Showing posts with label P N Junction Diode. Show all posts
Showing posts with label P N Junction Diode. Show all posts

Semi Conductor Electronics and Communication Systems Complete Lesson

Semiconducting material has conducting nature in between conductors and insulators. At absolute zero temperature the behaves like insulators and at increased temperature they can be have like conducting materials. Silicon and germanium are simple examples of the semiconducting materials which are also called as intrinsic type of semiconducting materials as well as called as pure semiconducting materials. 

 We can modify them as per requirement by adding a small amount of impurities and that phenomena is called as dooping. When you add impurities,we will be getting two types of semiconducting materials called P type and N type. Using this materials we can make electronic devices like diodes, transistors and integrated chips. Here we are going to identify how they are going to work. We are also going to discuss a little bit of digital electronics and  some discussion about logic Gates.

Communication system is a way of communicating information from one place to another place using a channel. There are basically three different types of communications like ground wave communication, sky wave communication and space wave communication.

Here we are going to list out all the posts that are linked with this topics is given below in a systematic order .

Intrinsic and Extrinsic Semi Conducting Materials

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Zener diode as Voltage Regulator

PN junction diode works very well in the forward bias and pass current through it. If you want to operate this PN junction diode in reverse Bias,we need to apply a voltage greater than the breakdown voltage at which the reverse Bias depletion layer can be dominated.

 Theoretically it looks fine but practically any PN junction diode cannot withstand that much of the high-voltage. Even before it practically reaches the breakdown potential,it will damage itself therefore we cannot use the PN junction diode in reverse Bias.

For the purpose of using the diode in reverse Bias,special diode is designed and its name is zeenar are diode. This is a highly doped PN junction diode. Its properties are similar to the PN junction diode in forward bias. But in reverse Bias its breakdown voltage is a low value, therefore it can pass current even in the reverse bias without breaking the device itself. Therefore we can use this  PN junction diode in reverse Bias without damaging it.

In reverse Bias once if you apply the voltage equal to breakdown voltage,the Zeenar diode start passing current.Here all charges got enough energy to cross the potential barrier.Hence we need not apply any further voltage.Thus in the breakdown region, the voltage remains constant and current will be keep on increasing since all the charges has got enough energy to cross from one side to another side of the depletion region. Taking this property is an advantage this zenar diode can be used to like a voltage regulator.

It is obvious that we can use this kind of the diode in reverse Bias and we prefer to use it only when it is required to be connected in reverse Bias. It is simply because if you want to connect it in the forward bias, we already had PN junction diode which is a much cheaper and easier option.

Zenar diode as a voltage regulator:

A zenar diode is connected in reverse Bias  as shown. The output is connected to a load resistance in parallel. Being the diode and the load resistance in parallel they always have the same voltage and current among them a share is inversely proportional to their resistance.


When the input voltage is less than the breakdown voltage, circuit will never start passing current. When the applied voltage is more than the zenar breakdown voltage, there is an extra voltage available in the circuit.This extra voltage can create unnecessary heat in the circuit. To  control is kind of the extra heat due to extra voltage available, we are going to connect a series resistance in the circuit. The very purpose of this resistance is to hold the extra voltage that is available in the circuit therefore the system will be working perfectly. 

 The value of the series resistance can be decided basing on the maximum possible input voltage that we are going to give and the corresponding current that is going to be generated in the circuit. We can identify the corresponding numerical values like current passing in the load resistance, zenar diode, series resistance as shown in the following diagram.

Applying the concept of voltage regulator to solve a Problem 

Here we shall consider solving a problem in understanding the concept of the Zeenar diode more clearly. Consider the following electronic circuit. Find the current passing to the load resistance as well as the zeenar  diode in the circuit.

While solving this problem we are actually analyzing almost all the concepts of the zeenar diode. We have to understand that the diode and the load resistance are in parallel to each other and hence they carry the same voltage across them. This is the basic concept that any of the two electric elements who are in parallel to each other will always carry the same voltage and the current between them a shared.

We also have to understand that only a portion of the voltage will pass to the  diode and all the remaining voltage that we have is passed across the input the series resistance. We also have to understand that the total voltage is going to the passed through the series resistance and it is going to be shared in between the load resistance and the diode in the inverse ratio of their resistances.



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P N Junction Diode as Full Wave Rectifier

Full wave  rectifier is a device which converts complete alternate current into direct current. The input of this circuit is very similar to the half wave  rectifier.It is nothing but a step down transformer. In this kind of the device the output consists of not one diode but two diodes. In between the two diodes there is a special arrangement called central tap. The purpose of the central tap is to reverse the phase of the current that is passing through it. When the input is connected to both diode D1 and D2 it first passes to first diode and then to the second diode via central tap.

When the positive cycle of the alternate current is given like input ,diode D1 will be in forward bias. Before the same positive peak is passed to the diode D2, it is first of all passed to the central tap therefore it will reverse that positive peek into negative peak. Hence  the diode D2 will be simultaneously will be in reverse Bias.

When the Negative peak of the alternate current has reached the system, the diode D1 will be in reverse Bias. The same negative peak before reaching the diode D2 is passed to the central tap therefore it will be converted as positive peek. This is the job of the central tap which is managed with the help of the phase reversal. 

 Thus the diode D2 will be in forward bias for the Negative peak of the alternate current that is reaching the circuit. All positive peaks are passed through the diode D1 and all negative peaks are passed  through the diode D2.Thus  all alternate current is converted into direct current.

Theoretically the full wave rectifier is supposed have an efficiency of hundred percent. Anyway as the output still contain some part of the alternate current components, it is not equal 100% and in the ideal case it is close to 81.2% as shown.



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P N Junction Diode as Half Wave Rectifier

Rectifier is a device which converts alternate current into direct current.The PN junction diode can act like a rectifier. There are two types of rectifiers. One is a full wave rectifier and another one is a half wave rectifier.

Half wave rectifier :

The device which converts half of the input alternate current into direct current  is called a half wave rectifier. Then input is given generally from a step down transformer which has a primary and secondary coils and in between them there is a soft core. The output of the step down transformer is connected to a diode as shown in the figure. When the diode is in the forward biased the alternate current will be converted into direct current and when it is in the reverse biased it is not going to allow the current passing through it.

Anyway the converted direct current is not a complete direct current but still it will be having some sort of alternate current components. This is the reason why the output is still shown with the small kind of waves.To eliminate the remaining the alternate current we need to use filter circuit which is a combination of capacitors and resisters.

Efficiency means the ratio of portion of the output that is converted into direct current when compared with the input alternate current given. The half wave rectifier is theoretically supposed have efficiency of 50%. Anyway because of the remaining alternate current components and output it is having an efficiency close to 40.6% . This is also possible only  when  the diode in the forward bias. In that case, ideal diode offers zero resistance.



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Solving Problems on the Concept of P N Junction Diode

PN junction diode is a electronic device which acts like a current passing device in forward bias and acts like a insulator when it is in the reverse bias. We had already analyzed the working the of the PN junction diode in the previous post.Here we would like to analyse some more problems on this concept.The core point that we have to understand is when the voltage applied to the P type material is more than that of the N type than the diode is in the forward bias and it is able to pass the current through it .

Let us consider solving the problem on the concept of a PN junction now.In the following problem we have to identify which of the diode is in forward bias in which of the diode is in reverse Bias.

In solving the problem we have to remember only one simple concept that if the voltage that is applied to the P type material of the PN junction diode is higher than that of the voltage that is applied to the N type material of the PN junction diode,than the diode is said to be in the forward bias and vice versa.


Let us solve one more problem on the same concept. In this problem calculate the current passing in the circuit.

While we are solving this problem we have to understand that the diode is in the forward bias and it can be able to pass the current through it. In the case of an ideal diode,the resistance of the diode in the forward bias is equal to zero.When no information is given in the problem the shall assume that diode is ideal .

Problem and Solution

Let us see one more problem in solving.In this problem we have to find the current passing through the hundred ohm resistor.

After having a look at the circuit we can understand that the diode two is in the reverse Bias and there is no way that the current is going to passing that diode. Thus that part of the circuit will become the dead part of the circuit and it is not going to pass any part of the current through it.


Here we would like to solve one more problem basing on the diodes and their way of connection. Find the equivalent resistance between the point A and B  as shown in the following circuit. 

 We need to solve the problem in the two following conditions. 

 What happens the effective resistance when the voltage at the point A  is greater than voltage at the point B and what happens to effective resistance when the voltage at the point B is greater than that of the voltage at the pointy A ?

In solving the problem we are to be little bit more careful. First of all we had understand that the if diode is in the forward bias it will pass the current through it with the zero  resistance.When the diode is the reverse Bias it will not pass the current through it.

When the voltage at the pointy A is greater than that of the voltage at the point B the current the directed from the pointy A , has two options either  to go through the 18 ohms are to pass to the diode D1. The entire current choose to pass through the diode D1 because the diode D1 is not offering any resistance and simply because it is in the forward bias. Current always prefer pass into the devise ways having less resistance and it will never pass through the diode who is having high resistance.
when there is an option of having a devise with zero resistance. Hence the current the distracted from the pointy a will pass through the diode D1 and then through the diode D2 and then to the diode D3 and finally reach the point be bypassing all the available resistances.

Hence the total resistance is simply sum of all these resistance because they are all in series.The same kind of analysis can be made even with the second case also.


Problem and Solution 

Find the current passing through the battery in the following circuit.
Here also we are going to follow the same technique.Tthe diode is in forward bias will pass the current in the diode who is in reverse bias,it will not pass the current is the simple concept that we need to know to solve this problem.



Problem and Solution 

Find the equal and resistance between the point A and B  when the voltage applied  at the pointy A is greater than that of the voltage at the point B.

The analysis of the problem with is done with the blue colour ink as shown in the diagram below. We had analyse the problem bit more carefully while we are solving. 

 What all explanation  being written in the diagram itself and we want you to have a look at that and ask the questions that if you have, you can just commented the end of the page so that , we will try to clarify.



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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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