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What is a P-N Junction?

A junction known as the p-n junction is an interface or a boundary that is present between two semiconductor material types. These material types are namely the p-type and the n-type inside a semiconductor.

 

The side which is known as the p-side or the positive side of the semiconductor has an excess of holes and the n-side or the negative side has an excess of electrons. In a semiconductor, we can say that the p-n junction is created by the method of doping. The process that is basically doping is explained in further detail in the next section.


What is PN Junction Doping?

As we already know that if we use different semiconductors or the different materials to make a p-n junction then there will be a grain boundary that would inhibit the movement of electrons from one side to the other by scattering the electrons and holes.  Thus, here we use the process that is known as doping. 

 

We will understand the process of doping with the help of this example. Let us now further consider a thin p-type silicon semiconductor sheet. Now, again If we add a small amount that is of pentavalent impurity to this thing, we can see that a part of the p-type Si will get converted to n-type silicon. This sheet which we are talking about will now contain both p-type region and n-type region and a junction which is between these two regions. The processes that we have talked about we need to follow after the formation of a p-n junction are of two types – diffusion and drift.  


Depletion Region

As we already have this thing in our mind that there is a difference in the concentration of holes and electrons at the two sides of a junction,  the holes from the p-side diffuse to the n-side, and the electrons from the n-side diffuse to the p-side. Now we can notice that these give rise to a diffusion current across the junction. Also, we will notice that when an electron diffuses from the n-side to the p-side then at that moment an ionized donor is left behind on the n-side that is immobile.

 

As this whole process goes on we can see a layer of positive charge is developed on the n-side of the junction. Similarly, we can say that when a hole goes that too from the p-side to the n-side then we can say that an ionized acceptor is left behind in the p-side which then results in the formation of a layer of negative charges in the p-side of the junction. This region which we have seen here is of positive charge and negative charge on either side of the junction is termed as the depletion region. 

 

Drift

These all things are due to this positive space charge region on either side of the junction. An electric field direction from a positive charge towards the negative charge is developed.  Now again we can see that due to this electric field that an electron on the p-side of the junction moves to the n-side of the junction. This motion which we have already seen is termed the drift. Here we can easily notice that the direction of the drift current is opposite to that of the diffusion current.


Application of PN Junction Diode

  • The junction which is the p-n junction diode can be used as a photodiode, the diode which is sensitive to the light when the configuration of the diode is reverse-biased.

  • It can be used as a solar cell.

  • When the diode is forward-biased, it can be used in LED lighting applications.

  • We can see that it is also used as a rectifier in many electric circuits and as a voltage-controlled oscillator in varactors.


There are two operating regions in the p-n junction diode:

  • The P-type

  • Then the N-type


There are also three biasing conditions which are generally for the junction which is the p-n junction diode and this is based on the voltage applied:

  • Zero Bias: there is no external voltage applied to the p-n junction diode.

  • Forward Bias: which is the positive terminal that is of the voltage potential is connected to the p-type while the negative terminal is connected to the n-type.

  • Reverse Bias: that is we can say that the negative terminal which is of the voltage potential is connected to the p-type and the positive is connected to the n-type.

 

Reverse Bias of P-N Junction

The p-n junction is said to be reverse-biased when the p-type is linked to the negative terminal of the battery and the n-type is attached to the positive side. The applied electric field and the built-in electric field are both in the same direction in this scenario. Because the generated electric field is in the same direction as the built-in electric field, the depletion area becomes more resistive and thicker. If the applied voltage is increased, the depletion region gets more resistant and thicker.


V-I Characteristics of PN Junction Diode

A curve between the voltage and current through the circuit defines the VI properties of PN junction diodes. Voltage is represented on the x-axis, and current is represented on the y-axis.

 

With the help of the curve, we can see that the diode works in three different zones, which are:

  • Zero bias 

  • Forward bias

  • Reverse bias

 

If the PN junction diode is zero biased, no external voltage will be supplied. This means that the junction potential barrier is blocking the flow of current.

FAQs on P-N Junction

1. How Do PN Junctions Work?

PN junction is said to be formed in a single crystal by joining two N-type and P-type semiconductors. That is we can say that when the junction diode that is said to be biased in the reverse direction which is the majority charge carriers that are attracted by the respective terminals away from the PN junction which is said to be thus avoiding the diffusion of electrons and holes at the junction.

2. What is the forward Bias of the P-N Junction?

When the p-type is linked to the positive terminal of the battery and the n-type to the negative terminal, the p-n junction is said to be forward-biased. The built-in electric field at the p-n junction and the applied electric field are in opposite directions when the p-n junction is forward biased. 

 

The resultant electric field is smaller than the built-in electric field when both electric fields are added together. As a result, the depletion region becomes less resistant and thinner. If the applied voltage is high, the resistance in the depletion region will be small. At 0.6 V, the resistance of the silicon depletion layer becomes completely negligible, allowing the current to flow unimpeded.

3. How is a PN Junction Formed?

The P-n junctions are formed by joining the n-type and p-type semiconductor materials. The n-type region has a high electron concentration and the p-type that is a high hole concentration.

4. What is the Depletion Layer in the PN Junction?

The depletion region or we can say that the depletion layer is a region which is in a P-N junction diode where no mobile charge carriers are present. The depletion layer acts like a barrier that opposes the flow of electrons from n-side and holes from p-side.

5. Why is PN Junction Used?

Applications of p n Junction Diode:


It can be used as a solar cell. Again here we can notice that when the diode is forward-biased then it can be used in LED lighting applications. Again we see that it is used as rectifiers in many electric circuits and as a voltage-controlled oscillator in varactors.

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