BJT (bipolar junction transistor)

A bipolar junction transistor is a three terminal semiconductor current controlled device with two P-N junctions. The three terminals are emitter(E), base(B) and collector(C). the emitter junction is heavily doped, base is less doped and made very thin and collector terminal is moderately doped. Collector has grater size than emitter and base terminal is thinner than both. (The thinner the base, the stronger the E-C electric field, and the larger the impact of a small current injected into the base. Explained clearly in active mode operation below) emitter terminal is moderate in size.

A BJT has two types of transistors:

1. NPN transistor


2. PNP transistor

NPN transistor :

In an NPN transistor a p-type material is sandwiched between two n-type materials.

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PNP transistor :

In a PNP transistor a n-type material is sandwiched between two p-type materials.

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Operation modes:

A BJT has three modes of operation

1. Saturation mode


2. Cut-off mode


3. Active mode


4. inverted active mode

Saturation mode:

In this mode transistor acts as a closed switch current freely flows from collector to emitter.

For this both emitter-base and collector-base junctions should be forward biased. This makes emitter and collector terminals short-circuited. For this base voltage must be grater than collector and emitter voltages.



Cut-off mode:

Cut-off mode is opposite of saturation mode. It acts as open switch when both the junctions are reverse biased. there is no collector current therefore no emitter current. the base voltage will be less than both the emitter and collector voltages.



Active mode:

In this region transistor acts an amplifier. For this emitter-base junction should be forward biased and collector-base junction should be reverse biased (emitter-base voltage is grater than zero and collector-base is negative).

Transistor as an amplifier:

A signal is amplified when it is sent from low resistance to high resistance. Here also same thing happens when emitter-base junction forward biased offers low resistance and collector-base is reverse biased offers high resistance.

Working of NPN in active mode:

• Emitter-base forward biased and collector-base reverse biased.

• The electron in emitter region(n-type) are repelled from the negative terminal of the battery towards junction and enters in to base region(p-type) as junction is in forward biased condition acts same as P-N junction diode.


• Width of the base region is very thin and lightly doped and hence only a few electrons will combine with holes in the p-region (i.e 2-5%).


• Remaining electrons are able to drift across the base and enter the collector region. They are swept by positive collector voltage.


• For every electron flowing out the collector and entering the positive terminal of battery, an electron from negative terminal of battery enters to emitter region.


• In this way electron conduction takes place continuously so long as two junctions are properly biased.

current conduction in NPN transistor is due to electrons. In case of PNP transistor conduction is due to holes and follows same mechanism.

                Here β is amplification factor.

               α is the common-base current gain.

you can relate both  α and β as follows

characteristic curves: