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BJT CIRCUIT CONFIGURATION

In any circuit, two terminals are required for input and two terminals are required for output.
Therefore, totally four terminals are required for both input and output sides.
But in a transistor only three terminals are there.
So in order to overcome this problem one terminal should be kept common to both input and output.
This is the need for transistor circuit configuration.
Based on the common terminal the transistor can be configured in three ways....
        Common-Base (CB)
        Common-Emitter (CE)
        Common-Collector (CC)

Let us see the working of both PNP & NPN transistors in all the three circuit configurations.
In circuit configuration we are going to see about current flow & voltage.
(NOTE: The current considered here is electron current)

RULES:

There are two currents & two voltages one is actual & other is conventional.
Actual: True behavior of the circuit.
Conventional: universally accepted behavior of the circuit.
RULE 1: Assume all the currents entering into transistor as positive. If the actual direction coincides with the conventional direction, then it is taken as positive. Otherwise take is as negative.
RULE 2: Assume voltage near the common terminal as negative or low potential. If the actual voltage coincides with the conventional voltage, then it is taken as positive. Otherwise take is as negative.

COMMON-BASE CONFIGURATION (CB):

In this configuration, the base is common to both input and output terminals. The input terminals are Emitter-Base and the output terminals are Collector-Base.
EMITTER CURRENT:
    NPN: Actual emitter current is output from the terminal because electrons (majority carriers) move towards base. The actual direction doesn't coincide with the above rule. Hence emitter current is taken as negative.
    PNP: Actual emitter current enters into terminal because electrons (minority carriers) move towards positive terminal. The actual direction coincides with the above rule. Hence emitter current is taken as positive.
BASE CURRENT:
    NPN: Actual base current enters into base region. It coincides with the above rule. Hence base current is taken as positive.
    PNP: Actual base current comes out from base region. It doesn't coincide with above rule. Hence base current is taken as negative.
COLLECTOR CURRENT:
    NPN: Actual collector current enters into transistor because electrons come out under the influence of positive potential. The actual direction coincides with the above rule. Hence collector current is taken as positive.
    PNP: Actual collector current comes out of the transistor because electrons move inward under the influence of negative potential. The actual direction doesn't coincide with the above rule. Hence collector current is taken as negative.
EMITTER-BASE VOLTAGE:
    NPN: The DC voltage is connected so as to make emitter terminal negative and base terminal positive such that emitter is having low potential. These polarities don't coincide with above rule. Hence Emitter-Base voltage is negative.
    PNP: The DC voltage is connected so as to make emitter terminal positive and base terminal negative such that emitter is having high potential. These polarities coincide with above rule. Hence Emitter-Base voltage is positive.
COLLECTOR-BASE VOLTAGE:
    NPN: The Dc voltage is connected so as to make collector terminal positive and base terminal negative such that collector is having high potential. These polarities coincide with above rule. Hence Collector-Base voltage is positive.
    PNP: The DC voltage is connected so as to make collector terminal negative and base terminal positive such that collector is having low potential. These polarities don't coincide with above rule. Hence Collector-Base voltage is negative.

COMMON-EMITTER CONFIGURATION (CE):

In this configuration, the emitter is common to both input and output terminals. The input terminals are Base-Emitter and the output terminals are Collector-Emitter.
EMITTER CURRENT, BASE CURRENT and COLLECTOR CURRENT are same as that of common base. The change only will be in voltage.
BASE-EMITTER VOLTAGE:
    NPN: The DC voltage is connected so as to make base terminal positive and emitter terminal negative such that base is having high potential. These polarities coincide with above rule. Hence Base-Emitter voltage is positive.
    PNP: The DC voltage is connected so as to make base terminal negative and emitter terminal positive such that base is having low potential. These polarities don't coincide with above rule. Hence Base-Emitter voltage is negative.
COLLECTOR-EMITTER VOLTAGE:
    NPN: The Dc voltage is connected so as to make collector terminal positive and emitter terminal negative such that collector is having high potential. These polarities coincide with above rule. Hence Collector-Emitter voltage is positive.
    PNP: The DC voltage is connected so as to make collector terminal negative and emitter terminal positive such that collector is having low potential. These polarities don't coincide with above rule. Hence Collector-Emitter voltage is negative.

COMMON-COLLECTOR CONFIGURATION (CC):

In this configuration, the collector is common to both input and output terminals. The input terminals are Base-Collector and the output terminals are Emitter-Collector.
EMITTER CURRENT, BASE CURRENT and COLLECTOR CURRENT are same as that of common base. The change only will be in voltage.
BASE-COLLECTOR VOLTAGE:
    NPN: The Dc voltage is connected so as to make base terminal positive and collector terminal negative such that base is having high potential. These polarities coincide with above rule. Hence Base-Collector voltage is positive.
    PNP: The DC voltage is connected so as to make base terminal negative and collector terminal positive such that base is having low potential. These polarities don't coincide with above rule. Hence Base-Collector voltage is negative.
EMITTER-COLLECTOR VOLTAGE:
    NPN: The Dc voltage is connected so as to make emitter terminal positive and collector terminal negative such that emitter is having high potential. These polarities coincide with above rule. Hence Emitter-Collector voltage is positive.
    PNP: The DC voltage is connected so as to make emitter terminal negative and collector terminal positive such that emitter is having low potential. These polarities don't coincide with above rule. Hence Emitter-Collector voltage is negative.

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)