What is Bipolar Junction Transistor BJT?

Bipolar Junction Transistor (BJT) is one of the very important electronic circuit components in the world of electronics. Scientist William Shockley discovered the first Transistor in 1948. In modern days, technology is better and many variety of transistor have been discovered. In this article, we are going to discuss what is Bipolar Junction Transistor (BJT)? What are its properties and uses?

Contents of this Article:

  • What is Bipolar Junction Transistor (BJT)?
  • Types of BJT
  • Construction of BJT
  • Uses of BJT transistor

What is Bipolar Junction Transistor (BJT)?

Transistor
Transistor

Bipolar Junction Transistor is a three terminal electronic circuit component or device used to amplify the input signal at the output of an electronic circuit. It is made of semiconductor materials. One can fabricate a BJT by doping a semiconductor substrate with n-type and p-type materials alternatively into three parts. These three parts are the Emitter, Base and Collector regions of the transistor.

Types of Bipolar Junction Transistor (BJT)

There are two types of BJT on the basis of doping in three regions. One is NPN Transistor and other is PNP Transistor.

NPN Transistor:

When a p-type region is sandwiched between two n-type region in a semiconductor substrate, a NPN transistor forms. The full form of NPN Transistor is Negative Positive and Negative Transistor. This negative and positive terms refer the doping type of the three regions of the Transistor. Doping type of Emitter and Collector are n-type and doping type of Base is p-type. Thus, free electrons are the majority carriers in NPN Transistor.

PNP Transistor:

One can form a PNP Transistor by inserting a n-type region between two p-type region in a semiconductor substrate. PNP Transistor has its full name as Positive Negative and Positive Transistor. Here also the negative and positive terms indicate the the doping type of the three parts or layers of transistor. Emitter, Base and Collector have the doping type as p-type, n-type and p-type respectively. Thus, holes are the majority carriers in PNP Transistor.

Construction of Bipolar Junction Transistor (BJT)

We already know that BJT consists of three doping regions on the semiconductor substrate. These regions are Emitter, Base and Collector. The size, doping type, doping concentration and direction of current flow in the three layers are the key terms to describe the construction of the Transistor.

Common things for all types of BJT –

  • All the Transistors have three regions – Emitter, Base and Collector. These three regions are alternatively doped with different types of doping (n-type or p-type). Thus, these three region forms two p-n junctions. The junction between Emitter and Base is the Emitter-Base junction or simply Emitter Junction and the junction between Collector and Base is the Collector-Base junction or simply Collector junction.
  • Collector has greater size to reduce heating effect, Emitter has medium size and Base has lower size.
  • Doping of Emitter region is greater than that of Collector and Base region. Base has smallest doping concentration to reduce electron-hole recombination.
  • Emitter emits the charges towards Base region and finally charges are collected in Collector region after passing through the Base region. Conventional current flow will be in the direction of the flow of Holes and in the opposite direction of electron flow.

Construction of NPN and PNP Transistor are almost the same, but there are some difference in doping types, direction of current flow, symbols etc.

Click here to check the difference between NPN and PNP Transistor.

Construction of NPN Transistor

Construction and Symbol of NPN Transistor
Construction and Symbol of NPN Transistor

Emitter region

Emitter has medium size and greater doping level. This region has n-type doping materials like Arsenic, Phosphorus etc. So, free electrons are the majority carriers in this region. Since Emitter emits charges to Base region, the free electrons flow from Emitter to Base of the NPN Transistor. So, the direction of current flow through Emitter junction is along Base region to Emitter region. Therefore, in symbol of NPN transistor we can see that the direction of current flow is outward from the Emitter terminal. The current at Emitter terminal is the Emitter current (IE).

Base region

Base region has smaller size and smaller doping concentration. In NPN Transistor the doping type of Base is p-type. Doping concentration of Base region is made very small to reduce the recombination of holes with the electrons coming from Emitter region.

Collector region

Collector of a Transistor collects the charges coming from Emitter via Base. This region has greater size and medium doping concentration. In NPN Transistor, collector region has n-type of doping. The electrons coming from Emitter enters in Collector region from Base region. So, electrons flow from Base to Collector region. Hence, Current through the collector junction flows from Collector region to Base region.

Construction of PNP Transistor

Construction and Symbol of PNP Transistor
Construction and Symbol of PNP Transistor

Emitter region

Emitter has medium size and greater doping level. This region has p-type doping materials. So, positive holes are the majority carriers in this region. Since Emitter emits charges to Base region, the holes flow from Emitter to Base of the PNP Transistor. So, the direction of current flow through Emitter junction is from Emitter region to Base region. Therefore, in symbol of PNP transistor we can see that the direction of current flow is inward at the Emitter terminal.

Base region

Base region has smaller size and smaller doping concentration. In PNP Transistor the doping type of Base is n-type. Doping concentration of Base region is made very small to reduce the recombination of electrons with the holes coming from Emitter region.

Collector region

Collector of a Transistor collects the charges coming from Emitter via Base. This region has greater size and medium doping concentration. In PNP Transistor, collector region has p-type of doping. The holes coming from Emitter enters in Collector region from Base region. So, holes flow from Base to Collector region. Hence, Current through the collector junction flows from Base region to Collector region.

Use of Transistor:

Transistors have two main uses. Most common purpose of a Transistor is to amplify input signal at the output of an electronic circuit. At this situation Transistor behaves like an Amplifier. Transistors are also used as an electronic switch. Check here the uses of Transistor as a Switch and also Transistor as an Amplifier.

This is all from the basic of Bipolar Junction Transistor (BJT). If you have any doubt on this topic you can ask me in the comment section.

Thank You!

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