What is BJT?

BJT full form is bipolar junction transistor. The BJT construction explains its operational features. After taking this lecture you should be able to understand following things

1. Transistor construction
2. Transistor Symbol
3. Transistor types, i mean NPN and PNP transistor
4. Transistor Biasing

Transistor Construction

Before reading this section i recommend you kindly take my lecture on pn junction diode.

We make three semiconductor regions (p-type and n-type) in BJT construction. As compared to diode which has only one pn junction, transistor has two pn junctions. (See Fig-1)

Fig-1: Bipolar Junction Transistor (BJT)

There are three terminals of transistor called emitter, base and collector. The base is sandwiched between emitter and collector. Emitter and collector are always made of same type of semiconductor material. The base has always different semiconductor material as compared to emitter and collector.

Collector is larger than emitter and base. The base is very thin as you can see in the above fig-1.

Transistor Types

There are two types of transistor i.e NPN BJT and PNP BJT

1. NPN BJT

In npn transistor emitter and collector are made of n-type semiconductor material whereas base is made of p-type semiconductor material. See fig-2

Fig-2: NPN BJT

2. PNP BJT

In pnp transistor emitter and collector are made of p-type semiconductor material whereas base is made of n-type semiconductor material. See fig-3

Fig-3: PNP BJT

Transistor Symbol

The following are the symbols for both npn and pnp transistors

Fig-4: NPN and PNP transistor symbols

Emitter, base and collector are denoted by E,B and C respectively. You can see the arrow is outward in case of NPN transistor. Whereas it is inward for PNP transistor.

Transistor Biasing

Biasing means to make transistor able to work by applying external dc voltages. If transistor is not biased properly, then it will not perform the required task. The following figure shows the common emitter biasing of both npn and pnp transistors.

Fig-5: Transistor Biasing

For proper transistor operation the B-E junction is forward biased and B-C junction is reversed biased.

For npn, the base is connected to +ve terminal of the battery (Vbb) and emitter is connected to -ve terminal of Vbb. The collector is connected to +ve terminal of battery Vcc.

The polarities are reversed in case of pnp. But remember the B-E junction is always forward biased and B-C junction is always reverse bias. Or in other words the collector is always has more potential as compared to base.

The fig-6 shows current flow inside npn transistor. When you apply proper biasing the electrons enters into the base from emitter. The emitter is highly doped.

Why Emitter Current is always Greater than Collector Current?

As the base is very thin so very few electrons combine with holes in the base. Whereas most of the electrons are attracted by the high potential of the collector. Very few electrons are also attracted by the +ve potential of the base.  

Fig-6: Flow of current inside transistor

You can observe is the above figure, that from emitter the electrons are going to base and collector. Another scenario is that two types of currents are entering into emitter i.e base current (Ib) and collectors current (Ic). So emitter current (Ie)is always larger than base and collector current.

This can be expressed by the following equation

$I_e=I_b+I_c$