Junction Field Effect Transistor is a semiconductor device in the family of field effect transistor. The field effect transistor is the type of transistor which being operated by the electric field applied across the junction of the device. There are mainly two types of field effect transistor. Junction Field Effect Transistor or JFET and Metal Oxide Semiconductor Field Effect Transistor or MOSFET. We shall discuss here in this article about Junction Field Effect Transistor.JFET is a voltage control device whereas BJT is a current control device. The current through JFET is caused due to the flow of majority carriers whereas in BJT flow of current is due to both majority and minority carriers. Since only majority carriers are involved in creations of current in JFET, it is a unipolar device. The input impedance of a JFET is very high.
Types of JFET
JFET is of two types – n channel JFET and p channel JFET.
N Channel JFET
An n channel JFET is made of Si or GaAs bar. The bar is doped with n-type impurities. One metallic terminal is attached to each of both ends of the bar. One of the terminals is called drain terminal, and the other is called the source terminal. Two sides of the bar are highly doped with p-type impurities. The region which doped with p-type impurities is called gate region. A metallic terminal is connected to the gate region, and the terminal is called the gate terminal.
P Channel JFET
Similarly, a p channel JFET is made of Si or GaAs bar doped with p-type impurities. The sides of the bar are highly doped with n-type impurities. Here also the drain, and the source terminal are connected to two ends of the bar. The terminal attached to the side n-type region is the gate terminal.
NB:- Here in both types of junction field effect transistor both drain and source terminal can be interchangeable.
If a voltage is applied between drain and source terminal, a current starts flowing through the device. The space between two oppositely doped regions is referred to as channel of the device. The current flowing through the channel due to drift of majority carriers. The majority carriers enter into the channel through the terminal is referred as to source terminal and the terminal through which the majority carriers leave the channel is referred as to drain terminal.
In normal operating condition the drain terminal of n channel JFET is applied with positive potential and the drain terminal of a p channel JFET is applied with negative potential. The gate voltage is kept such in a JFET that the PN junction between the gate region and the channel is in reverse biased condition. The width of the depletion layer of this PN junction can be varied by varying gate terminal voltage. The opening of the channel depends on the width of the depletion layer.
If by changing the gate terminal voltage, the width of the depletion layer increases, it is extended into the channel and reduces the opening of the channel and therefore the current through the channel gets decreased. Hence we can conclude that by controlling gate voltage we can control the drain current. Depending on that typical property of the JFET we can use this JFET for many different electronic applications. A JFET can be used as a switch, as an amplifier etc.
Properties of JFET
- This is a voltage control device since the current through the channel gets controlled by gate voltage. In other words, the electric field across the junction effects the operation of the transistor and that is why it is named as junction field effect transistor.
- As in normal operating condition, the junction between input gate region and channel remains to reverse biased, the input impedance of the transistor is high.
- Ideally, there will be no gate current in JFET.
- Majority carriers only contribute the current through the channel in the device, i.e., free electrons in n channel and holes in p channel and these are the reason why a transistor is called unipolar device.