BJT (Bipolar Junction Transistor)

In this article, we will discuss about the bipolar junction transistor, which is also known as BJT. The BJT is a solid-state device and used for amplifying, controlling and generating electrical signals.

What is the meaning of “bipolar”? Bipolar word is used because of the current through the BJT is by both the carriers; the electrons and the holes. The junction stands for, inside the BJT, there is PN junction and Transistor is stands for transfer of resistance.

Construction:

The transistor has three terminals; emitter (E), base (B) and collectors (C).

There are two types of transistors;

              1)      NPN transistor

              2)      PNP transistor

      Symbol:

1)  NPN transistor: In this type of BJT, three regions are used; one P region and two n region. P region is sandwich between two N regions. Here, emitter and collector are connected with N type of region and base is connected with P type region. The base terminal is always lightly doped and thin area. The collector terminal is highly doped and wide area.

As shown in above figure, there are three types of current; emitter current (IE), base current (IB), and collector current (IC). As we discuss earlier, base is lightly doped and thin, value of base current is very low (in terms of micro-amps). The value of collector current and emitter current is nearly equal. By the above figure, we can say that, the collector current is addition of the base current and the emitter current.

2)  PNP transistor: In this type of BJT, two P type of region and one n type region is used. The collector and emitter terminal of transistor is connected as P reigon and the base region is connected as N type region.

As shown in above figure, the emitter current is addition of the base current and the collector current.

Configuration:

Transistor has three terminals. With respect to these terminals, transistor can use with common emitter, common base and common collector configurations. The basic two configurations are common emitter and common base. By this configuration, we can define current gain of transistor. Current gain is defined as ratio of output current and input current.

For common emitter configuration, the output current is IC and the input current is IB. so, gain is IC/IB and it is denoted as α.

For common base configuration, the input current is IE and the output current is IC. So, gain is IC/IE and it is denoted as β.

Modes of operation;

The transistor will work on three regions;

                1) Active

a.       Forward active

b.       Reverse active

                2) Saturation

                3) Cut-off

Mode	NPN	PNP	Feature	Usage
Cut-off	Reverse	Reverse	No current	Switch
Forward active	Forward	Reverse	Voltage controlled current	Amplifier
Saturation	Forward	Forward	Low resistance	Switch
Reverse active	Reverse	Forward	Voltage controlled current	Usually not used