Hey, friends welcome to the YouTube channel ALL ABOUT ELECTRONICS. So in this video, we will learn about the Bipolar Junction Transistor. The invention of the transistor, led the invention of the many other semiconductor devices including the integrated circuits.
And in fact, due to these integrated circuits, the modern day computers, and other electronic gadgets, which we are using is possible. So the bipolar Junction transistor or the BJT is a three terminal semiconductor device, which can act as a conductor or insulator based on the applied input signal. And due to this property, the transistor can be used as a switch in the digital electronics.
Or it can be used as an amplifier, in the analog electronics. So nowadays, the field effect transistors, are widely used in the electronic industry. But still the BJTs are extensively used.
And anyone who is interested in the electronics, should have some basic knowledge of this BJT. So in this bipolar Junction transistor, there are three doped regions. The Emitter, the Base and the Collector.
And based on the doping of these three regions, it is known as either NPN or PNP transistor. So in case of the NPN transistor, both Emitter and the Collector are doped with the N-type impurity, and the Base is doped with the P-type impurity. On the other end, in the PNP transistor, the Base is doped with the N-type impurity, and the Emitter and the Collectors are dope with the P-type impurity.
And here, the term bipolar indicates that, both electrons and holes contributes in the flow of current. Now if you look inside this BJT, then there are two PN junctions. One is between the Emitter and the Base, and second is between the Base and the Collector.
And it appears as if, the two back-to-back diodes are connected in the series. But actually, it won't behave like that. Because when we connect to back-to-back diodes, then we are assuming that, there is no interaction between the two diodes.
That means these two diodes are operating independently. But in case of the BJT, actually there is an interaction between the two regions. So if we connect the two back to back diodes like this, then it won't behave like a BJT.
Now if we talk about the internal construction of the BJT, then the Emitter is heavily doped, and the function of the Emitter is to supply the electron. And in fact, that is why it is known as the Emitter. Then if you talk about the Base, then it is lightly doped, and the doping concentration of the Collector is between the Emitter and the Base.
That means the Collector is the moderately doped. And if we talk about the width of these three regions, then the Base region is much narrower, compared to the two regions. So in terms of the width, the Collector region is wider, than the other two regions.
Because the job of the Collector region is to collect the electrons, which is supplied by the Emitter. And in fact, that is why, it is known as the Collector. Now depending on the biasing, the BJT can be operated in three regions.
The Active Region, the Cut-off Region and the Saturation Region. So in case of the Active Region of operation, the Emitter Base Junction, is forward biased. and the Base Collector Junction is reversed biased.
So let's say, the voltage at the Emitter is VE, the voltage at the Base is VB, and the voltage at the Collector is VC. And to forward bias this Base Emitter Junction, the voltage of Base should be greater than the Emitter. And similarly, to reverse bias this Collector Base Junction, the voltage at the Collector should be greater than Base.
That means, to operate the BJT, in the active region, we can say that, the Collector voltage should be greater than Base voltage, and the Base voltage should be greater than Emitter voltage. So once this condition is satisfied, then the BJT will operate in the active region. Similarly, in the Cut-Off region both Base-Emitter junction and the Base Collector junctions are reversed biased.
So to operate the BJT in this region, the Emitter voltage, should be greater than Base voltage, and at the same time, the Collector voltage should also be greater than Base voltage. Similarly, in case of the Saturation Region of operation, both Base Emitter, and the Base Collector junction of the BJTs, are forward biased. That means in this region of operation, the Base voltage VB is greater than Emitter voltage, and at the same time, this Base voltage, is also greater than Collector voltage.
So these are the three regions of operation, in case of the BJT. Apart from that, there is a one more region of operation, which is known as the Reverse Active Region of Operation. So in this region of operation, instead of Base Emitter Junction, here the Base Collector Junction is forward biased, and the Base Emitter Junction is reverse biased.
But in this region of operation, the gain provided by the BJTs very less, and due to that, this region of operation is usually avoided. Similarly, if we talk about the PNP transistor, then in case of the Active Region of Operation, this Base Emitter Junction is forward biased, and the Collector Base Junction is reverse biased. But in this case, now the Emitter voltage is greater than the Base voltage, and similarly the Base voltage is greater than Collector voltage.
So we can say that, in case of the PNP transistor, to operate it in the active region, the Emitter voltage should be greater than Base voltage, and the Base voltage should be greater than the Collector voltage. And similarly, this PNP transistor can also be used in the different regions. So whenever, the BJT is used for the amplification, then it is used in this Active Region.
And whenever it is used as a switch, and it is used in the Saturation in the Cut-off Region. And in the upcoming videos, we will see in detail about these different regions of operation. Now if we talk about the symbol, then this is the symbol of the NPN transistor.
So these three terminals, are the Base Collector and the Emitter. And here, this arrow indicates the direction of the current during the Active Region of Operation. So in case of the NPN transistor, the current will flow from the Base towards the Emitter.
On the other end, if you see the symbol of the PNP transistor, then it is similar to the NPN transistor, but here the direction of the arrow will get reversed. So now, the current will flow from the Emitter towards the Base region. Now as I said, when the BJT is used for the amplification of the signal, then it is operated in the active region.
And there are different ways to configure it. So in case of the common Emitter configuration, the Emitter terminal is common between the input and the output. That means, in this configuration, the input signal is applied between the Base and the Emitter, and the output is measured, between the Collector and the Emitter terminal.
Similarly in case of the common Collector configuration, the Collector terminal is common between the input and the output side. And likewise in the common Base configuration, the Base terminal is common between the input and the output side. So each configuration has its own advantage and disadvantage, and we will see all these configurations in detail in the upcoming videos.
But in short, depending on the requirement, and the application, the BJT can be configured in any of these three configurations. All right, so now let's understand the working of the BJT, whenever it is operated in the active region. And here, we will take the example of the NPN transistor.
Now before we understand the working, let me just clear the notations, which is used for the supply voltages. So as you can see were here, for the Base and the Collector supplies, double subscription is used. That means here, this VBB is a source voltage for the Base.
And the VCC is the source voltage which is connected to the Collector terminal. And this voltage VBE defines the difference between the voltage at the Base and the Emitter terminal. So this voltage VBE can be defined as voltage VB minus VE.
So if the voltage at the Base terminal is let say VB and the voltage at the Emitter terminal is VE, then the voltage VBE can be defined as voltage VB minus VE. And here, this voltage VB and VE are measured with respect to the ground terminal. Similarly, this voltage VCE can be defined as the Collector voltage VC minus VE.
So instead of VBE if we write voltage VEB, then it can be written as voltage VE minus VB. And this voltage, will be negative of the VBE. So throughout our discussion on the BJT, we will use these notations.
So here these Base voltage, and the Collector voltage, is applied in such a way that, the base Emitter Junction will get forward biased. And the Base Collector Junction will get reversed bias. That means over here, the BJT is biased in such a way that, this voltage VBE is positive, and this voltage VCB is also positive.
Now if you notice over here, this is the PN Junction. So whenever this PN Junction is forward bias, and the typical photo voltage drop across this diode is in the range of 0. 6 to 0.
7 volt. That means, whenever we forward bias these Base Emitter Junction, then the typical voltage drop between these Base and the Emitter Junction, will be roughly around 0. 7 volt.
Now once we apply this biasing voltages, and the electrons from the Emitter, will be pushed towards the Base terminal. Because as I said earlier, the Emitter is the heavily doped. That means the Emitter has the large number of electrons, as the majority carriers.
And once we apply the biasing voltage, then this negative voltage will push the electrons towards the Base region. So due to that, the electrons will starts moving towards the Base region. Now once the electrons enters this Base region, and there are two paths for them.
One is they can flow towards the positive terminal on the left, and the second is they can flow into the Collector region. But most of the electrons will enter into the Collector region. Because if you see over here, the Base is lightly doped.
That means a number of holes in this Base region, is very small compared to the electrons, which are coming from the Emitter region. That means the free electrons, which has come into this Base region will see the longer lifetime. And the second reason is, the width of this Base region is very thin.
That means most of the electrons, will be able to escape this Base region, and they can go into the Collector region. That means in this Base region, only few electrons will recombine with these holes, and they will be get attracted towards the positive terminal of this VBB. And the remaining electrons will enter into the Collector region.
Now, if you notice over here, once the electrons from the Emitter enter into the Base region, then they will become the minority charge carriers. And if you see over here, the Base Collector Junction is reversed biased. So due to the applied electric field at the Collector terminals these minority charge carriers, or the electrons will get attracted towards the Collector terminal.
So once the electrons enters into this Collector region, then they will get attracted by the positive terminal of this VCC. So if you see the direction of the flow of electron, and from the emitter the electrons will flow in this direction, and most of the electrons which is emitted by the Emitter will get collected by the Collector region. And very small amount of electrons only will flow in this direction.
And if we see the direction of the holes, then it will be exactly opposite to the flow of electrons. And in fact, the conventional current, will also flow in the same direction. That means, the Base current IB will flow in this direction, while the Collector and the Emitter current will flow in this direction.
So now let us establish the relationship between all these currents. So if you apply the KCL, then we can say that this current IB plus IC, that is equal to Emitter current. That means the Emitter current is the summation of this Base current and the Collector current.
As I said, only fraction of electrons, are able to go in this direction. That means the Base current will be very small. Or we can say that, this Collector current IC, is approximately equal to IE.
And exactly it can be defined as Collector current IC is equal to alpha times IE. And this alpha defines, what fraction of the Emitter current is flowing through the Collector terminal. Now if we put this value of IC in this expression, then we can say that, this Base current IB plus alpha times IE is equal to IE.
That means the Base current IB is equal to 1 minus alpha times Emitter current. And once again if we put the value of IE in terms of the Collector current, then we can say that, the base current IB is equal to 1 minus alpha times IC divided by alpha. That means the Base current IB is equal to 1 minus alpha divided by alpha times Collector current.
Or we can say that, the collector current IC is equal to alpha divided by 1 minus alpha times Base current. And let's say this is equal to beta. That means the Collector current IC is equal to beta times IB.
So this beta is known as the current gain of the BJT. And typically, the value of beta varies from 50 to 400 for the different transistors. So from this we can say that, IB plus IC that is equal to Emitter current.
That means Base current IB plus beta times IB is equal to IE. That means the Emitter current IE can be given as beta plus 1 times IB. So this is the relationship between the Base current, Emitter current and the Collector current.
Now if you notice over here, in this common Emitter configuration, this Base current is the current on the input side, while the Collector current is the current on the output side. And these two currents, are related by this expression. That means by controlling this Base current on the input side, it is possible to control the collector current.
And that is why, these Bipolar Junction Transistors are known as the current control device. That means just by controlling the input current on the Base side, it is possible to control the output current. On the other end, if you see the other type of transistor, that is the field effect transistor it is the voltage control device.
That means in that case, by controlling the input voltage, it is possible to control the output current. Also if you notice over here, in this configuration of the BJT, the output collector current gets amplified by the factor of beta. And if we connect the resistor between the Collector and the Emitter terminal, then it is possible to amplify the input signal.
That means after biasing by BJT, in this configuration, if we apply the AC signal at the input, then it is possible to amplify that signal. And we will discuss about it in the detail in the upcoming videos. Now during our discussion, we haven't considered the current due to the minority charge carriers in this collector region.
Because if you notice over here, this Base collector Junction is reversed bias. So for a moment, if we remove this Emitter connection, then the current which is flowing through the Collector is only due to the minority charge carriers. And let's say, this current is equal to IC O.
So this current is similar to the reverse saturation current, which we have seen in the PN junction diode. So the total Collector current ICT will be equal to the IC plus ICO. Where this ICO is the current due to the minority charge carriers.
And typically this current is in the range of microamperes. While this Collector current IC is in the range of milliampere. So this is all about the different types of currents in the BJT.
So in the upcoming videos, we will see the different configurations of the BJT, as well as the input and output characteristics of the BJT. And we will also see, how the BJT can be biased using the different techniques. But I hope in this video, you got a brief overview about the BJT.
So if you have any question or suggestion, do let me know here in the comment section below. If you like this video hit the like button, and subscribe the channel for more such videos.
