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.
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 gate voltage is applied:
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.