In this content, you can learn about introduction of bipolar junction transistor and it's construction.


Bipolar Junction


 Transistor ( B J T )


A bipolar junction transistor (B JT) is a semiconductor device which is generally used to amplify a low signal. The word "bipolar" is made by two words" bi" and" polar" . Where" bi" stand for two and polar stand for polarity. 


In this device the conduction is due to two type of polarity of charge carrier i.e electrons and holes. The electrons is negative polarity of charge carrier where hole is positive polarity of charge carrier.


              Bipolar junction transistor is also known as current control device ( CCD). It consists three semiconductor layer and three terminals. It is low input resistance device.


 Leakage current is existing due to the presence of minority charge carriers. It is noisy device, due to presence of minority carriers. Bipolar junction transistor is temperature sensitive device.


Types of Bipolar junction transistor


There are two types of Bipolar junction transistor.


(a) NPN Transistor


( b) PNP Transistor


Construction:-


A bipolar junction transistor is constructed by using three semiconductor layer and it's construction is known as sandwich type construction. It consists two outer layer of same semiconductor material and one inner layer ( middle layer) different and semiconductor layer.


 In NPN Semiconductor as shown in the given figure:-




Circuit symbol of NPN transistor


The circuit symbol of NPN transistor as shown in given figure:-




In PNP Transistor, both outer layer is of P-type semiconductor but middle layer is of N-type semiconductor as shown in the given figure.




Circuit symbol of PNP Transistor


The circuit symbol of PNP transistor as shown in the given figure.





The transistor has three region, namely; emitter, base and collector.


(a) Emitter


(b) Base


(c) Collector


(a) Emitter:-


Emitter is left hand region of the transistor and it's main function is to supply majority charge carriers (electrons in case of NPN and holes in case of PNP Transistor) to the base.


 The emitter is always forward biased w.r.t base so that it is able to apply majority charge carriers to the base. The emitter is heavily doped so that it may be able to inject a large number of charge carriers. The emitter region is of moderate size.


(b) Base:-


Base is middle region of the transistor. The base is much smaller in size than emitter and collector region. It is very lightly doped region to reduce the recombination with this region so as it increases collector current.


(c) Collector:-


Collector is the right hand region of the transistor and it's main function is to collect majority charge carriers. Collector is always reverse biased so as to remove the charge carriers away from its junction with the base. It is moderately doped and this region is large in size to withstand the temperature generated at the collector.



The bipolar junction transistor consists two pn-junction i.e. emitter-base junction (j1) and collector base junction (j2).


Note:-


Both type of (NPN and PNP) are same in charge concentration and in size of semiconductor layer except that type of semiconductor layer is used.


Conclusion:-


You have learnt about introduction of bipolar junction transistor and it's construction and it's symbol.