- JFET Definition: A Junction Field Effect Transistor (JFET) is defined as a voltage-controlled semiconductor device that controls current flow using majority carriers.
- JFET Construction: JFETs consist of a channel of either p-type or n-type semiconductor material, surrounded by the opposite type of semiconductor.
- Types of JFET: There are n-channel and p-channel JFETs, determined by the type of semiconductor in the channel.
- Working Principle of JFET: The working principle of JFET involves controlling the drain current by applying a voltage to the gate, which affects the depletion layer and the channel’s resistance.
- Pinch-Off Voltage: At pinch-off voltage, the depletion layers in the channel almost touch, maintaining a constant drain current despite further increases in drain voltage.
A Bipolar Junction Transistor (BJT) is a current-controlled device where the base current regulates its operation. In BJTs, both minority and majority carriers are used. In contrast, a Junction Field Effect Transistor (JFET) is a voltage-controlled device that operates with only majority carriers. Before we explore the working principle of a JFET, let’s review its basic construction to better understand it.
In a JFET, a channel of either p-type or n-type semiconductor is created within the opposite type of semiconductor. If the channel is p-type, it is surrounded by n-type material, and if the channel is n-type, it is surrounded by p-type material. Based on the semiconductor used in the channel, there are two types of JFETs: n-channel JFET and p-channel JFET.
To understand the basic working principle of a JFET, we will use an n-channel JFET as an example. The working principle is similar for both n-channel and p-channel JFETs.
The terminal connected to one end of the n-channel is called the drain terminal, and the terminal at the other end is called the source terminal. The metallic terminal connected to the layer surrounding the channel, which is of the opposite semiconductor type (p-type in this case), 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.
When the drain current drops to zero, there is no voltage drop across the channel, causing the reverse bias of the junction to disappear. As a result, the drain current starts flowing again, and the voltage drop reappears. This cycle ensures the depletion layers never fully touch, maintaining a narrow channel for 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.
