فهم ترانزستور ثنائي القطب BJT: الوظائف والتشغيل والتكوينات

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.