Junction field effect transistor
The junction field effect transistor or JFET is a voltage controlled
three terminal unipolar device semiconductor device available in
N-channel or p-channel
The JFET is a three terminal device semiconductor device in which
current conduction is by one type of carrier that is electrons and
holes
The current conduction is controlled by means of an electric field
between the gate and the conducting channel of the device
The JFET has high input impedance and low noise level
Construction
The JFET consists of a p-type or n-type silicon bar containing two
pn junction at the sides as shown in figure the bar forms the
conducting channel for the charge carrier
The two p-n junction forming diode are connected internally and a
common terminal called gate is taken out
Other terminal are source and drain taken out the bar
Thus a JFET has three terminal device such as gate ,source and
drain
If the bar is of p-type it is called as p channel JFET
And if the bar is of n-type it is called as n-channel JFET
JFET working
In each case the voltage between the gate and the source is such
that the gate is reverse biased the source and the drain terminal
are reverse biased or interchangeable
The following points may be noted as:
The input circuit of a JFET is reverse biased this means that the
device has high input impedance
The drain is so reverse biased with respect to source that the
drain current flows from the source to drain
In all JFET source current is equal to the drain current
Principle and working of JFET
Principle of JFET: the circuit of n-channel JFET with normal
polarities the two p-n junction at the sides form two depletion
regions the current conduction is by one type of the charge
carrier is through the channel between the two depletion layers
and out of the drain the width and hence resistance of this
channel can be controlled by changing the voltage .
The greater the reverse volt5age the wider will be the depletion
layer and narrower will be the conducting channel the narrower
channel means greater resistance and hence source to drain
current decreases
Reverse will happen when gate source voltage decreases. thus
JFET operates on the principle that
Width and hence resistance3 of the conducting channel
decreases and can be varied by changing the reverse voltage
In other word the magnitude of the drain current can be changed
by altering
Working of JFET
The working of the JFET can be explained as follows :
Case:1 when a voltage is applied between drain and source
terminals on the gate is zero as shown in figure at the sides of the
bar establish depletion layers
The electrons will flow from the source to drain
