JFET

JEFT refer to junction field effect transistor .A bipolar junction has major two disadvantage ,one is low input impedance because of forward biased of transistor and high noise level.To over come those two disadvantage JFET is used in many application instead of BJT.Bipolar junction transistor is a current controlled electronics devices that means the output characteristics of BJT are controlled by input base current where as FET is a electronics devices whose output characteristics of its is controlled by input voltage (i.e electric field).JFET is a one type of FET (i.e field effect transistor).It is used in IC (i.e integrate-circuits) application so it is very very important semiconductor devices in this field. The JFET have three terminal and conduction is done by either electrons or holes charge carriers where as in BJT(i.e bipolar junction transistor ) the current conduction is done by both charge carriers (i.e holes and electrons) .

Symbol of JFET:

Construction of JFET:

This figure shows the construction of N-channel JFET .Here is only one type (i.e n-type ) semiconductor material is used as bar with two PN junction layer at the side of the this bar as shown in figure below.Conducting channel for the charge carriers is formed by the bar material . At the side of the bar material P-type material is diffused in proper amount if the bar is N-type material .If this conducting channel is N-type s/c material and diffused material is P-type then this type of JFET is called N-channel JFET where as If the conducting channel is P-type material and diffused material is N-type then this type of JFET is called P-channel JFET.

This above figure is N-channel JFET. In this construction two PN junction forms such as two diodes connection internally at the side of the bar.The terminal which is taken out from diffused material is called Gate terminal (G). And other two terminal which are taken out from Bar material is called Drain terminal (D) and source terminal (S).So JFET has three terminals such as gate ,drain and source terminals.

Polarities of JFET:

This above figure shows the general polaritie

s of N-channel JFET where as the polarities of P-channel JFET is just opposite is that of N-channel JFET.
JFET is three terminal electronics devices. It consists of Gate terminal ,Drain terminal and Source terminal .In put gate voltage or potential between the gate terminal and the source terminal is always reverse biased in the both type of the JFET and source terminal is such that way internal PN junction diode becomes reverse biased connection .That why JFET has high input impedance .The drain terminal and source terminal can interchange But one things can be always take care that the drain terminal is so c

onnected w.r.t source that the drain current is always flows from source terminal to the drain terminal and the amount of source current is equal to the drain current.

Principle of JFET:

This JFET circuit shows N-channel JFET with general polarities where gate input voltage is reverse bias with source.In this JFET circuit the two depletion layers are formed at the side of two PN junction in its internal body .This is N-channel JFET circuits so current conduction is

done by free electrons where as in the case of P-channel JFET circuits polarities is just opposite that of N-channel JFET circuits and current conduction is done by hole charge carriers . In the case of fixed width of conduction channel its resistance can be controlled by varying input gate voltage or potential. If the reverse input gate voltage is high the area of depletion layers is increase and conducting channel is decrease .Hence drain current is decrease .So conducting channel is varied by changing the reverse input gate voltage (i.e amount of drain current can be change according to input gate voltage .

Working of JFET:It has two condition as below:

• When Gate voltage is zero :

When the input of the biased is open (i.e potential deference between the drain and source terminal is zero or gate voltage is zero as shown in figure below .The depletion layers is form at the side of the bar by the two PN junction .Under this condition current conduction from source to drain via conduction channel between two depletion layers determine the width of the conducting channel and amount of current flow.

• When gate voltage is applied:

Connection of the gate terminal and source terminal is reverse biased .If the input voltage is applied or closed to gate and source terminal then the width of the depletion layers is increased .Hence the width of the conducting channel is decreased and resistance of N-type conducting channel bar is increased .As a result flow of current from source terminal to the drain terminal is decrease because of its resistance .Just opposite action will happen if input gate or reverse voltage is decrease .Hence the drain current is increased because of its resistance decrease.Hence it is note that flow of current from source to drain can be controlled by reveres gate input voltage .This is similar way , in the case of P-channel JFET but current conduction is done by holes instead of electrons in N-channel JFET and polarities is just opposite that of N-channel JFET.

Features of JFET: JFET has following features;

• In JFET gate-source PN junction of JFET is always worked in reverse biased connection.
• JFET is voltage controlled semiconductor devices with three terminals (i.e gate ,drain and source termianl)
• In JFET the output drin current is controlled by varying the width of the conducting channel.
• Gate current in JFET is zero.
• Since in JFET drain current is equal to the source current at that condition.
• JFET is not temperature sensitive devices.
• In JFET depletion layers of Pn junction increase or decrease by an equal amount because of same potential in two input gate terminals.
• JFET is working in two conditions that is when gate termianl voltage is zero and when gate terminal voltage is applied.

Advantages of JFET:

BJT is current controlled devices where as JFET is not current controlled devices but JFET is voltage-controlled semiconductor devices (i.e change in input voltage controlled the output drain current) .JFET is similar to a vacumm pentode but it has more advantages;

• In JFET input impedance of its is very high.
• It has high isolation between input circuits and output circuits.
• JFET is negative temperature co-efficient of resistance devices.
• JFET has no risk of thermal runway.
• JFET has long life.
• JFET has high efficiency .
• JFET is smaller in size.
• Power gain of JFET is very high ,Hence no needed of other amplifier stage.