PN Junction Diode in Forward Bais and Reverse Bias

Di means two and odes means terminals. Thus PN junction diode is a device who has a  junction made up of P and N type materials with two terminals. The P type material consists of a lot of positive charges and N type material consists of negative charges.The junction doesn’t means a sandwich of the two materials and there is a procedure to make this PN junction diode.

A pure semiconducting material is taken and from one side trivalent impurity and from other side a pentavalent impunity has been added. At the junction there is a region formed depletion region. At this region some holes from the P material will shift towards the N side. It is simply because there is a repulsion from the P-side and there is attraction from the other  N side material. 

 This kind of the transfer will happen only with the first batch of the charges and it is not going to happen further. The logic behind this is simple. The first batch of the charges who have transferred to the opposite side will oppose the flow of the further transfer of the charges. 

Thus there is a region formed which is a neutral region and the region is called as depletion region. This region obstruct the further flow of any of the charges from one site to another side hence it is going to behave like insulator.

 There is a potential difference developed at the junction which is opposing the flow of the current because of this depletion region and the potential difference can also be called as barrier potential. The value of the barrier potential depends on the nature of the material.For silicon it is around 0. 5 to 0.7 volts whereas germanium it is around 0.2 V.

 Forward Bias

When you don’t connect a battery there is no possibility for the flow of the charges from one side to another side and it behaves like insulators.Not only this device, every electronic device behaves like a insulating material when you don’t connect a battery because batteries the driving force who can make the current flow from one place to another place.

When you are connecting the battery how to options.The positive plate of the battery can be connected to the P type material of the diode and the negative plate connected to the N type material. This way of connection is called as forward biased connection and this enables the flow of the charges from one site to another side. It is simply because the positive plate of the battery a positive charge starts.It starts repelling the positive charges of the holes and push the width of the depletion layer to a smaller value. The same will happen from the other side also.

 Simultaneously from the negative plate of the battery electron starts and they start repelling the electrons of the N type material.Hence the thickness of the depletion layer will decrease significantly.Thus there is a convenient environment for the flow of the charges and hence the device is going to behave like a conducting material.

In forward biased connection there will be flow of current in the devise only when the applied voltage is more than the barrier potential.Until then the applied voltage is not sufficient to overcome the barrier potential and hence there is no flow of current. That means when the applied voltage is less than barrier potential the device is going to behave like insulating material.

 Once if the applied voltage is more than the barrier potential with the increase of the applied voltage the charges will get more energy to flow from one side to another side and hence the current flow will be automatically increasing.The corresponding graph is as shown.

 Reverse Bias

If the positive plate of the batteries connected to the N type material and a negative plate of the batteries connected to the P type material that kind of the connection is called as reverse biased connection. In reverse biased connection the charges the battery will attract the charges of the P and N type materials back towards the plates and hence the width of the depletion layer increases further.This makes the flow of the current  further very difficult and hence it is going to behave like insulating material.

 Break Down Potential

Therefore the PN junction diode in forward bias acts like a conducting body and in reverse bias it behaves like insulating material.Though it is behaving like insulating body during the reverse biased connection, with the increase of the voltage in the reverse biased connection the width of the depletion level further increase and makes the flow of the correctness even more difficult. 

 This will be keep on happening until the depletion layer width has reached its maximum. Once if this is happened, there is no further scope for the increase of the depletion layer and if the charges were given enough energy to cross that larger depletion layer, we can make flow of current from one side another side. This is possible only when you apply a very high-voltage the reverse biased connection and that high-voltage is called as break down potential. 

Once if the break down potential is applied all the charges at both the sides will get enough energy to cross from one side to another side and hence the current flow increases abnormally and there is no need of the increase of the voltage further.

Thus if the breakdown voltage is applied,there is an abnormal increase in the current and the charges doesn’t need any further voltage therefore.Hence the voltage further remains constant and there is a big increase in the flow of the current. This can be noticed in the graph drawn that after reaching the breakdown voltage the graph is just right line parallel to y-axis along which the current is shown.

There is one practical problem with the reverse biased connection of the PN junction diode. Though it is theoretically possible that it can flow the current in the reverse bias, when the applied voltage is more than the breakdown voltage.But practically that is not possible to reach.The PN junction diode is made with such a sort of material that it cannot withhold the breakdown voltage. Even before you apply the breakdown voltage the diode will damage therefore we can never operate in the reverse Bias and for all practical purposes can operate it only in the forward bias.




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Intrinsic and Extrinsic Semi Conducting Meterials

Basing on the ability of a material that whether it can pass the current through it are not,the materials can be divided into three categories.

The materials that can pass the current through them because they’re having the free availability of the charges are called conductors. Almost all the metals and human bodies of the some of the examples for the conductors .

The materials who cannot pass the current through them because they are not having the availability of the free charges are called insulators.Plastic, rubber, cotton and paper are some of the examples of the insulators.

The materials who behave  like insulators at absolute zero temperature but behaves like conductors once if the temperature is raised a little bit is called a semiconducting material. Silicon and germanium are some of the examples of the semiconducting materials.

Here we are going to have further detailed discussion regarding the semiconducting materials.Many are for modern-day electronic gadgets are having a lot of the integrated parts of the semiconducting materials. It is impossible to imagine electronic gadgets without having a semiconducting material inside it. Diodes, transistors, integrated circuits(IC’s) are very much made up of semiconducting materials.

Properties of the semiconducting materials:

1. The resistivity is more than that of the conductors but less than that of the insulators.
2. They have negative temperature coefficient which means with the increase in temperature corresponding resistance is going to decrease.
3. At absolute zero there is no availability of the free charges and hence it behaves like insulators.
4. Its conductivity can be increased by adding a little bit of impunity to it and this phenomena is called doping.

Energy band theory:

To explain the conducting nature of a given material we can depend on the energy band theory.We know that it electrons are distributed in the different orbits like S, P, D,F. The electrons in the outer orbits are called as valency electrons and they are always having a chance of going to some other body if they got enough energy. The valency electrons in the outermost orbits are bit close to each other and their form together like a band. This band of electrons is called valency electrons and it is either completely filled are partially filled in any kind of material.

Once if this electrons becomes free they’re ready to conduct and all that kind of electrons together could be called as conduction electrons.The conduction band is either partially filled are completely empty depending on the nature of the material.

The band gap between the valency band in the conduction band is called forbidden band gap. The size of this gap depends on the nature of the material.



In the case of the conductor valency band and conduction band overlap with each other electrons from valency band can easily move to the conduction band and hence there is a flow of current quite easily.

In the case of insulators there is a big gap between these two bands and hence it becomes very difficult for the valency electrons to travel from that band to the conduction band and hence it always behaves like insulators.

In the case of the semiconducting material the band gap is somewhere in between the conductors and insulators. By default the charges have not enough energy to cross this band and hence it behaves like insulators. Once if the temperature is raised a little bit, the charges get enough energy to cross the and hence the start behaving like a conductors.



Types of semiconducting materials:

Semiconducting materials are broadly of two types. Those who are naturally available are called pure semiconducting materials or intrinsic semiconducting materials. If a electron leaves a place in a energy band , there is an empty place left in the band that empty spaces called hole. In the case of a intrinsic semiconducting material ,number of electrons is equal to the number of holes. The flow of current in this kind of semiconducting materials is equal due to both electrons and holes.

At 0° Kelvin in the crystalline structure of the semiconducting material who is having four valency electrons  and each electron is having a covelent bond with its neighboring electron. There’s no electron is free and hence it is behaving like insulators. Once if a little bit of temperature is given, by obserbing that heat energy some of the covalent bonds can break and hence it starts behaving like a conductor because of the availability of the free charges.

Anyway practically use of this kind of intrinsic semiconductor is very much limited because they are by default behaving like insulators. We are looking for a semiconducting material or can pass the current directly without raising the temperature and for that sake we are going to use a phenomena called doping. Doping is the phenomenon of adding a little bit of impunity to the pure semiconducting material therefore there will be availability of the free charges.

When we are doping we have two options.We can add tiavalent  impunity and hence there will be one excess hole and hence it start behaving like a P type semiconducting material who is having access holes.

We can also had a pentavalent and impurity to the semiconducting material and hence there will be availability of excess electrons in the material. This type of the material is called N type. These are called impure semiconducting materials or  extrinsic semiconducting materials.


In practical life we use extrinsic semi conductor materials in making electronic devices because of its ability to conduct current at ordinary temperatures.

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