p-n Junction class 12th


There are many types of semiconductors like intrinsic, extrinsic, p-type and n-type. I have already covered about it in a seperated article about Types of a semiconductor.

That's why in this article we will be focusing on combinations of the p type and n type semiconductor also known as p-n junction diode

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.

 

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.

These semiconductors are fused together where one side of it is n-type and the other type is of p-type semiconductor. The point where two things come into contact with each other is known as junction in physics and that's why this type of semiconductors are called p-n junction. 

There are many practical application of a p-n junction, one of which is LED light which we all use in our houses nowadays are possible due to p-n junction semiconductors and it's property. 

The property and characteristic of a p-n junction is what makes it unique and in order to understand these characteristics, we will first need to take a loot at how is p-n junction 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:

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. Due to this motion from one side to another, it gives rise to diffusion current across the junction.

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 

F.A.Q (Frequently asked questions)

1. What is meant by p-n junction? 

A p-n junction is an interface between p type and n type semiconductors. The p-side of the semiconductor has an excess of holes and the n-side has the excess of electrons.

2. What causes the depletion region? 

Depletion region in the p-n junction is caused by diffusion of charges. 

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