DAY 6 OF ELECTRONICS

BIPOLAR JUNCTION TRANSISTOR (BJT)

               

BJT is also called as junction transistor.
It consists of two P-N junctions.
The junctions are arranged in such a way that one type of semiconductor material is common to both the junctions.
In that way, it provides two possible types to construct a transistor.

• NPN transistor
• PNP transistor

NPN: One P-type semiconductor is sandwiched between two N-type semiconductors.
PNP: One N-type semiconductor is sandwiched between two P-type semiconductors.

THREE TERMINALS:

It has three terminals Emitter, Base and Collector.

EMITTER: This region is on one side of transistor and it is heavily doped. In NPN emitter is of N-type and in PNP emitter is of P-type.

BASE: This region is in middle of transistor and it is lightly doped. In NPN base is of P-type and in PNP base is of N-type.

COLLECTOR: This region is on other side of transistor and its doping lies between emitter and base. In NPN collector is of N-type and in PNP collector is of P-type.

The junction between Emitter and Base is called as Emitter-base junction. And junction between Collector and Base is called as Collector-base junction.

WORKING:
UNBIASED TRANSISTOR:

An unbiased transistor is called as open-circuited transistor.
Under unbiased condition diffusion of majority carriers takes place across two junction due to concentration gradient (i.e. to achieve equilibrium)
Due to diffusion of majority carriers depletion region is formed.
The diffusion of carriers continues and finally attains equilibrium condition. Thus thereafter no diffusion of carriers takes place.

BIASED TRANSISTOR:

When two junctions are biased using external DC voltages, the transistor is called biased transistor.
As there are two junctions, two Dc sources are required.
Based on the biasing a transistor have three modes of operation.

• Active mode
• Saturation mode
• Cut-off mode

(NOTE: All the modes are explained here with NPN transistor)

ACTIVE MODE:

In this junctions are biased as, Emitter-base junction: Forward-biased & Collector-base junction: Reverse-biased.

The negative potential between emitter-base causes the electrons to flow towards base. This constitutes emitter current.

The electrons that enter into the base recombine with holes and constitute base current.However, only few electrons recombine with holes and remaining crosses the junction enters into collector region due to the fact that the base is lightly doped.

The electrons that entered into the collector region + electrons already exist in the collector region get attracted towards the positive terminal because of reverse bias.

So that it produces collector current. Also there will be a continuously flow of current from one terminal to another such that forming closed loop.The current will flow without any interruption for this reason only this is called as active mode.

SATURATION MODE:

In this junctions are biased as, Emitter-base junction: Forward-biased & Collector-base junction: Forward-biased.The negative potential between emitter-base cause electrons to flow towards base. This constitutes emitter current.Likewise negative potential between collector-base cause electrons in collector region to flow towards base.

Because of flow of both the region's electrons towards base more recombination takes place and this abruptly increases base current. The width of junctions is small.Hence there will be high current. The transistor in this mode is used for closed switch. 

CUTOFF MODE: 

In this junctions are biased as, Emitter-base junction: Reverse-biased & Collector-base junction: Reverse-biased The positive potential between emitter-base and collector-base causes electrons to flow towards the terminal. This increases the width of the junction.Hence there will be no chance of recombination and no current flow.

The transistor in this mode is used for open switch.

(NOTE: Normally the operation of transistor will be explained in active mode)