Bipolar Junction Transistor (BJT)

Bipolar Junction Transistor (BJT)

In the article of diode, we have seen that it is a two-terminal simple PN Junction device. If we connect two diodes back to back in such a way that, it shares a common P or N terminal. Hence, a device formed with two junctions, three-layer that is known as Bipolar Junction Transistor or BJT.

The BJT was invented by William Shockley in 1948. The bipolar junction transistor allows current to flow by both charge carriers (electrons and holes). In unipolar transistors, like FET (Field Effect Transistor), the current flow by only one charge carrier.

The semiconductor material can act as either insulator or conductor by the application of small-signal voltage. Hence, it offers low resistance, and as it allows current to flow in one direction. And it offers high resistance as it opposes to flow of current in the opposite direction. The transistor has this ability to change between these two states enables the function as a switch and an amplifier.

Table of Content

  • Type of BJT
  • Construction of BJT
  • Two Diode Analogy of BJT
  • Working of BJT
  • Operating Regions of BJT
  • Configuration (Characteristic) of BJT
  • Advantages of BJT
  • Disadvantages of BJT

Types of BJT

There are two types of bipolar transistors;

  • Bipolar Point Contact Transistor
  • Bipolar Junction Transistor

Out of these two types, the bipolar junction transistor is widely used compared to the point-contact transistor.

The Bipolar Junction Transistor is also classified in two types according to the arrangement of semiconductor layers;

  • PNP Transistor
  • NPN Transistor

In PNP transistor, an N-type semiconductor is sandwiched by two P-type semiconductors. And in NPN transistor, a P-type semiconductor is sandwiched by two N-type semiconductors. The basic construction diagram and symbol of PNP and NPN transistors are as shown in the below figure.

Construction Diagram and Symbol of PNP and NPN Transistors
Construction Diagram and Symbol of PNP and NPN Transistors

Construction of BJT

The BJT consists of two PN junctions, three layers, and three terminals. Three terminals are Emitter, Base, and Collector.

An emitter is used to supply the charge carriers. Because of the highly doped region, it can inject a large number of charge carriers into the base terminal.

The base terminal is a very thin and lightly doped layer. It is placed between the emitter and collector. The doping level is also very light compared to other terminals.

The collector is used to collects the charge carriers. It is a moderately doped region. Hence, BJT is formed by three differently doped semiconductor regions.

The below figure shows the construction of transistors.

Construction of Transistors
Construction of Transistors

Two Diode Analogy of BJT

The construction of BJT easily explained by two diode analogy as shown in the below figure. This figure shows an equivalent circuit of BJT in terms of diodes.

For the NPN transistor, the P-type base region is formed by connecting the anode of both diodes back-to-back. And for PNP transistor, the N-type base region is formed by connecting the cathodes of both diodes back-to-back.

Two Diode Analogy of Transistor
Two Diode Analogy of Transistor

Working of BJT

The BJT is a three-terminal device. These terminals form two junctions. One junction is between base and emitter terminal and the second junction is between base and collector terminal.

As shown in the two-diode analogy, when we supply voltage between emitter and collector, one diode always remains in reverse bias. And hence, the current cannot flow through the transistor.

In other words, the base-emitter junction is in forward bias and base-collector junction is in reverse bias. Because of the forward biasing, the majority charge carrier flows from emitter to base. But this charge carriers cannot cross the junction between base-collector as it is in reverse bias.

Now to flow the current, we use the second source connected between the base and emitter terminals. This source is known as the base source. The voltage of this source is increasing gradually and the second diode comes into forward bias.

So, the majority charge carrier will flow through the base to a collector by crossing the base-collector junction. And current start flowing through the circuit.

The working of NPN and PNP transistors is the same. The only difference is in the majority charge carrier. In the PNP transistor, the majority charge carrier holes. And in NPN transistors, the majority charge carrier is electrons.

The working of the NPN transistor is as shown in the below figure.

Working of Transistors
Working of Transistors

Operating Regions of BJT

When the DC supply is given to the transistor, the junctions are in forward bias or reverse bias. According to the biasing of a junction, the transistors operate in four regions;

  • Active Region
  • Saturation Region
  • Cutoff Region
  • Inverse Active Region

Out of these four regions, the inverse active region is not useful for any applications.

Active Region

This region is also known as the linear region. And in most of the applications, the transistor operates in this region. In many applications, the transistor operates in this region.

The transistor act as an amplifier. The below figure shows the active region.

Active Region Transistor
Active Region Transistor

When the base-emitter junction is in forward bias and base-collector junction is in reverse bias, the transistor operates in an active region. In an active region, the ratio of collector current to the base current shows the current gain or current amplification factor. And it represents as β.

    \[ I_C = \beta I_B \]

Where,
IC = Collector Current
IB = Base Current

So, from the above equation, the collector current is β times the base current.

Saturation Region

In this region, transistor behaves like a closed switch as the collector and emitter terminals are shorted. The below figure shows the representation of the saturation region.

Saturation Region of Transistor
Saturation Region of Transistor

In this mode of operation, both junctions connected in forward bias. And for this situation, the collector current is the same as the emitter current.

    \[ I_C = I_B \]

Cutoff Region

In this region, the transistor behaves as an open circuit and base, emitter, and collector current all are zero. The below figure shows the working of a transistor in the cut-off region.

Cutoff Region of Transistor
Cutoff Region of Transistor

In this mode of operation, both the base-emitter and base-collector junctions are in reverse bias. Therefore, no current will flow through the transistor and it acts as OFF switch.

    \[ I_C = I_B = I_E = 0 \]

The below table summarizes the operating modes of transistors.

Operating Region Base-Emitter Junction Base-Collector Junction
Active Region Forward bias Reverse Bias
Saturation Region Forward bias Forward bias
Cut-off Region Reverse Bias Reverse Bias
Inverse Active Region Reverse Bias Forward bias

Configuration (Characteristic) of BJT

The transistor is a three-terminal device. But we need four terminals to operate the transistor; two for input and two for output. Hence, we use one terminal as a common terminal for input and output.  Therefore, in a transistor, three types of configurations are possible. And that is selecting a common terminal as a base, collector, and emitter.

In different configurations, a gain of the system is different and it is as below list;

Configuration Voltage gain Current gain
Common-Base (CB) configuration YES NO
Common-Collector (CB) configuration NO YES
Common-Emitter (CE) configuration YES YES

Advantages of BJT

The advantages of BJT is as the below list;

  • Better voltage gain
  • Low forward voltage drops
  • High current density
  • Large gain bandwidth
  • Suitable for high and low power applications
  • Performed well in high-frequency applications

Disadvantages of BJT

The disadvantages of BJT is as listed below;

  • Low thermal stability
  • Complex base control
  • Low switching frequency
  • More noise produces
  • Effect of radiation

Related Article:

Comparison of SCR, Power BJT, Power MOSFET, IGBT

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