similar to a simple semiconductor resistor and drain current, Id increases linearly with Vds.


The developed depletion region penetrates deeper into the channel near the drain and less towards the source. Therefore, reverse bias is high at the drain than at the source. Therefore, the effective width of the channel is decreased by the growing depletion region. Eventually, Vds is reached at which the channel is pinched off. This is the voltage at which the drain current Id begins to level off and approaches a constant value. Thus, the channel width is varied by varying Vds at constant Vgs.


Characteristics of N-channel JFET:


Characteristic curves are a family of curves that shows the relation between current and voltage.


There are two important characteristics of JFET. They are

  1. Drain or VI characteristics

  2. Transfer characteristics

1.Drain or VI characteristics:


The relation between drain to source voltage, Vds and drain current, Id is given by Drain #characteristics.



When Vds is increased, the drain current, Id also increases linearly up to a point known as knee point. This region is called the ohmic region.

With the increase in the drain to source voltage, the drain current also increases the drain current is increased slowly compared to the ohmic region.


It is because of the fact that there is an increase in voltage Vds. Due to the increase in Vds the reverse bias across the gate-source junction increases. As a result, the width of the depletion region is increased and the effective channel width is decreased.


All the drain to source voltage corresponding to point the channel width is decreased to a minimum value and is known as pinch-off. The drain to source voltage at which the channel pinch-off takes place is called pinch-off voltage (Vp).


Pinch-off region:


This is the region shown by the curve as the saturation region. It is also known as saturation or constant current region. The channel width is occupied by the depletion region, and it is more towards the drain than towards the source. As a result, the channel is limited and only a few carriers cross the channel from source to drain, causing a constant current in this region. FETs are operated in saturation while working as amplifiers. The drain current in this region is constant and the maximum value of it is IDSS.


Breakdown region:


In the Breakdown region with the increase in the drain to source voltage the drain current increases rapidly due to the avalanche effect.


Transfer Characteristics:


Transfer Characteristic curves show the relationship between drain current (Id) and gate to source voltage (Vgs) for different Vds values.


Transfer characteristics of N-channel JFET are shown below:



The value of Vgs at the cutoff point is termed as Vgsoff.

The upper end of the curve indicates the drain current which is equal to Idss. That is, the drain current is maximum when Vgs=0V.

The lower end indicates a voltage that is equal to Vgsoff.






 

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Written By: Nagapuri Swathi


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