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How a Transistor works?

Constructional details: 

                   The transistor is a solid state device,whose operation depends upon the flow of electric charge carriers with in the solid.The transistor is capable of amplification and in most respect it is analogous to a vacuum triode. The main difference is between the two is that the transistor is a current controlled device where as vacuum triode is voltage controlled device. the transistor can replacing vacuum tubes in almost all applications.The advantages obviously small size,light weight, rugged construction, more resistive to shocks and vibrations, instantaneous operation, low operating voltage, high operating efficiency and long life with essentially no ageing effect if operated within permissible limits of temperature and frequency.
            The drawbacks in comparison to vacuum tubes are loud hum noise, restricted to operating temperature 75℃ and operating frequency upto a few MHZ only.


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A P-type material is sandwiched between two N-type materials.We have one junction between emitter and base is called as Emitter-Base junction,and another junction is between base and collector i.e Collector-Base junction.

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An N-type material is sandwiched between two P-type materials. this also having two junctions as explained above.

Transistor Terminal Theory: 

Transistor is a single crystal device in which there are two P_N junctions as shown above in diagram. The idea behind is to have first section to supply charges either electrons or holes to be collected by the third section through the middle section. 


                It is placed at left hand section of the transistor and its main function is supply majority charge carriers.i.e electrons in NPN transistor and holes in PNP transistor. The emitter is always forward biased w.r.t base for always supplying the charges to the base. The emitter is heavily doped so that it may inject a large number of charge carriers.  


       It is the middle section of the transistor and is very lightly doped and it is very thin in comparison to either either emitter or collector. So that it may pass most of the injected charge carriers to the collector. 


         It is in the right hand section of the transistor and its main function is to collect majority charge carriers. Collector is always reverse biased so as to remove the charge carriers way from its junction with the base. It is moderately doped.

Transistor action:

Transistors of both types N-P-N and P-N-P behave exactly in the same way except change in biasing and majority charge carriers. In P-N-P transistors the conduction is by holes whereas in N-P-N transistors conduction is by electrons. However N-P-N transistors are preferred due to their better high frequency response.

Working of N-P-N transistor:

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N-P-N Transistor

The emitter is forward biased and as a result a large forward current flows across the emitter junction due to flow of majority carriers i.e electrons in this case enters from the emitter region to base region and as well as holes from the base to emitter region. However since the conductivity of emitter region is larger than that of base region, the electrons from the emitter out the number of holes from the base region. It may be assumed with good accuracy that forward current in the emitter junction flows due to movement of electrons from the emitter to base. 
Therefore we can say that, the electrons from the emitter region due to forward bias in the emitter junction crosses into the base region where electrons are in minority.Injection of electrons makes the electrons concentration on the emitter junction very large and on the collector junction extremely small. 

Injected electrons diffuse into collector region due to extremely small thickness of base which is much less than the diffusion length. Most of the electrons cross into the collector region. Collector is reverse biased and creates a strong electrostatic field between base and collector. The field immediately collects the diffused electrons which enter the collector junction. Flow of electrons into the base region when confronted with the holes, a few electrons combine and neutralize and rest of the electrons say 95 to 99% of the injected electrons diffuse into the collector region and collected by the collector electrode. To maintain the base neutrality base electrodes provides equal number of electrons which have combined with holes and results in a base current. 

Thus the emitter current Ie is equal to the sum of collector current Ic and base current Ib. The ratio of collector current to emitter current is ∝  (current amplification factor).

Working of P-N-P Transistor:

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P-N-P Transistor

In a P-N-P transistor , the forward bias causes the holes in the P-type emitter to flow towards the base. This reduces the potential barrier at the junction and thus holes cross the junction and penetrate into the N-region. This constitutes the emitter current Ie. The width of the base region is very thin and it is lightly doped hence only 1 to 10% of the holes recombine with free electrons of the N-region. This constitutes the base current Ib, which, of course is very small. Rest of the holes drift across the base and enter the collector region and are swept away by the negative collector electrode. This constitutes the collector current Ic. Thus the current conduction in P-N-P transistor is by movement of holes. However, the current conduction in the external circuits is by electrons.   

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