The bipolar junction transistor is a current controlled device. In this transistor mainly base current controls the operation of the device. In BJT both minority and majority carriers are involved in the operation. On the other hand, a junction field effect transistor is a voltage controlled device, and only majority carriers take part in the operation. Before going to the basic working principle of Junction Field Effect Transistor, we first review the basic construction of the device because it helps us to understand the matter well.
Here channel of either p-type or n-type semiconductor is created between opposite type of semiconductor. That means if the channel is of p-type the surrounding would be of n-type and if the channel is of n-type the surrounding would be of p-type. Depending on the type of semiconductor used in the channel, there are two types of Junction Field Effect Transistor namely n channel JFET and p channel JFET.
For understanding the basic working principle of Junction Field Effect Transistor, we take here an n channel JFET although the working of P channel JFET is the same as that of n channel FET.
The terminal connected to one end of the n channel is called drain terminal and the terminal connected to the other end of the channel is called the source terminal. The metallic terminal connected to the layer surrounding the channel of opposite type semiconductor material (here it is p-type) is known as the gate terminal.
Now let us connect the external circuit with these three terminals. Here we connect the positive terminal of a voltage source circuit at the drain of the transistor. The negative end of the voltage source would be grounded. The gate terminal is also connected to the ground as shown.
Now at that condition n channel gets higher potential than gate region hence junction between the p-type gate region and n-type channel region would be in reverse biased condition. As a result, the depletion layer of this junction becomes thicker, and apparently, the thickness of the depletion layer depends on the voltage difference between these two regions.
Now if we look into the channel, we see the potential of the channel towards drain terminal is more than that towards source terminal. Because the positive terminal of the voltage source gets connected at the drain terminal and source terminal is grounded. Because of voltage distribution along the channel, the portion of the junction nearer to the drain gets more voltage stress than the lower portion of the junction. As a result, the width of the depletion layer nearer to the drain would be more than the lower portion. At that condition flow of majority carriers (here in n channel majority carriers are free electrons) through the channel continuous due to the applied electrical field between drain and source. If we slowly increase the drain voltage, the current through the field effect transistor channel increases linearly. However, this linearity does not continue after a particular drain voltage. That voltage is called pinch-off voltage. When we increase the drain voltage, the channel to gate voltage difference also increases. However, this difference is more towards the drain terminal. Hence depletion layer towards drain terminal get thicker faster than that towards source terminal. At the pinch-off voltage, the depletion layers touch each other and theoretically blocks the channel. So theoretically drain current that is current through the channel becomes zero but practically the current would not be zero rather it gets a constant value.
Because as soon as the drain current becomes zero no voltage drop occurs in the channel so reverse biasing of junction vanishes and just then again drain current starts flowing and voltage drop again gets established. Because of this phenomenon, the depletion layers never touch, and there would always be a narrow channel to facilitate the drain current to flow.
As much as the drain voltage gets increased beyond pinch off value, the depletion layers come more and more close. As a result, the resistance of the channel increases proportionately which keeps the drain current almost constant.
Now we fix the drain voltage at a certain level and apply a negative voltage at gate terminal and slowly increase the negative gate terminal voltage and let us see what happens. If we increase the negative gate terminal voltage from zero to a certain negative value, the voltage difference between the channel and gate region increases hence the width of the depletion layer gets increased. Hence here also opening of the channel gets reduced which causes a decrease in drain current even at fixed drain voltage. So it is now clear to us how we can control the drain current by controlling gate voltage. Hope you got the idea about the basic working principle of Junction Field Effect Transistor.