In the previous lecture, we talked about voltage multiplier, what is doubler, tripler, quadrupler. We discussed about all these circuits and their working. Now, in this lecture, we are going to talk about another main important topic, on which most of the time, question comes in the question paper.
And that is called Zenit doubt. What is Zenit doubt? What is the working symbol?
What is the VI characteristic? And the main important thing, which is put up here, that is what are the different breakdown mechanisms used in our Zenit doubt. So, let's discuss one by one all the topics in our lecture that is on Zener diode.
So, Zener diode. Zener diode is basically heavily doped PN junction diode. Here, basic PN junction diode is there. But here, a term is used, a word is used, heavily doped.
So, that means, the P type and N type semiconductor that we had studied, we had studied doping in that. How P and N type were made? P is formed by adding a trivalent impurity. and N is formed by adding a pentavalent impurity, right? So, now here the amount of impurity added to make these P-type and N-type is very very high, right?
This is what we call heavily doped. So, when PN junction diode is heavily doped, then it is called the Zener diode and also a second property which is marked here, that is, it operates in the breakdown region. Now, what is this breakdown region? We will see that later.
but zener diode is a heavily doped p-nitrogen diode which works in breakdown region we have read ordinary diode after breakdown region, as soon as voltage breaks down in reverse passing you apply so much voltage that diode breaks down after that diode becomes useless but zener diode operates in that breakdown region that is the main functioning of zener diode which is why we are studying zener diode now if I talk about symbol so this is a symbol of your zener diode same to same ordinary diode but here the change in the cable symbol that the next portion will be like this that this thing is not straight line that will be differ from your pn junction diode right, so this is the symbol used for zener diode now if I talk about next thing about zener diode, so the same thing we have studied in pn junction that is VI characteristic yes, if you talk about forward bass so the same thing we have studied in pn junction diode same to same So, here we have a V-I curve, voltage current characteristic. It shows when there is a zero external current, when the applied voltage is zero, then in that case, our depletion layer does not vanish. Depletion layer does not vanish, due to which current does not flow.
So, there is no current flow due to the junction potential. Now, after this zero external voltage, we had studied that if we apply positive voltage, then positive voltage, forward bass will decrease the depletion layer and the current will not flow till the depletion layer vanishes and at what point does the depletion layer vanish? that is the voltage called NE voltage concept so here we have same to same VIE characteristics that is no current is flowing at zero voltage forward bass will flow after the NE voltage and at what point does the voltage vanish?
which after this your current starts flowing and that the relation is given by I0 e to the power V upon eta Vt minus 1 which we had read in diode current equation that same current equation is applicable here too in case of forward pass now if I talk about reverse pass then it is the same thing what happens in reverse pass? in reverse pass our depletion layer increases depletion layer cannot vanish because when we apply reverse pass then depletion layer increases. So, increasing the depletion layer means there is no current flow.
But we have already studied that practically there will be a very small current flow due to the junction potential. On the junction, our net negative charge is there. On one side, our net positive charge is there. So, because of this, our current flow is there. That is a very small current called reverse situation current.
So, reverse situation current flow is there which is shown in your characteristic. And if I increase the reverse voltage, if we increase while doing reverse voltage, one particular point will be there where diode was said that is breakdown occurs that is diode is breakdown and after that it will not function but if we are talking about zener diode then zener diode after this reason, zener diode's working is defined because the voltage at which the breakdown occurs is called the breakdown voltage now after this breakdown voltage, if I talk about zener diode So after breakdown, Zener diode operates in the region called Zener region. Yes, after breakdown, Zener diode works in a special region, which we call Zener region. Now, what is there in this region? In this region, Zener varies across its current.
Yes, Zener diode varies across its current. from the minimum to a maximum value and that is called IZ based on your supplied voltage so if you remember I told you about AC to DC conversion where we started rectifying there I said at the end there is a voltage regulator so what is the work of regulator? if input supply is changing or if input supply is going up and down then what should be the output? constant so how will the output be constant? in that case we apply ZENED OUT here because it is the property of ZENED OUT if it is changing in input supply up and down then it will up and down its cross current so that the current which will go to the output side IL it keeps its value constant so how does the functioning of Zener diode how does it work like a voltage regulator we will see that in the next lecture but for now I am giving you an introduction you just have to remember that what is Zener diode if to explain the VI characteristic of Zener diode then you can see that Zener diode is a special diode which works after breakdown for a special reason where the current can be vary across it with respect to the applied supply voltage right this is the VI characteristic of Zener diode the another main important thing which is put up on Zener diode question that is the breakdown mechanism here many times this question is asked in the question paper that what are the different types of breakdown mechanism so here there are two types of breakdown mechanism one is called Zener breakdown and one is called avalanche breakdown now.
What is breakdown you already know? So many already happily correct. I'm reverse bass. We're going to increase cut it out there reverse voltage Go increase cut it out there to a particular point pay particular voltage as he will a gi Jhapa, Apka Junction, Kajaga break, Ojega Junction break on a gum lab depletion layer increase cut a cut a cut a cut a Kajagi break ojegi and we have studied also give reverse current rises sharply to a large value This particular condition is called breakdown Now, the value at which the breakdown occurs on the particular reverse voltage is called breakdown voltage. So, this breakdown is divided into two categories in Zener diode.
One is called Zener breakdown which is a kind of normal breakdown and the second one is called Avalanche breakdown. So, here a question is put up many times. What is the difference between these two breakdown mechanics?
Yeah, explain, write a short note on Zener breakdown, Avalanche breakdown. So basically you should know about these two, only then you can write there. So let's see one by one, basically what is it?
So if I talk about Zener breakdown, then it is written in Zener breakdown, when a reverse voltage of 5 volt or less, right? When you are applying a voltage of 5 volt or less, then the breakdown occurs, that is called the Zener breakdown. So when we apply this voltage across Zener diode, so due to this very high reverse voltage, a large electric field develops across the depletion region. now this electric field is developed here this is strong enough to pull the electrons from the valence band to the conduction band now as I said this is a normal breakdown so when we are applying a reverse voltage 5V or less, so that reverse voltage is increasing the depletion layer so across that so much electric field is developed that will be enough to pull the electrons from the valence band to the conduction band. Now, how does valence move to conduction?
We had taught in the first lecture that basically, the semiconductor is tightly bounded. All the electrons are tightly bounded with their neighbouring item by forming covalent bond. So, when your electric field is so high, this electric field breaks your covalent bond.
Due to this, electrons become free and available for conduction. Due to these large number of electrons, a large reverse current is produced through the zener diode. and the breakdown occurs that is called the zener breakdown so basically what we are talking about till now that when we apply the reverse voltage and increase it one particular voltage comes where the depletion layer breaks so why depletion layer breaks because here your covalent bond breaks so much high electric field is there in your depletion laser that it breaks the covalent bond when covalent bond breaks electron becomes free and available for conduction so that's why Due to these large number of electrons, a large reverse current is produced through the Zener diode and that is called the Zener breakdown.
So, this is your VI characteristic which we had read. That is why I had said it as normal breakdown. When we apply reverse voltage then basically reverse situation current flows practically. And when the reverse voltage equals to this breakdown voltage, then here your current sharply increases. And in this Zener breakdown, because I said in the starting that you know how to write difference.
So, for difference, you you can make a point that this is the 5V or less when this is applied then Zener breakdown occurs Zener breakdown's VI characteristics changes sharply Second, the increase in the temperature causes the decrease in the breakdown voltage If you increase the temperature, then the voltage of Zener diode will decrease. So this is about the Zener breakdown. Now what is avalanche breakdown? In avalanche breakdown, it is written that when a reverse voltage higher than 8V is applied to the Zener diode. So here you can see the difference that we talked about in Zener diode.
When a reverse voltage 5V or less is applied to the Zener diode, then the breakdown occurs. It is called Zener breakdown. So if your reverse voltage is higher than 8V, then the breakdown will be avalanche breakdown why? because when we apply such a high reverse voltage then what happens? minority charge carriers tends to accelerate what are minority charge carriers?
if we talk about minority charge carriers suppose you have a p-type semiconductor then in p-type semiconductor, your holes are majority and there are some electrons which are minority charge carriers so when we apply the reverse voltage 8V or greater than 8V So, these minority charge carriers are exhalated. Exhalation means that this reverse voltage is increasing the kinetic energy of these charge carriers. Now, this kinetic energy increase means that these are the exhalated minority charge carriers.
They will further collide with the stationary atoms and transfer some of their kinetic energy to the valence electron present in the covalent bond. Right! This means that these stationary atoms will further and transfer the kinetic energy so that stationary atom will break the covalent bond and jump into the conduction band and become free for conduction so ultimately what is the difference if I talk about the difference in one line so when we are applying higher than 8 volts in avalanche breakdown so the charge carriers here minority charge carriers are also accelerating they are starting to cohesion in each other Due to this additional energy is generated and the additional energy is converting your stationary atoms into free electrons.
So, due to this collision, basically the free electrons that are formed due to this collision will accelerate the other more valence electrons by means of collision. Due to this a large number of free electrons will be available for conduction. This effect is called avalanche effect. i.e. one electron hit, it hit the other, the third one collided. Similarly, as the collision increases, the kinetic energy of all will increase.
So, there will be a more number of electrons when this higher voltage is applied to your zener diode. So, this effect is called avalanche effect. And a large reverse current starts flowing through the zener diode and the breakdown occurs called the avalanche breakdown.
So, if we show this breakdown in characteristics, then this is basically increasing gradually. it is gradually increasing whereas in general breakdown we have read that it is sharp the second difference is if you raise the temperature here then this will cause increase in breakdown voltage whereas in general we have read that increase in temperature will decrease the breakdown voltage right so if you get this question which most of the time comes to write notes then you can write in the form of notes like here I have copied it straight forward so that you can write it by yourself and the second thing if you know how to write difference then you can note the difference point here that here reverse voltage is 5 volt or less under the inner breakdown but here it is higher than 8 volt here kinetic energy is involved to make the reverse current here there is no concept of kinetic energy we can give difference on the basis of VI characteristic here sharp is changing and gradually changing here the breakdown voltage is increasing in avalanche here it decreases in zener doubt So, you can create these 3-4 points in a table form to make a difference between Zener and Avalanche. So, basically after this lecture, your two questions are basically complete on Zener. One is, what is Zener Dowd? If you are asked what is VI curve in respect to Zener Dowd, then you can explain.
By the way, most of the time the question put up here is put up in breakdown mechanism. What are the Zener breakdown and Avalanche breakdown mechanism? So, this topic is very important in this lecture.
Now, I will explain. Now, in Zenit Out, there is another topic on which question is put up most of the time and that is called, how does your Zenit Out work like a regulator, like a voltage regulator. So, we will discuss this thing in our next lecture with some numerical problem because most of the time, you get problems in this paper.
So, stay with me. See you in the next lecture. Thank you.