What is Field Effect Transistor?
Field Effect Transistor is a three terminal unipolar device. It is a voltage controlled device unlike the BJT which is the current controlled device. FET also have three terminals such as Drain, Source and Gate. The voltage which is given in the gate terminal as input controls the current passing through it.
The input voltage is proportional to the output current. The operation of the FET depends on the electric field, thus it is called as Field Effect Transistor. The advantage of FET is it is smaller than the BJT, consumes less power when compared to BJT and power dissipation is also less.
What is Junction Field Effect Transistor (JFET)?
JFET is one of the types of FET. It is voltage controlled device. JFET is used as a voltage controlled resistor or switch or as an amplifier. There are two types of JFET namely N-Channel JFET and P-Channel JFET.
In N channel the generation of the current is due to the movement of electrons, in P channel the generation of current is due to the movement of holes. Since the movement of electrons is faster than the movement of holes, N- Channel JFET is preferred more than the P-Channel JFET.
Symbol of N channel and P channel JFET:
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| Symbol of N channel and P channel JFET |
JFET has three terminals Drain, Source and Gate equivalent to Collector, Base and Emitter in BJT. BJT have PN Junctions but JFET have channels made of either P type or N type. In P-Channel JFET arrow points outside and in N-Channel JFET arrow points inside.
Three terminals of JFET:
Source:
The majority carriers enters the Field Effect transistor through the source terminal.
Gate:
By controlling the Gate voltage the flow of majority carriers from Source to drain can be controlled.
Drain:
The majority charge carriers leaves the channel through drain. This current is designated by the source gate voltage VGS
Channel:
The region between source and drain and also between the gate region is called the channel.
Construction of N channel and P channel JFET:
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| Construction of P channel |
As it is shown in the figure in P Channel JFET in the P type substrate drain and source terminals are taken by creating ohmic contacts.
N type material is used at the either side of the channel and the gate terminals are taken from the N type materials.
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| Construction of N channel |
In N Channel JFET in the N type substrate drain and source terminals are taken by creating ohmic contacts and P type material is used at the either side of the channel and the gate terminals are taken from the P type materials.
The drain and source terminal is taken from the both the sides of the channel. A small voltage at the Gate terminal controls the current flow between source and drain.
Working of N channel and P channel JFET:
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| Working of N channel when Bias is zero |
The Gate to source voltage controls the flow of current from source to drain. When no gate to source voltage is applied or when bias is zero the resistance in the channel is low. So when a small drain to source voltage is applied it causes the current to flow from source to drain.
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| Working of N channel when moderate bias is applied |
When the gate terminal is reverse biased or when negative gate source voltage is applied the size of the depletion layer increases and the resistance in the channel increases. Thus it controls the flow of current in the channel.
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| Working of N channel when bias greater than Pinch off is applied |
When the reverse bias at the gate terminal is increased the depletion region increases and they meet at a point of time. This is called Pinch off mode. At this point the source to drain current is zero, but some reverse saturation current exists. The gate to source voltage at which Pinch off occurs is called Pinch off voltage.
V-I Characteristics of JFET:
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| V-I Characteristics of JFET |
Ohmic Region:
It is the region when gate source voltage is zero VGS=0
Saturation Region:
In this region JFET is ON and active.
Cut Off Region:
JFET is OFF in this region and there is no drain current flowing.
Breakdown Region:
When the drain voltage exceeds the maximum necessary voltage, current flows from drain to source without any resistance and the device breakdown and the JFET enters the breakdown region.
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