What is a Bipolar Junction Transistor & Its Working

BJT was invented in 1948 by William Shockley, Brattain, and John Bardeen which has remolded not only the world of electronics but also in our day-to-day life. The Bipolar junction transistor uses both charge carriers that are electrons and holes. Indifference, the unipolar transistors such as field-effect transistors use only one kind of charge carrier. For the operation purpose, BJT uses two semiconductor types n-type and p-type between two junctions. The main basic function of a BJT is to amplify current it will allow BJTs are used as amplifiers or switches to produce wide applicability in electronic equipment include mobile phones, industrial control, television, and radio transmitters. There are two different types of BJTs are available, they are NPN and PNP.

What is a Bipolar Junction Transistor (BJT)?

The Bipolar junction transistor is a solid-state device and in these transistors, the current flow in two terminals are emitter and collector, and the flow of current controlled by the third terminal is the base terminal. It is different from the other type of transistor,i.e. Field-effect transistor which is the output current is controlled by the input voltage. The basic symbol of the BJTs n-type and p-type is shown below.

BJT or Bipolar Junction Transistor Symbol
BJT or Bipolar Junction Transistor Symbol

Why BJT is Called Bipolar?

A BJT is a 3-terminal semiconductor device, as the name suggests, the term bipolar is taken from the truth that this kind of transistor includes two kinds of semiconductor materials like P-type (positive type) and n-type (negative type) where the current flows from these regions Usually, these transistors include Silicon.

Construction of Bipolar Junction Transistor

The construction of BJT will determine its working characteristics. So, the construction of BJT can be done through three doped semiconductor sections which are separated through two PN-junctions. This transistor includes three layers namely base, emitter, and collector. These transistors are available in two types namely PNP and NPN and their physical structure is shown below.

Construction of Bipolar Junction Transistor
Construction of Bipolar Junction Transistor

In NPN, it includes two N-regions separated by one P-region whereas, in PNP, it includes two P-regions and separates with one N-region. In BJT, the term bipolar refers to the utilization of both the charge carriers like electrons and holes within the structure.

When PN-junction connects the base and emitter region then it is known as BE-junction. Similarly, once the pn-junction connects the base & the collector region then it is known as the BC-junction. A wire lead unites to every region and these leads are branded with C, B & E for the collector, base, and emitter respectively.

The base (B) – region is slightly doped & it is extremely thin as compared to the emitter terminal that is heavily doped & the somewhat doped collector region. The schematic symbols of the NPN and PNP BJTs include the following.

Terminals of BJT

The BJT includes three terminals like base, Emitter, and Collector which are discussed in brief. The NPN and PNP transistors symbol representation can be done like the following. The arrow symbol on the emitter terminal is always there and the arrow direction will represent the current flow because of the charge carriers.

Emitter Terminal

The emitter terminal emits the charge carriers like holes or electrons to the other two terminals. The base terminal is always reverse biased with respect to other terminals like emitter so that it can produce huge majority carriers. In BJT, the emitter is the most heavily doped region.

The emitter-base junction must be connected in forwarding bias within both the transistors like PNP & NPN. The emitter terminal provides electrons toward the EB-junction within NPN transistor whereas it supplies charge carriers like holes to the same junction within the PNP transistor.

Collector Terminal

The part on the reverse side of the Emitter terminal will collect the emitted charge carriers called a collector. So, this terminal is heavily doped however the doping level of this terminal is among the base terminal which is a lightly doped & heavily doped emitter terminal. CB-junction must be reversely biased in both the transistors.

The main reason for this biasing is to eliminate charge carriers from the CB-junction. The collector terminal of the NPN transistor is to collects electrons that are emitted through the emitter terminal whereas, in the PNP transistor, it gathers holes that are emitted through the emitter terminal.

Base Terminal

The base terminal is the center part among collector & emitter terminals which forms two PN junctions among them. The base terminal is the most lightly doped segment in the transistor. So, being the central part of the BJT will allow it to manage the charge carriers flow among emitter & collector terminals. The BE junction shows high resistance as this junction can be connected in reversed bias.

Working of BJT

Bipolar junction transistor is classified into two types namely PNP and NPN based on types of doping of the terminals. In NPN transistor, two semiconductor junctions are there which have a thin anode region doped with ‘P’ whereas PNP transistor includes two semiconductor junctions that include a thin cathode region doped with ‘N’.

The flow of charge within a transistor is because of the charge carrier’s diffusion among the two sections which belongs to different concentrations of charge carriers. The emitter section is doped highly as compared to the remaining layers.

Both the layers like base and collector include the same concentrations of charge carriers. So, between these three junctions, the BE junction can be connected in forwarding bias & the BC junction is reverse biased. The operating of BJT can be done in three different regions like active, saturation, and cut-off.

Active Region

In this region, one junction is connected in a forward bias whereas the other one is connected in reverse bias. Here, the base current (Ib) is used to control the quantity of Ic (collector current). So, the active region can be mainly used for the purpose of amplification wherever this transistor performs like an amplifier through a gain ‘β’ with the following equation;

Ic = β x Ib

The active region is also called a linear region which lies in between the two regions like the cutoff & the saturation. The typical operation of this transistor happens within this region.

Saturation Region

In this region, the transistors both the junctions are connected in forwarding bias. So this region is mainly used for the ON-state for a switch wherever;

Ic = Isat

Here, ‘Isat’ is the saturation current and it is the highest quantity of current flow among the two terminals like emitter as well as collector once this transistor is connected in the saturation region. As these junctions are connected in forwarding bias then the transistor works like a short circuit.

Cutoff Region

In this region, both the transistor junctions are connected in reverse bias. Here this transistor works like the off condition of a switch wherever

Ic = 0

In the cut-off region, the operation is totally reverse as compared to the saturation region. So, there are no exterior supplies connected. If there is no collector current then there is no emitter current.
In this method, the transistor works like an off-state of the switch, and this mode can be achieved by decreasing base voltage to below the voltage of both the emitter as well as collector.
Vbe < 0.7

Types of Bipolar Junction Transistor

As we have seen a semiconductor offer less resistance to flow current in one direction and high resistance is another direction and we can call transistor as the device mode of the semiconductor. The Bipolar junction transistor consists of two types of transistors. Which, given us

  • Point contact
  • Junction transistor

By comparing two transistors the junction transistors are used more than point type transistors. Further, the junction transistors are classified into two types which are given below. There are three electrodes for each junction transistor they are emitter, collector, and base

  • PNP junction transistors
  • NPN junction transistors

PNP Junction Transistor

In the PNP transistors, the emitter is more positive with the base and also with respect to the collector. The PNP transistor is a three-terminal device that is made from semiconductor material. The three terminals are collector, base, and emitter, and the transistor is used for switching and amplifying applications. The operation of the PNP transistor is shown below.

Generally, the collector terminal is connected to the positive terminal and the emitter to a negative supply with a resistor either the emitter or collector circuit. To the base terminal, the voltage is applied and it operates transistor as an ON/OFF state. The transistor is in the OFF state when the base voltage is the same as the emitter voltage. The transistor mode is in ON state when the base voltage decreases with respect to the emitter. By using this property the transistor can act on both applications like switch and amplifier. The basic diagram of the PNP transistor is shown below.

NPN Junction Transistor

The NPN transistor is exactly opposite to the PNP transistor. The NPN transistor contains three terminals which are the same as the PNP transistor which are emitter, collector, and base. The operation of the NPN transistor is

Generally, the positive supply is given to the collector terminal and the negative supply to the emitter terminal with a resistor either the emitter or collector or emitter circuit. To the base terminal, the voltage is applied and it operated as an ONN/OFF state of a transistor. The transistor is in an OFF state when the base voltage is the same as the emitter. If the base voltage is increased with respect to the emitter then the transistor mode is in an ON state. By using this condition the transistor can act like both applications which are amplifier and switch. The basic symbol and the NPN configuration diagram as shown below.

PNP & NPN Bipolar Junction Transistor
PNP & NPN Bipolar Junction Transistor

Hetero Bipolar Junction

The Hetero bipolar junction transistor is also a type is the bipolar junction transistor. It uses different semiconductor materials to the emitter and base region and produces heterojunction. The HBT can handle the singles of very high frequencies of several hundred GHz generally it is used in ultrafast circuits and mostly used in radiofrequency. Its applications are used in cellular phones, and RF power amplifiers.

Working Principle of BJT

The BE junction is forward bias and the CB is a reverse bias junction. The width of the depletion region of the CB junction is higher than the BE junction. The forward bias at the BE junction decreases the barrier potential and produces electrons to flow from the emitter to the base.

The base is thin and lightly doped, it has very few holes and less amount of electrons from the emitter about 2% it recombine in the base region with holes, and from the base terminal it will flow out. This initiates the base current flow due to the combination of electrons and holes. The leftover large number of electrons will pass the reverse bias collector junction to initiate the collector current. By using KCL we can observe the mathematical equation

IE = IB + IC

The base current is very less as compared to emitter and collector current


Here the operation of the PNP transistor is the same as the NPN transistor the only difference is only holes instead of electrons. The below diagram shows the PNP transistor of the active mode region.

Working Principle of BJT
Working Principle of BJT

BJT Configuration

A bipolar junction transistor includes a three-terminal device so it can be connected to a circuit in three possible ways through one terminal being common between others which means one terminal in between input as well as output is common. Every connection simply responds in a different way to the input signal.

Common Base Configuration

In the CB configuration of the transistor, the base terminal is common among the input & output signals. Here, the input signal can be given among the two terminals like base as well as emitter whereas the output can be obtained among the two terminals like base & collector. At the collector terminal, the output signal is low as compared to the input signal at the emitter terminal.

CB Configuration
CB Configuration

Thus, its gain is less than 1, so, it attenuates the signal. This output of this configuration is non-inverting, so both inputs, as well as output signals, will be in phase. This common base configuration is not commonly used due to its high voltage gain. Because of its extremely high-frequency response, this kind of configuration is used only for 1- phase amplifier. These 1-phase amplifiers can be used like RF amplifiers and microphone pre-amplifier.

Gains of CB Configuration

Voltage Gain = Av = Vout/Vin = Ic x RL/IE X RIN

Current Gain = Ic/ie

Resistance Gain = RL/Rin

Common Emitter Configuration

In this configuration, the emitter terminal is common among input & output. The input is given among the two terminals like base and emitter whereas the output can be obtained among collector and emitter terminals.

It can be simply identified through observing the circuit. When the emitter terminal is connected to GND then input & output are obtained from the base & collector terminals respectively. The CE configuration includes the maximum current & power gain between all types of configurations.

CE Configuration
CE Configuration

The main reason is because of the input at the junction of forwarding bias, so its input impedance is extremely less whereas the output can be received from reverse bias junction. Thus, its output impedance is extremely high. In CE configuration, the emitter current is equivalent to the amount of base & collector currents. So the equation is

Ie= Ic + Ib

From the above equation, ‘Ie’ is the emitter current. So, this configuration has maximum current gain like Ic/Ib because the load resistance can be connected within series through the collector terminal.
It can be observed from the above equation that a small increase within the base current will effect in very high current at the output region.

This CE configuration works like an inverting amplifier wherever the output signal is totally opposite within polarity toward the input signal. Thus, the output signal can be shifted at 180° with respect to its input signal.

Common Collector Configuration

The CC configuration is also called an emitter follower or voltage follower that includes a collector which is grounded. In CC configuration, the connection of the collector terminal can be done to ground toward the supply.

CC Configuration
CC Configuration

For both input & output, the collector terminal is common. The output is obtained from the emitter through load which is connected in series whereas the input is provided toward the base terminal directly. This configuration includes high input as well as output impedance.

This permits it to execute like an impedance matcher. Thus this kind of configuration is extremely useful within the impedance matching method.

BJT Characteristics

The behavior of the bipolar transistor in every circuit configuration is extremely different & generates dissimilar circuit characteristics with respect to input & output impedances and gains like the voltage, power, and current. The fixed characteristics of a BJT can be separated into three main groups which are mentioned below.

Input Characteristics

Common base (CB) = ΔVEB / ΔIE
Common Emitter (CE) = ΔVBE / ΔIB

Output Characteristics

Common Base (CB) = ΔVC / ΔIC
Common Emitter (CE) = ΔVC / ΔIC

Transfer Characteristics

Common Base (CB) = ΔIC / ΔIE
Common Emitter (CE) = ΔIC / ΔIB

The different transistor configuration characteristics are given below.




Input Impedance




Output Impedance

Very HighLow


Phase Angle



Voltage Gain

Current GainLowMedium


Power GainLowVery High


Advantages of BJT

The main advantages of bipolar junction transistors include the following.

  • High driving capability
  • High-frequency operation
  • The digital logic family has an emitter-coupled logic used in BJTs as a digital switch
  • It has a high gain bandwidth
  • It gives good performance at high frequency
  • Voltage gain is good
  • It operates in low power or high power applications
  • It includes maximum current density.
  • Forward voltage drop is low


The main disadvantages of bipolar junction transistors include the following.

  • Thermal stability is less
  • It generates more noise
  • The BJT is more an effect of radiation.
  • Less switching frequency
  • Base control is complex so needs skillful handling.
  • The time taken for switching is not fast as compared to a high flashing frequency of voltage & current.

Applications of BJT

Following are the two different types of applications in BJT they are

  • Switching
  • Amplification
  • Converters
  • Automatic switch
  • Temperature sensors
  • Electronic switches
  • Amplifiers
  • High driving capability
  • Detection circuits
  • High-frequency operation
  • Demodulator and modulator
  • Digital switch
  • Clippers
  • Oscillation circuit

This article gives information about what is bipolar junction transistors, their types, advantages, applications, and characteristics of bipolar junction transistors. Thus, this is all about an overview of the bipolar junction transistor. These are the most broadly used devices to amplify all kinds of electrical signals within discrete circuits which are designed with separate components instead of ICs. These are available in several types in different shapes like BUH515, 2N3055, 2N2219, 2N6487, BD135, BD136 & 2N222. Here is a question for you, what are the different kinds of transistors available in the market?

I hope the given information in the article is helpful to give some good information and understanding of the project. For furthermore, if you have any queries regarding this article or on the electrical and electronic projects you can comment in the below section. Here is a question for you, if transistors are used in digital circuits they generally operate in which region?


  1. Ismail musa muhd says:

    Thank you very much

  2. Roshan mandal says:

    I have cleared my all doubt

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