Skip to main content

JFET :-Junction Field Effect Transistor

We had finished learning all the concepts of Bjt from basics and in the end, we had designed an amplifier using BJT. From this post, we will start learning about new transistors i.e. JFET.
We will see the following things in this post:-
-Jfet full form.
-Types of jfet.
-Working of jfet.
-Why jfet is called unipolar and bjt as bipolar device?

So the full form of JFET:-Junction Field Effect Transistor. Right now I'm not explaining to you that what is field-effect transistor but by the end of this post you will come to know what exactly it is?
Different types of JFET:-

Similar to Bjt JFET is also divided into two types i.e. N-channel Jfet and P-channel JFET.
In N-channel current will flow because of electrons and in P-channel the current will flow because of holes. But we will go with N-channel Jfet, as discussed in bjt that electrons are lighter in weight and basically current never flows because of holes it only flows because of electrons. Since in P-channel, there is fewer electron so when an electron moves it creates void space or a hole at that area, therefore, it is assumed that in PNP transistor and P-channel Jfet transistor the current flows because of holes. You can say that if there are fewer electrons in the P-channel then-current produced would be a little less as compared to N-channel Jfet. Therefore in practice or in practicals, we use N-channel Jfet.

Construction of JFET:-
As we had seen in the above figure that there are 2 types of JFET now we will see it's construction

So in N-channel Jfet the P-type material is doped in N-type material. As you can see above in N-channel JFET both the P-channels are connected together to form a gate terminal. Two terminals that are coming out from N-channel are known as drain terminal and source terminal. The upper terminal is known as the drain while the lower one is known as a source terminal.
Similarly, in P-channel Jfet N-type material is doped in P-type material rest all the things are the same for the P-type region.
Before starting with the working of Jfet we will see the significance of its 3 terminals.
Jfet has three terminals Gate, Drain, and Source.
Gate:-The function of the gate is the same as that of the base terminal in BJT transistor i.e. by varying the gate voltage we can control the working of the Jfet. As we saw in Bjt that by varying
the base current we performed 2 operations i.e. Switching on and off and as an amplifier.
Drain:-The drain terminal of jfet is the same as that of the collector terminal in bjt. Since the electrons draining from the source terminal to drain therefore it is known as the drain terminal.
Source:- It is the same as the emitter terminal of BJT. Current flows from drain to source.

Working of Jfet:-
When you provide a positive voltage to drain and negative voltage to the source it is seen that a current flows from drain to source or in other words electron moves from source to drain.
Now, if we provide negative voltage to the gate and positive voltage to the drain and source is kept common then we get some interesting results.
Let's say if we provide 2v to the drain i.e.(Vds = 2v) and zero or no voltage to the gate terminal i.e. (Vgs = 0v) then the electrons will move very quickly from source to drain.

In this case, the electrons move from source to drain due to which the current ID is generated from drain to the source terminal. Although we are assuming that the gate voltage is zero (Vgs=0) ideally but practically the gate terminal is getting some negative voltage due to which the formation of the depletion region takes place. 

Now If we keep the same voltage across the drain-source terminal and if we provide -ve voltage to the gate-source terminal i.e. (Vds=2v) and (vgs=-1v) then the depletion region present between the gate and source increases [acts as a reverse bias] due to which the passage through which the electrons flowing from source to drain or width of N-channel decreases.
Please Consider Here ID as ID1
Now as you can see in this figure that only a few electrons can pass from source to drain therefore ID decreases to form ID1. Thus we can conclude that as we decrease the gate voltage or in other words if we provide negative voltage to the gate terminal then the current decreases.
Now you may think that:-
Why the depletion region is wider at the top and narrower at the bottom?
Now as you see in the figure that the depletion region is moved a little upwards or in other words we can also say that the depletion region is wider at the top portion while it is narrower at the bottom. So this is because as you can see that the gate voltage is negative and we all know that in the p-region there are few electrons as minority charge carriers due to which these electrons are repelled and takes part in the formation of depletion region at the same time these electrons are under the influence of +2v of drain terminal and due to attraction of these electrons moves upwards. Therefore in Jfet the depletion region is wider at the top and narrower at the bottom. 

Why jfet is called unipolar and bjt as bipolar device?
As we saw above that in Jfet the current(ID) was generated only when majority carriers i.e electrons that were present in N-channel were moving from source to drain.
While in Bjt the current flows because of holes and electrons i.e minority carriers and majority carriers.
Therefore Jfet is considered a unipolar device while bjt as bipolar.

Why Jfet is preferred over Bjt?
I will explain all the concepts in-depth after 2-3 posts but for now, just remember that all these factors like (power dissipation, heat generation, noise generation, thermal runaway) were taking place in Bjt.

VIDEO:-
You will understand the above concept in depth by watching this video
I hope you understood this concept if you have any doubt then please ask me via comments or via mail or through WhatsApp.



Comments

Recent Posts