p-n Junction class 12


There are many types of semiconductors like intrinsic, extrinsic, p-type and n-type. This topic has already been covered in the other article Types of a semiconductor.

That's not the point of this article, many different types of semiconductor are combined with different combinations to form something more useful and one of those things is p-n junction diodes about which we will be learning in this article.

A diode is a combination of semiconductors where one part is negatively charged having a lot of free electron charge carriers and the other side is positively charged having holes or vacant space in the atoms of semiconductors.

p-n junction diode is also something same, in a p-n junction diode, two types of semiconductor (p-type and n-type) are fused together where one part consist of n-type semiconductors and the other one consists of p-type semiconductor. The point where both semiconductors meet is called junction i.e "p-n junction". 


What is p-n junction?

As I mentioned earlier, a p-n junction is the diode made with p-type and n-type semiconductors where n-type is negatively charged and p-type is positively charged semiconductor respectively.

A p-n junction is made with the process known as doping about which we will learn further in this article about how p-n junction is made.

How p-n junction is made ?

A semiconductor is made from the process known as doping where we add suitable impurity in a natural semiconductor to get desirable semiconductor.

Now, a p-n junction has both p-type and n-type semiconductors. I have talked about these semiconductors and doping in more detail in this article so please read it to get a clear understanding.

Now, to understand the formation of the p-n junction, let's understand how its made. So a pentavalent impurity (5 valence electrons) is doped with a p-type semiconductor.

Doping of pentavalent impurity in p-type, a part of p-type semiconductor is now converted into n-type and it contains both the p-type region and n-type region. 


p-n junction

The junction between both these regions is called a p-n junction. Now, you will see that formation of the p-n junction was pretty simple, we just needed to add a pentavalent impurity in p-type semiconductor and now you will have a p-n junction, but during the formation of these p-n junctions, there are many things which occur inside the semiconductor and the two most important processes occur during the formation of the p-n junction which is diffusion and drift. 

So now, let's understand what is diffusion and drift during the p-n junction formation.

Diffusion and drift in the formation of p-n junction:

We already know that, in n-type semiconductors, electrons are the majority charge carriers and therefore the concentration of electrons is more in n-type semiconductor.

Same way, in a p-type semiconductor, holes are the majority charge carriers and therefore you will find the concentration of holes is more in p-type semiconductors.

When a p-n junction is created, holes from the p-side diffuses to the n-side and electrons from the n-side diffuses to the p-side. TDue to this motion from one side to another, it gives rise to diffusion current across the junction.

Now, when an electron diffuses from n-side to the p-side, it leaves behind an ionized donor or a positive charge on the n-side can't move. As this diffusion continues from the n-side to the p-side, a layer of this ionized donor (positive charge) is created at the n-side.

In a similar way, when a hole is diffused from p-side to n-side, it leaves behind an ionized acceptor or a negative charge behind it, as this process of diffusion also continues from p-side to n-side, a layer of the ionized acceptor (negative charge) is created on the p-side. 

This region of space-charge between p-side and n-side of p-n semiconductor is known as the depletion region. 

Inside this depletion region, an electron from the p-side moves to the n-side and holes from the n-side moves to the p-side due to an electric field which is created between them, this motion of electron and holes inside depletion region is known as drift. 

You can also watch this video, to understand about depletion layer and formation of p-n junction more clearly:


Biasing in a p-n junction:

Now, I know you may have got confused with the term "biasing", but don't worry guys, I am here for that only.

So Now after the p-n junction and our desirable semiconductor is made, it's time to use it depending upon our need, but before that we need to learn it's property about how it behaves in different conditions and from different condition I mean to say that how will a p-n semiconductor a.k.a diode will respond if we applied voltage from different directions.

Applying voltage to a semiconductor is we what call it as biasing.

Now, in a p-n semiconductor, as you already know it is made up of p-type and n-type semiconductors, it behaves differently depending upon which side we are applying a voltage from.

p-n semiconductor or diode has 3 different types of biasing: 

1. Zero Biasing: There is no external voltage applied to the p-n diode.

2. Forward biasing: In this, the positive terminal of potential voltage is connected to the p-type semiconductor side and the negative terminal is connected to the n-type semiconductor side. 

3. Reverse biasing: In this, the positive terminal of potential voltage is connected to the n-type side of semiconductor and the negative terminal is connected to the p-type semiconductor side. 

So, now you must have known that zero biasing will not have any effect on p-n diode as we are not applying any voltage. Therefore, we will be only looking at forward and reverse biasing.

Forward bias:

When the positive terminal voltage is connected to the p-side of p-n semiconductor and the negative terminal voltage is connected to the n-side of p-n semiconductor then it's in forward bias. 

In forward bias, the built-in electric field of p-n junction diode and applied electric field are in the opposite direction which reduces the thickness of the depletion layer in the semiconductor. 

If the applied voltage is small, the depletion barrier will be reduced slightly and only a few electrons from the n-side and holes from the p-side can cross the junction and only a small amount of current can flow. 

If the applied voltage is large enough, the depletion barrier will reduce more and more electron from n-side and holes from p-type can cross the junction resulting in more flow of current.


Reverse bias:

When the p-side of p-n semiconductor is connected to the negative terminal and n-side of p-n semiconductor is connected to the positive terminal of applied voltage, then it's in reverse bias.

In this case, the built-in electric field of p-n junction diode and applied electric field both are in the same direction which increases the thickness of the depletion layer in the semiconductor.

If the applied voltage is small, the depletion layer will increase slightly but if the applied voltage is more then the depletion layer will increase more and resistance of the depletion layer increases



V-I Characteristic of p-n junction:

V-I characteristics where "V" stands for Voltage and "I" denotes current in the diode is essential to know and understand the behaviour and characteristic of diode and semiconductor on different voltage in forward and reverse bias.

Here is the graph which shows the V-I characteristic of p-n junction:

V-I characteristic of p-n junction diode

Application of p-n junction:

A semiconductor has a wide variety of applications in many industries so does p-n junction semiconductors.

Here are some of the applications of p-n junction diode:
  • Computers, radio, radar circuits and many more
  • As switch in digital logic design
  • Used as a rectifier in DC power supply manufacturing. 
  • As voltage multiplier
  • In LED 

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