pn junction diode principle
In this blog, we discuss the pn junction diode, its VI characteristics and discuss biased pn junction diode and unbiased pn junction diode. Understanding the forward and reverse biasing of the pn junction diode is fascinating. Let’s understand what a pn junction diode is.
What is a pn junction?
The previous blog taught us about majority and minority charge carriers and the doping process.
By doping one side of silicon crystal with p-type impurity and another with an n-type impurity, we can convert this silicon crystal into the p-n junction.
Junction is the border where the p-type and n-type region meets.
This combination of two junctions is also called the p -n junction diode because the p-type and n-type region acts like two electrodes.
Unbiased pn junction diode
There are large no’s of electrons in the n- side but very few on the p- side, i.e., holes are minority carriers, and electrons are majority carriers.
And for the p-side, holes are majority carriers while electrons are minority ones.
Whenever the electrons enter the p-type region, it becomes minority carrier and being surrounded by holes and combines with the holes.
When the electrons leave this n-region, the pentavalent atoms become positive ions due to the short of electrons. As soon as it enters the p region, it combines with a hole; after capturing these electrons, the trivalent atom becomes a negative ion.
Depletion region:– When an electron diffuses from the p- side to the n- side, it creates positive and negative ions near the junction. Due to this recombination, these ions near the junction become immobile (motionless). It depleted from free charge carriers, because of which, a region is created near the junction; this region is called a depletion region.
Some free electrons in the depletion region are thermally generated. The ions in the depletion region set up an electric field whose direction is from negatively charged ions to positively charged ions.
Due to this, some minority carriers from both sides can cross the junction. But the electron flow due to minority and majority charge carriers will cancel out each other. And hence no current will flow.
Biased pn junction diode
We need some external supply to create this electron flow, so we connect the p n junction to the supply (battery).
according to connection biasing is divided into two types
- Forward biasing
- Reverse biasing
Forward biasing of pn junction diode
In the forward bias condition, the battery’s positive terminal is connected to the p- side, and the negative terminal of the battery is connected to the n-side. This connection is called forward bias.
In forward biasing external electric field is opposite to the built-in electric field.
The electrons from the n- side get attracted toward the p-side, and holes from the p- side get attracted towards the n-side; this causes both the holes in the p- side and electrons in the n-side move closer to the junction. Consequently, the width of the depletion region decreases, causing holes and electrons to cross the junction.
As we increase the external electric field, more and more electrons and holes will cross the junction and contribute to the flow of electric current.
Before moving forward, let me clear one concept for you holes are the absence of electrons, so when electrons are moving from one place to another in a way, holes are also moving at the same time, so we can say that flow of current is due to electrons as well as holes.
Reverse biasing of pn junction diode
The positive terminal is connected to the n- side; the negative terminal is connected to the p-side, due to which holes are attracted toward the negative terminal and electrons toward the positive terminal.
More ions get created near the junction. As we increase, the reverse voltage depletion region will also increase.
During this process, minority carriers from the p-side and n- sides will cross the junction, and some current will flow due to minority carriers. Still, there will be no current flow due to majority carriers, as the current flows due to minority carriers; its amount is minimal.
V-I Characteristics
In V-I characteristics, the name indicated a relationship between the current flowing through the diode and voltage across it.
The V-I characteristics of the P-N junction Diode are divided into two parts: a forward biasing mode and a reverse biasing mode.
Forward biasing mode
When the forward biasing is applied to the diode, the small current will flow until the applied voltage becomes equal to the threshold voltage of the diode after that diode starts conducting.
Barrier potential
During the forward bias condition, Due to the creation of an electric field, holes, and electrons cannot cross this junction. And the potential required to cross this junction is called barrier potential.
Threshold voltage or Knee voltage
Knee voltage or threshold voltage is the voltage across the diode at which the forward diode current increases rapidly.
threshold voltage is 0.7v for silicon and 0.3v for germanium.
It is close to the barrier potential. As soon as knee voltage is achieved diode starts conducting in forward biased condition.
Reverse biasing mode
When the diode is reverse biased, a small amount of leakage current will flow through the diode due to the movement of minority charge carriers, and it is independent of the supply voltage.
Reverse Breakdown voltage
Once the reverse bias is made too high, the current through the P-N junction diode increase abruptly, and the voltage at which this phenomenon occurs is breakdown voltage.
After this breakdown, the diode will be damaged, and the magnitude of the current will increase rapidly.
Conclusion
Thus, we see that diode is a unidirectional device, i.e., it allows current to flow only in the forward direction( conducting in forward bias ) and does not allow the current to flow in reverse bias condition. (Non conducting in reverse bias).
Thus diode acts as a switch.
In the upcoming blog we will dicuss about zener diode.
