Bipolar Junction Transistor (BJT): Principles & Types

Bipolar junction transistor (BJT) is the type of transistor and three-terminal semiconductor device, which has two p-n junctions. They are mainly used as amplifiers or current controlled devices in electronic circuits. Both the electrons and holes will act as the charge carriers in the Bipolar junction transistor. Usually, the BJT transistors do not require any external DC sources. This article explains transistors and types, characteristics and working principles of BJT and types of BJT in detail. 

What is Transistor and Its Types? 

The transistors are semiconductor devices, which will conduct and resist the electric current and voltage.  Usually, transistors will act either as a switch or as an amplifier. The main function of transistors is to regulate and control the flow of current in the electronic circuit. The transistors are mainly classified into three types based on p-n junctions. They are unipolar junction transistors, bipolar junction transistors and field-effect transistors.  

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BJT Characteristics and Types

Usually, BJTs will have three terminals and two p-n junctions. The three terminals are namely, base, emitter and collector. In BJT, only less current will flow between base and emitter terminals and a larger current will flow between collector and emitter terminals. 

Based on the doping BJTs are mainly classified into two types. They are PNP transistors and NPN transistors. 

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BJTs are classified into two types, PNP transistors and NPN transistors. Let us understand these types of transistors in detail.

NPN Transistor

In NPN transistors, one p-type semiconductor is placed between two n-type semiconductors and it forms the two p-n junctions. These NPN transistors are widely used in many electronic devices, mainly to amplify weak signals.  In the NPN transistors, the current flow will be usually from the emitter to the collector region. 

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PNP Transistor 

In PNP transistors, one n-type semiconductor is sandwiched between the two p-type semiconductors and creates two p-n junctions. The PNP transistors are mainly used to control current flow through the circuit. Usually, the p-n junction is considered a diode. So, the transistors look like two crystal diodes connected in series. In the PNP transistor, the left side diode is known as the emitter-base diode. The right side diode is known as the collector-base diode.

Bipolar Junction Transistor Configurations

A BJT can be configured into three types, they are a common collector configuration, common base configuration and common emitter configuration. 

In the common base characteristics,  the base of the transistor is grounded, then the emitter turns as input and the collector turns as output.  

Input characteristics of CB \[\Delta V_{CB}\] at constant:  Rin = \[\frac{\Delta V_{BE}}{\Delta I_{E}}\]

Output characteristics of CB  \[\Delta I_{E}\] at constant:  Rout =\[\frac{\Delta V_{CB}}{\Delta I_{B}}\]

Current Transformer characteristics of CB \[\Delta V_{CB}\] at constant: 𝞪 = \[\frac{\Delta I_{C}}{\Delta I_{B}}\]

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In the common collector characteristics,  the collector of the transistor is grounded, then the base turns as input and the emitter turns as output.  

Input characteristics of CC \[\Delta V_{CB}\] at constant:  Rin = \[\frac{\Delta V_{CB}}{\Delta I_{B}}\]

Output characteristics of CC \[\Delta I_{B}\] at constant:  Rout = \[\frac{\Delta V_{CE}}{\Delta I_{B}}\]

Current Transformer characteristics of CC \[\Delta V_{CE}\] at constant: 𝞪 = \[\frac{\Delta I_{B}}{\Delta I_{E}}\]

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In the common-emitter characteristics,  the emitter of the transistor is grounded, then the base turns as input and the collector turns as output.  

Input characteristics of CC  \[\Delta V_{CE}\] at constant:  Rin = \[\frac{\Delta V_{BE}}{\Delta I_{B}}\]

Output characteristics of CC  \[\Delta I_{B}\] at constant:  Rout = \[\frac{\Delta V_{CE}}{\Delta I_{E}}\]

Current Transformer characteristics of CC \[\Delta V_{CB}\] at constant: 𝞪 =\[\frac{\Delta I_{C}}{\Delta I_{B}}\]

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BJT Working Principle

The NPN transistor is a biased active region. Here, the base-emitter junction is forward biased and the collector-base junction is reversed biased. So, the width of the depletion region of the base-emitter junction is small, while compared to the width of the collector-base junction.  The forward biased BE junction will reduce the barrier potential and help the current to flow from the emitter to the base. 

Usually, the base of NPN transistors are thin and lightly doped, so it has fewer holes while compared with the electrons in the emitter.  The recombination of holes in the base with electrons in the emitter region will constitute the flow of the base current.  Usually, the direction of conventional current flow will remain opposed to the flow of electrons. 

Then the remaining large number of electrons in the emitter will cross the reverse-biased collector junction in the form of collector current.

According to Kirchhoff's Current Law, the emitter current is equal to the sum of collector current and base current. Generally, the base current IB will remain small when compared to the emitter current IE and the collector current IC

IE = IC + IB

The only major difference between the NPN and PNP transistors are their majority charge carriers. The majority charge carriers of NPN transistors are electrons and the majority charge carriers of PNP transistors are holes.  All other working principles and their doping ratio will remain the same for both NPN and PNP transistors.

In the transistor, if the collector current increases, then the collector junction temperature will increase. So, the resistance provided by the collector also gets reduced. As a result the collector current increases. This phenomenon is known as the thermal runway in BJT transistors. 

Advantages of BJT Transistors

1. BJT have better voltage gain and high current density

2. BUT also have a low forward voltage 

3. BJT can operate in low to high power application

4. BJT have a large gain bandwidth

5. It will show better performance at high frequency

Disadvantages of BJT Transistors

1. Thermal stability of BJT transistor is low

2. The radiation of BJT transistors are effective 

3. BJT transistors will produce huge noise. 

4. They have a low switching frequency

5. BJT has a complex control

6. The switching time of BJT is slow while compared to the high alternating frequency of current and voltage 

This article explained what is BJT, its working principle, types and characteristics in detail.