Draw the construction of BJT (NPN) and explain its working principle. State and explain different operating regions.

Draw the construction of BJT (NPN) and explain its working principle. State and explain different operating regions.
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1 Answer

Answer :

Construction of BJT (NPN):

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Working of NPN Transistor : The circuit diagram of the NPN transistor is shown in the figure below. The forward biased is applied across the emitter-base junction, and the reversed biased is applied across the collector-base junction. The forward biased voltage VEB is small as compared to the reverse bias voltage VCB.

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The emitter of the NPN transistor is heavily doped. When the forward bias is applied across the emitter, the majority charge carriers move towards the base. This causes the emitter current IE. The electrons enter into the P-type material and combine with the holes. 

The base of the NPN transistor is lightly doped. Due to which only a few electrons are combined and remaining constitutes the base current IB. This base current enters into the collector region. The reversed bias potential of the collector region applies the high attractive force on the electrons reaching collector junction. Thus attract or collect the electrons at the collector. 

The whole of the emitter current is entered into the base. Thus, we can say that the emitter current is the sum of the collector or the base current. 


Active region. 

The region between cut off and saturation is known as active region. In the active region, collector-base junction remains reverse biased while base-emitter junction remains forward biased. Consequently, the transistor will function normally in this region. 


Saturation. 

The point where the load line intersects the IB = IB(sat) curve is called saturation. At this point, the base current is maximum and so is the collector current. At saturation, collector base junction no longer remains reverse biased and normal transistor action is lost.

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If base current is greater than IB(sat), then collector current cannot increase because collector-base junction is no longer reverse-biased.

OR 

Both junction are forward bias and ouput current change with output biasing voltage transistor in saturation region. In this region transistor act as closed switch.


Cut off. 

The point where the load line intersects the IB = 0 curve is known ascut off. At this point, IB = 0 and only small collector current (i.e. collector leakage current ICEO) exists. At cut off, the base-emitter junction no longer remains forward biased and normal transistor action is lost. The collector-emitter voltage is nearly equal to VCC i.e. VCE (cut off) = VCC


OR

Both junction are reversed bias and ouput current is Zero with input current is Zero transistor in cut off. In this region transistor act as open switch

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