Bipolar Junction

Basic diodes are made up of two pieces of semiconductor material joined together to form a simple pn-junction.

When we connect two independent signal diodes back to back, we get two PN-junctions connected in series that share a common Positive, (P) or Negative, (N) terminal. The combination of these two diodes results in a three-layer, two-junction, three-terminal device that serves as a framework for a Bipolar Junction Transistor, or BJT for short.

Transistors are three-terminal active devices built of various semiconductor materials that, when a modest signal voltage is applied, may operate as an insulator or a conductor.

Because of the transistor’s capacity to move between these two states, it can perform two basic functions: “switching” (digital electronics) or “amplification” (analogue electronics).

What is a Bipolar Junction Transistor?

A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that consists of two p-n junctions that may amplify or magnify a signal. It is a device that is controlled by current. The base, collector, and emitter are the three terminals of the BJT. A bipolar junction transistor (BJT) is a kind of transistor that employs both electrons and holes as charge carriers.

When a tiny amplitude signal is applied to the transistor’s base, it is amplified and accessible at the transistor’s collector. The BJT provides amplification in this case. It should be noted that the amplification procedure does necessitate the use of an external DC power supply.

Types of Bipolar Junction Transistor 

BJTs are classified into two categories.

NPN Transistor and PNP Transistor

NPN

An npn-transistor is made up of two n-type semiconductor materials separated by a thin p-type semiconductor layer. The two terminals, Emitter and Collector, are taken from the two n-type semiconductors, whereas the Base terminal is taken from the p-type semiconductor.

The arrow on the emitter terminal of the BJT symbol represents the direction of conventional current in the emitter with forward bias. The conventional current flows out of the emitter of a npn-transistor, as represented by the outgoing arrow.

PNP

A pnp-transistor is made up of two p-type semiconductors separated by an n-type material layer. The Emitter and Collector terminals are obtained from the two p-type semiconductor layers, whereas the Base terminal is taken from the n-type semiconductor. The conventional current goes into the emitter of the pnp-transistor, as represented by the inward arrow.

Bipolar Junction Transistor Working principle and Applications

Working Principle

The EB(Emitter Base) junction is forward biassed, whereas the CB(Collector Base) junction is reverse biassed. The breadth of the CB junction’s depletion zone is greater than that of the EB junction. The forward bias at the EB junction lowers the barrier potential, causing electrons to flow from the emitter to the base.

The base is narrow and slightly doped, with very few holes and a small quantity of electrons from the emitter that recombine in the base area with holes and flow out from the base terminal. The combination of electrons and holes causes the base current to flow.

To start the collector current, a huge number of electrons will travel through the reverse bias collector junction. We can see the mathematical equation using kirchhoff’s-law.

IE = IB + IC

The base current is quite small in comparison to the emitter and collector currents.

IE ~ IC

The PNP transistor operates in the same way as the NPN transistor, with the sole difference being that holes are used instead of electrons. 

Applications

The bipolar junction transistor has the following applications:

1) These are the transistors that are commonly used in logic circuits.

2) It is used in amplification circuits.

3) These are used in oscillator circuits.

4) These are used in multi-vibrator circuits.

5) These are preferred for wave shaping circuits in clipping circuits.

6) It is used in timer circuits and time delay circuits.

7) These are used in switching circuits.

8) Used in detector circuits or as demodulation.

Conclusion 

The combination of these two diodes results in a three-layer, two-junction, three-terminal device that serves as a framework for a Bipolar Junction Transistor. A Bipolar Junction Transistor is a three-terminal semiconductor device that consists of two p-n junctions that may amplify or magnify a signal. The base, collector, and emitter are the three terminals of the BJT. A bipolar junction transistor is a kind of transistor that employs both electrons and holes as charge carriers. The two terminals, Emitter and Collector, are taken from the two n-type semiconductors, whereas the Base terminal is taken from the p-type semiconductor. The Emitter and Collector terminals are obtained from the two p-type semiconductor layers, whereas the Base terminal is taken from the n-type semiconductor. The conventional current goes into the emitter of the pnp-transistor, as represented by the inward arrow.