What is a bipolar junction transistor (BJT)?

A BJT is a semiconductor device with 3 terminals that consist of a p-n junction diode that can amplify the signal or current. It is a current-controlled device. The three BJT terminals are base, collector, and emitter. The BJT works with the charge carrier, the two type of carriers in the BJT are holes and electrons, the charge carriers flow by the process of recombination. The signal to be amplifier is generally given as the input at the base terminal and an amplified form of it is available at the collector terminal.

What is the construction of bipolar junction transistor (BJT)?

BJT is a three-terminal semiconductor device with doped semiconductor the three terminals are i.e. base, collector & emitter divided in a way that they form two P-N junction diode.

Bipolar transistors are manufactured in two types, PNP BJT and NPN BJT, and they both use different materials for the fabrication as charge carrier is different for both the types. The main purpose or function of this transistor is to maximize the current. The properties of BJT makes them useful as switches or amplifiers. They have a wide range of applications in electrical equipment such as cell phones, televisions, radio transmitters, and industrial controls.

Working of BJT

That is the performance of BJT is divided into two parts: PNP BJT and NPN BJT. In PNP BJT the charge carrier is hole and in the NPN BJT the charge carrier is electron.

NPN transistor

The NPN transistor has a layer of p type material doped between the layer of the N type material. The electrons flow from the collector to the emitter terminal, thus the electrons flow from the collector to the emitter while the base controls the flow of the electrons. The emitter then redirects the electrons towards the load.

In an NPN transistor the three regions have different doping like the doping at the base is light and the doping at the collector is medium. While the emitter being heavily doped. In NPN transistor the base is forward biased with respect to the emitter and the collector is also forward biased with respect to the base.

The current equation for the NPN transistor is:

$I_e=I_b+I_c$

Here, the emitter current is $I_e$, the base current is $I_b$ and the collector current is $I_c$. 

PNP transistor

PNP transistors operate internally in the same way as the NPN but here the current and voltage polarities are reverse-biased. The emitter current is created when the emitter-base junction is forward biased, the emitter pushes holes straight into the base. When electrons enter an n-semiconductor or base, they meet through holes. The base is lightly doped and small in comparison to emitter and collector. So only a few holes are attached to the electrons and the rest are taken to the charger layer of the collector's space.

This bipolar PNP junction transistor is made up of three layers of semiconductor material, consisting of two p-type circuits and one n-type circuit.

It includes three terminals:

Emitter: The emitter component in the transistor allows you to offer carriers a lot of charge. The emitter is always looking forward bias with respect to the base. Therefore, most charge carriers are provided in the base. The emitter region of the transistor is highly doped and limited in size.

Collector: Mos of the charge carrier that are sent by the emitter are collected by the collector. The assembly of the collector base is reverse biased.

Base: The central part of the transistor is known as the base. The base consists of two circuits, an input circuit with an emitter and an output circuit and collector. The emitter-base is forward-biased and provides low resistance to charge. The collector-base junction is reverse biased and provides high resistance to the circuit. The base of the transistor is lightly doped and very thin, as a result of which it provides the most charge carrier.

Configuration of BJT

The BJT's are configured in different connection depending on the type of characteristics needed. The different type of gains are current gain, voltage gain, or one gain configure the transistor to obtain the unity gain or no gain.

The BJTs are connected in three different configurations as follow:

Common-Base configuration

In the common-base configuration, the base terminal remains between the output and input terminal. The input is applied between the emitter and the base. The output is obtained between the collector and the base. The base is thus given as common terminal and gives the name to the configuration as common base.

The common base configuration has the advantage of high impedance at the input side and the low impedance at the output side. The configuration provide high voltage gain.

Common-Emitter configuration

In the common emitter configuration, the output is obtained between the collector and the emitter terminals. The input is provided between the base and the emitter terminal. Thus, the emitter terminal is kept common for both the input and the output.

The common emitter configuration has the advantage of the high current gain and the high voltage gain. The input resistance of the common emitter is not too high or low, it is in between the common base and the common collector configuration.

Common-Collector configuration

In the common collector configuration, also known as the voltage follower circuit. The input is given between the collector terminal and the base terminal. The output is obtained between the emitter terminal the base terminal.

The common collector configuration provides high impedance at the input and low at the output. The common application of the common collector configuration is impedance matching.

Applications of bipolar junction transistor(BJT)

Applications for bipolar junction transistors are as follows:

• These transistors have their application in sensing circuits.
• Used in amplification circuits.
• In clipping circuits, these are selected in wave-forming circuits.
• BJT's work better in high load conditions and higher frequencies compared to Metal oxide field effect transistors (MOSFET's)
• BJT's have higher reliability and better profitability inline areas as compared with MOSFET's.
• Compared to MOSFET's, BJT's are much faster due to the lower capacity. But MOSFET can withstand heat and can work as a good resistor.
• Current MOSFET mirror circuits are much better than those with BJT.
• MOSFET have a lower chance of heat dissipation than BJT.

Common Mistakes

Remember that, BJT switching time is low compared to MOSFET's in terms voltage and current with high alternating frequency.

Context and Applications

In each of the expert exams for undergraduate and graduate publications, this topic is huge importance and is mainly used for:

• Bachelor of technology in the electrical and electronic department
• Bachelor of Science in physics
• Master of Science in physics

Related Concepts

• UJT
• Mosfet
• Fet
• PNP junction diode
• NPN junction diode

Practice Problems

Q1 The current I_b is _____________.

(a) Base current

(b) Hole current

(c) Donor ion current

(d) Acceptor

Correct option- (a)

Explanation- The base current is represented as I_b.

Q2 Which of the following is not a part of a BJT_____________.

(a) Emitter

(b) Base

(c) Collector

(d) Gate

Correct option- (a)

Explanation- BJT consists of three terminals that are emitter, collector and base. The gate is present in power transistors like in MOSFET.

Q3 In which of the following modes can a BJT be used_____________.

(a) Cut-off mode

(b) Active mode

(c) Saturation mode

(d) All of these

Correct option- (d)

Explanation- The BJT can work in cut-off mode, active mode and saturation mode.

Q4 If a BJT is to be used as an amplifier, then it must operate in______________.

(a) cut-off mode

(b) active mode

(c) saturation mode

(d) all of these

Correct option- (b)

Explanation- A BJT operates as an amplifier in active mode and as a switch in cut-off or saturation mode.

Q5  If a BJT is to be used as a switch, it must operate in_______________.

(a) cut-off mode or active mode

(b) active mode or saturation mode

(c) cut-off mode or saturation mode

(d) all of these

Correct option- (c)

Explanation- A BJT operates as an amplifier in active mode and as a switch in cut-off or saturation region mode.