What is BJT

A Bipolar Junction Transistor, also known as a BJT or BJT Transistor, is a semiconductor device with three terminals and two p-n junctions that can amplify or magnify a signal. It is controlled by current. The base, the collector, and the emitter are the BJT’s three terminals. A BJT is a type of transistor that carries charges using both electrons and holes.

At the transistor’s collector, an amplified signal of a small amplitude can be applied to the base. The BJT’s amplification consists of this. Take note that in order to carry out the amplification procedure, an external DC power supply is necessary.

PNP transistors and NPN transistors are the two types of bipolar junction transistors. The following is a diagram of these two kinds of bipolar junction transistors. Bipolar Junction Transistor, or BJT. As depicted above, each BJT consists of three components: the emitter, base, and collector. JE and JC address the intersection of producer and intersection of authority separately. At this point, it is sufficient for us to be aware that collector-base junctions exhibit reverse bias while emitter-based junctions exhibit forward bias. The two types of these transistors will be discussed in the following section.

NPN Bipolar Junction Transistor In an n-p-n bipolar transistor (also known as an npn transistor), two n-type semiconductors are sandwiched between one p-type semiconductor. The diagram below depicts an n-p-n transistor. Now, IE, IC, and VEB and VCB represent the emitter-base voltage and collector-base voltage, respectively. The convention states that the current’s sign is taken as positive if it enters the transistor for the emitter, base, and collector currents (IE, IB, and IC), while the current’s sign is taken as negative if it exits the transistor. The n-p-n transistor’s various currents and voltages can be tallied.

Bipolar Junction Transistor Symbol

Construction of Bipolar Junction Transistor

A BJT is a semiconductor device with two p-n junctions that separate three doped semiconductor regions—the Base, the Collector, and the Emitter—from one another.

Bipolar semiconductors are made in two kinds, PNP and NPN, and are accessible as isolated parts, normally in enormous amounts. Amplifying current is this type of transistor’s primary use or function. Because of this, they can be used as switches or amplifiers. They are used in a variety of electronic devices, including televisions, radio transmitters, mobile phones, and industrial control systems.

Operation of Bipolar Junction Transistor

A bipolar junction transistor operates in three distinct regions:

Active area: The area wherein the semiconductors work as an intensifier.
Region of saturation: The part of the transistor that is completely on and functions as a switch, with the collector current being the same as the saturation current.
Limiting region: The district where the semiconductor is completely off and gatherer current is equivalent to nothing.

Types of Bipolar Junction Transistor

There are two types of bipolar junction transistors:

  • PNP bipolar junction transistor
  • NPN bipolar junction transistor


The two p-type semiconductors are sandwiched between the n-type semiconductor in PNP BJT. The n-type semiconductor serves as a base, while the two p-type semiconductors act as emitters and collectors, respectively. The image below depicts this.

The transistor receives current through the emitter, resulting in forward bias at the collector-base junction and reverse bias at the emitter-base junction.


The two n-type semiconductors are sandwiched between a p-type semiconductor in NPN BJT. The p-type semiconductor serves as a base, while the two n-type semiconductors act as emitters and collectors, respectively. The image below depicts this.

Current entering the emitter, base, and collector has the sign convention of positive while the current that leaves the transistor has the sign convention of negative

Function of Bipolar Junction Transistor

Based on the doping types of the three main terminals, BJTs are divided into two categories: NPN and PNP. A NPN semiconductor comprises of two semiconductor intersections that have a slim p-doped anode district and PNP semiconductor likewise comprises of two semiconductor intersections that have a dainty n-doped cathode locale.

NPN and PNP Transistors The diffusion of charge carriers between two regions with different charge concentrations causes a bipolar transistor’s charge flow. The base, collector, and emitter are the three BJT regions.

The producer district is exceptionally doped when contrasted with different layers. The concentrations of charge carriers in the collector and base layers are the same. The base-emitter junction is biased forward, while the base-collector junction is biased in the opposite direction. The p-doped region has more potential than the n-doped side, which is known as forward biased.

Voltage, Charge Control and Current

The collector-emitter current dictates the base-emitter current. This end is drawn by the current-voltage connection of the base-producer intersection. Minority carriers are concentrated in a base region of collector current.
The charge distribution that explains a transistor’s behavior is the function of transistor models like the Glenn Poon model.

Configuration of Bipolar Junction Transistors

There are three ways to connect a Bipolar Junction Transistor to an electric circuit because it has three terminals, with one terminal serving as the same for both input and output. Each strategy for association answers diversely to the info signals inside a circuit.

The following table lists the characteristics of various transistor configurations: Common Emitter Configuration – has both voltage and current gain Common Collector Configuration – has no voltage gain but has a current gain Common Base Configuration – has no current gain but has a voltage gain

Characteristics of different transistor configurations are given in the following table:

Applications of BJT

A bipolar junction transistor can be used as a switch, an amplifier, a filter, and even an oscillator, as we are aware. The following is a list of additional uses for the bipolar junction transistor:

BJT can be used as a demodulator or a detector.
In order to shape the waves, BJT is used in clipping circuits.
BJT is used in logic and switching circuits.

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