Bipolar Junction Transistor (BJT)

Bipolar Junction Transistor

A bipolar junction transistor is a three terminal semiconductor device consists of two P-N junctions which are able to amplify a signal. It is a current controlled device. The three terminals of BJT are the base, the collector & the emitter. A BJT is a type of transistor that uses both electrons & holes as charge carriers.

Types of BJT

• NPN Transistor
• PNP Transistor

From the above figures, we can see that every BJT has three parts named emitter, base & collector. JE & JC represent the junction of emitter & junction of collector respectively.

1. NPN Transistor

In an NPN transistor one P-type semiconductor resides between two N-type semiconductors.

Now IE, IC is emitter current & collector current respectively & VEB & VCB is emitter-base voltage & collector-base voltage respectively. 

According to the convention if for the emitter, base & collector current IE, IB & IC current goes into the transistor the sign of current is taken as positive & if current goes out the transistor the sign of current is taken as negative.

2. PNP Transistor

In an PNP transistor one N-type semiconductor resides between two P-type semiconductors.

For PNP transistor, current enters into the transistor through emitter terminal. Like any bipolar junction transistor, the emitter-base junction is forward biased & the collector-base junction is reverse biased.

Working of BJT

Consider an NPN transistor, the BE junction is forward biased & CB junction is reverse biased. The width of the depletion region at the junction CB is higher as compared with BE junction.

When the BE junction is forward biased it decreases the barrier potential, hence the electrons starts flowing from the emitter to the base. As the base is thin & lightly doped it consists of very few holes so some of the electrons from the emitter recombine with the holes present in the base region & flow out the base terminal. 
Similarly, when you consider the PNP transistor, is same as of the NPN transistor, the only difference in that the majority charge carriers are holes instead of electrons.

Advantages of BJT

• It has a better voltage gain.
• It has high current density.
• It shows better performance at high frequency.
• The BJT has a large gain bandwidth.
• There is a low forward voltage drop.
• It can be operated in low or high power applications.

Disadvantages of BJT

• It more noise produced.
• It has a low thermal stability.
• The switching frequency of a BJT is low.
• The BJT is more an effect of radiation.

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