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What Is A Transistor ?

A transistor is a semiconductor device that provides a necessary amplification, it can be used as a switch, not only that in-fact various processors and controllers which we use in mobiles and various machines consist of millions or billions of transistors. But this blog will explain to you some basic information about the BJT transistor.


The name transistor is given to this particular model because it is widely used in amplification and while amplifying you are converting the low or weak signals into magnified or strong form i.e. you are transferring weak signal into amplified form (trans). This transfer of signal takes place from high resistance to low resistance (istor). The BJT transistor is further divided into 2 NPN and PNP transistors as shown below.
As we all know n-type has more amount of negative charges while p-type has less number of negative charges i.e. (e-). Therefore they are aligned in such a way that the desired output from it can be obtained. Since all the 3 regions are separated from each other therefore a BJT transistor has 3 terminals collector, base, and emitter.
Emitter:-The name emitter is given to one of the terminals of BJT because the emitter emits electrons. It is highly doped and its area is small as compared to a collector but its area is large as compared to the base.
Collector:-The name collector is given to one of the bjt terminals because it collects the electrons emitted from the emitter. The doping concentration of the collector region is less as compared to the emitter region but it is high as compared to the base region and it has the largest area among all the 3 terminals of transistors.
Base:-The doping concentration in the base region is very less as compared to the above 2 regions also the area of the base is less. Even though it is lightly doped and has very less area it plays a very significant role in the operation of transistors.


The area of the emitter is small than that of the collector and the doping concentration is high as compared to the collector while the area of the collector region is large and the doping concentration is less as compared to the emitter because this creates a pressure and the electrons get quickly emitted from emitter region into collector region. Imagine it as a spray container the small force given to the tip of the spray container results in the release of spray at high pressure. Similarly, a small base current at the base of the transistor produces a large amount of collector or output current.
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Why the collector region of BJT is greater than the emitter and base? 
The main reason why the collector region is large is that the emitted electrons from the emitter are more in number and it needs to be accumulated and then passed forward so if the collector region is equal to the emitter region then the electrons emitted from the emitter region gets stored in the collector region for more amount of time which in-short leads to some heating issues and if the number of electrons accumulated are more then the collector region may burn out completely and therefore the collector region is made little large than that of emitter region.
The major difference between NPN and PNP transistor is the flow of current in NPN the current flows from collector to emitter while in PNP the current flows from emitter to collector. In an NPN transistor, the current flow is because of electrons while in a PNP transistor the current flows because of holes, actually, there are very less number of electrons in PNP transistors therefore when electrons move in PNP transistors it creates a vacant space known as holes and due to this reason, we say that holes or positive charge are responsible to produce current in PNP transistor.
But NPN transistors are used in many applications this is because the electrons are lighter in weight as compared to holes also electrons can move or travel very quickly as compared to holes or in short you can assume that PNP has less number electrons and therefore the current produced will be less while in NPN transistor the electrons are more and therefore current produced in NPN transistors is more.
Out of 3 terminals, 1 terminal is kept common at the input and output side and therefore 3 configuration takes place. But before going into much detail just remember that BJT is treated as a current-controlled current source.Therefore the gain is calculated as (current gain) = output current(o/p I) / input current(i/p I)
1) Common Base:-
@ The gain obtained in this configuration is less than 1.
@ This configuration has moderate power gain.

2) Common Emitter:-
@ The gain obtained in this configuration is high.
@ Therefore the power gain is high in this configuration.

3) Common Collector:-
The gain obtained in this configuration is high.
But the power gain is less in this configuration.
So by looking at the above configuration it is clear that common-emitter provides high power gain therefore this configuration is used in various amplification applications

If you want to understand the concept in-depth visit the link given below.

Now the major problem is how we can recognize the terminals of BJT transistors?
1) For that there is a trick to remember it but this trick is only applicable to BJT(bc547), everyone knows the alphabet abc for a second just remember it as ebc.
2) You write this alphabet from left to right similarly you hold the flat side of the transistor in your left hand such that all the terminals should point out from your left hand.
3) The upper terminal is e-emitter, the middle one is b-base and the bottom one is c-collector.


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