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What is a bjt?


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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 (BJT)

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.

What is BJT and its working?

A Bipolar Junction Transistor (BJT) is a three-terminal device which consists of two pn-junctions formed by sandwiching either p-type or n-type semiconductor material between a pair of opposite type semiconductors. The primary function of BJT is to increase the strength of a weak signal, i.e., it acts as an amplifier.

What is the type of BJT?

BJTs are of two types namely NPN and PNP based on doping types of the three main terminals. An NPN transistor consists of two semiconductor junctions that have a thin p-doped anode region and PNP transistor also consists of two semiconductor junctions that have a thin n- doped cathode region.

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.

Types of BJT

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.

How is BJT used as a switch?

A bipolar transistor can be used for switching or amplification by allowing a tiny current to be injected at one of its terminals and controlling a much greater current flowing between two other terminals.

What is the symbol of BJT?

The basic symbols of BJT are n-type and p-type. Electronic current is conducted by both free electrons and holes in bipolar junction transistor.

What are the 4 regions of BJT?

BJTs consists of three differently doped semiconductor regions: the emitter region, the base region and the collector region. These regions are, respectively, p type, n type and p type in a PNP transistor, and n type, p type and n type in an NPN transistor.

What are the sizes of BJT?

Modern BJTs have base widths of about 0.1 µm. This is much smaller than the typical diffusion length of tens of microns (see Example 4–4 in Section 4.8).

what is bjt?

What is the difference of BJT and FET?

A type of transistor which uses two types of charge carries viz. electrons and holes for conduction is known as bipolar junction transistor (BJT). A type of transistor in which electric field is used to control the flow of current in a semiconductor is known as field effect transistor (FET).

What are 5 applications of BJT?

Application of bipolar junction transistor (BJT)

  • The bipolar junction transistor (BJT) is used in logic circuits.
  • The BJT is used as an oscillator.
  • It is used as an amplifier.
  • It is used as a multivibrator.
  • For wave shaping it is used in clipping circuits.
  • Used as a detector or demodulator.
  • It is also used as modulator.
Application of BJT

What is the application of BJT in electronics?

There are some applications of bipolar junction transistor (BJT) which are given below. The BJT is used as an amplifier. The BJT is used as an oscillator. It is used for wave shaping in chipping circuits.

What is the function of bjt?

The function of a BJT (Bipolar Junction Transistor) is to amplify and/or switch an electrical signal. BJTs are used in electronic circuits to control the flow of current, and to amplify or switch signals. The BJT consists of three regions of doped semiconductor material – the emitter, base, and collector. When a small current is applied to the base-emitter junction, it allows a larger current to flow from the collector to the emitter in the case of an NPN transistor, or from the emitter to the collector in the case of a PNP transistor. This behavior allows the BJT to act as a switch or amplifier in electronic circuits. Overall, the function of a BJT is to provide a way to control and manipulate electrical signals in electronic circuits.

What is the function of BJT and FET?

Thus, we can think of the BJT and the FET as two fundamental variations on the theme of semiconductor amplification and switching: a BJT allows a small current to regulate a large current, and an FET allows a small voltage to regulate a large current.

What is the use of bjt in electric vehicle?

BJTs (Bipolar Junction Transistors) are not commonly used in electric vehicles (EVs) today, as they have largely been replaced by newer power semiconductor technologies like IGBTs (Insulated Gate Bipolar Transistors) and MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors).

However, BJTs were used in some older EV models in the past, primarily in the motor control circuitry. In the motor control circuit, BJTs were used as power amplifiers to control the speed and torque of the electric motor.

Overall, while BJTs have been used in some older EV models, newer power semiconductor technologies like IGBTs and MOSFETs are now more commonly used in modern EVs due to their higher efficiency, faster switching speed, and lower power loss.



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