Bipolar Junction Transistor-BJT

Posted By on December 19, 2014

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he bipolar junction transistor, BJT, is the cornerstone of much of today’s semiconductor electronics industry.

This form of transistor has been in existence for many years and is still very widely used in electronic circuits.

The bipolar transistor is very versatile and finds applications in many applications and at a wide range of frequencies.

Modern surface mount, SMD transistor on a printed circuit board.
Modern surface mount transistor on PCB

Bipolar transistor circuit symbol

The circuit symbol for the transistor is seen in very many circuits.

The circuit symbol shows the three connections to the transistor, namely: base, collector and emitter.

The base is the connection that is connected to the centre of the main element within the symbol. The emitter is the line entering at an angle and has an arrow on it. An inward facing arrow denotes a PNP transistor, whereas an outward facing one denotes a PNP transistor. The collector is the third electrode and it is designated by the line at an angle with no arrow on it.

The circuit symbol of a bipolar junction transistor or BJT showing collector, emitter and base electrodes for both NPN and PNP versions.
Bipolar transistor, BJT circuit symbol

BJT Beginnings ….

The bipolar transistor dates back to the middle of the twentieth century when three scientists named Bardeen, Brattain, and Shockley working at Bell Laboratories in the USA discovered it. They had been researching an idea for a semiconductor field effect device, but they had been unable to make it work. They had not succeeded in making this idea work and as a result they decided to follow other lines of research and in doing this they developed the bipolar transistor. They succeeded in making it work in late 1947, and only a week after their initial discovery they demonstrated it in front of a group of executives at Bell.


Note on Transistor History:

The transistor history forms an interesting story. The bipolar transistor was first discovered in 1949, although at the time the main researchers, Bardeen, Brattain and Shockley had wanted to make a field effect device, but had not succeeded and followed another line that lead to the development of the bipolar transistor.


Image of old discrete germanium transistors: and OC71 (type originally introduced in 1954) and OC200 dating from the early 1960s.
Old germanium transistors

Today the semiconductor industry is enormous and vast quantities of money are being invested in new semiconductor device developments. Although there are many new types of transistor, the bipolar junction transistor is still in very widespread use.

Semiconductor basics

The bipolar transistor is a semiconductor device and as such it is based around the basic of semiconductor technology.


Note on Semiconductor Theory:

Semiconductors form the basis of today’s electronics industry. Everything from simple discrete diodes and transistors through to state of the art microprocessors all relay on semiconductors. The technology relies upon a group of materials called semiconductors which can be doped to give p-type variants where charge carriers are holes and n-type semiconductors where the charge carriers are electrons.

Bipolar transistors, although much simpler than integrated circuits utilise the same basic technology in many respects. They can be manufactured from a variety of materials of which silicon is by far the most popular. Germanium has been used in the past and gallium arsenide is used for some high frequency devices.

Transistor circuit design

Bipolar transistors are used in many circuits as they provide an excellent level of performance for many circuits. Circuits can be designed for many applications for frequencies over many ranges from DC up to microwaves.


Note on transistor circuit design:

Bipolar transistors, BJTs can be used in a whole variety of circuits. There are three main basic circuits: the common collector, common emitter and common gate. These form the basis of transistor circuits.

Click on the link for further information about Bipolar transistor circuit design


The transistor is a current operated device. It is the amount of current flowing in the base circuit that controls the amount of current flowing in the collector circuit. The other major three terminal semiconductor device, the field effect transistor is a voltage operated device. Here it is the potential on the gate that controls the current passing between the source and drain.

Summary of BJT key features

The table below summarises some of the key features of the bipolar junction transistor.


Junction Transistor Key Features
Feature Details
Device type Current amplifier
Number of PN junctions Two – each transistor has two back to back PN junctions.
Electrodes Three: collector, emitter and base
Polarities NPN and PNP varieties available to allow for operation from rails of either polarity, etc.
Gain Forward current gain denoted by the character β. Figures vary between types and actual devices but may typically be between 20 and 1000.

Bipolar transistors are used in many areas of circuit design, particularly within circuits using discrete components. Although field effect technology is often used within integrated circuits because of the lower power dissipation levels that can be achieved, bipolar technology is used on occasions when it is more suited to the application.

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Posted by Akash Kurup

Founder and C.E.O, World4Engineers Educationist and Entrepreneur by passion. Orator and blogger by hobby