Hey Guys! Hope you are doing great. Today, I am going to discuss the details on the Introduction to BJT (Bipolar Junction Transistor). It is an electronic component mainly used for amplification and switching purpose. As the name suggests, it is composed of two junctions called emitter-base junction and collector-base junction.
Don’t confuse BJT with regular transistors. A transistor is a semiconductor device, comes with three terminals that are used for external connection with electronic circuits. A transistor is termed as a trans resistor which is used as switch or gate for electronic signals. Small signals applied between one pair of its terminals are used to control much larger signals at the other pair of terminals.
Actually, transistors are divided into two categories called unipolar transistor and a bipolar transistor. Bipolar junction transistor uses two charge carries i.e. electrons and holes while unipolar transistor like FETs (Field Effect Transistors) uses only one charge carrier. I hope you are aware of another type of transistors called MOSFET.
I’ll try to cover each and everything related to this bipolar junction transistor, so you find all information at one place. Let’s get started.
Introduction to BJT
- Introduced in 1948 by Shockley, BJT is an electronic component mainly used for switching and amplification purpose.
- It is composed of three terminals called emitter, base, and collector, denoted as E, B and C respectively.
- This transistor comes with two PN junctions. The PN junction exists between emitter and base is called emitter-base junction and the PN junction exists between collector and base is called collector-base junction. Emitter-base junction is forward biased and the collector-base junction is reverse biased.
- In the start BJTs were made from germanium, however, recent transistors are made from silicon.
- BJT comes in two types called NPN transistor and PNP transistor.
- It is a bipolar device where conduction is carried out by both charge carriers i.e. electrons and holes. The number of electrons diffused in the base region is more the number of holes diffused in emitter region. Electrons behave as a minority carrier in the base region.
- Under normal conditions, when the emitter-base junction is forward biased it allows the current to flow from emitter to collector. When a voltage is applied at the base terminal, it gets biased and draws current, which directly affects the current at the other terminals.
- BJT is called a current controlled device where small current at the base side is used to control the large current at other terminals. All three terminals of the BJT are different in terms of their doping concentrations. The emitter is highly doped as compared to base and collector.
- The collector is moderately doped and its area is larger as compared to emitter area, allowing it to handle more power.
- When a voltage is applied, the majority of electrons from emitter are diffused into the base where these electrons act as minority charge carriers, making the holes in the base region majority charge carriers.
- As the base is very thin and lightly doped it cannot hold the number of electrons for too much time, allowing the electrons to diffuse from base to collector.
- Making a slight change at the voltage applied at the base-emitter terminals can cause a significant change at the current between emitter and collector terminals.
- This is the process used for amplification purpose.
- When the emitter-base junction is not forward biased the amount of current at the base and collector terminal is zero, no matter how much voltage is applied at the base terminal.
- Common-Emitter current gain is a term mostly used for BJTs. It is a ratio between collector current and base current. Similarly, a common-base current gain is defined as a ratio between collector current and emitter current. Most of the time its value is taken as unity.
- Construction of BJT is not symmetrical in nature. The lack of symmetry of BJTs is due to the difference in doping concentration between the terminals.
- Generally, BJTs are operated in forward-biased mode. Interchanging the emitter and collector allows the forward biased mode to change to reverse biased mode. This interchange causes a wide impact on the values of current gains, making them much smaller as they are in forward-biased mode.
- The mode of operation where an emitter-base junction is forward biased and the collector-base junction is reverse biased is called active region.
Types of BJT
BJTs are divided into two types based on the nature and construction of the transistor. Following are two main types of the BJT.
- NPN (negative-positive-negative) is a type of BJT where a P-doped layer of semiconductor exists between the two layers of N doped material.
- The P doped region represents the base of the transistors while other two layers represent emitter and collector respectively.
- NPN transistors are also called minority carrier devices because minority charge carriers at the base side are used to control large current at other terminals of the transistor.
- The current moves from an emitter to the collector where electrons act as a minority carrier at the base side.
- PNP (positive-negative-positive) transistor is a type of BJT where N doped semiconductor layer which acts as a base, is housed between the two layers of P doped material.
- The base uses small base current and negative base voltage to control large current at the emitter and collector side and voltage at the collector side is larger than the voltage at the base side.
- In PNP transistor current direction and voltage polarities are reversed as compared to NPN transistors.
- PNP transistors work in a similar way like NPN transistor with some exception i.e. holes are diffused through the base from an emitter and are collected by the collector.
- This transistor is rarely used for applications as conduction carried out by the movement of electrons is considered fast and holds more value as conduction by movement of holes.
Regions of Operations of BJT
Bipolar junction transistors come with different regions of operation. These modes of operations set a tone for current flowing from emitter to collector.
Forward Active Mode
- BJT comes with two junctions called emitter-base junction and collector-base junction. Emitter-base junction is forward biased and the collector-base junction is reverse biased.
- For amplification purpose, most of the transistors come with high common emitter current gain which shows the exact current and power gain required for amplification purpose.
- The collector-emitter current is largely dependent on the base current where small current at the base side is used to control the large current at the emitter and collector side.
Reverse Active Mode
- By interchanging the emitter and collector, transistor goes from active mode to reverse active mode.
- Most of the transistors are designed to afford high current gain, but reversing the role of emitter and collector makes the current gain very small as compared to forward biased region. This type of mode is rarely used unless a failsafe condition is required.
- BJT exhibits saturation mode when both junctions are forward biased. This mode of operation is referred as a closed circuit which allows a large amount of current flowing from emitter to collector side.
- When the emitter-base junction is not forward biased, the transistor is said to have in the cut-off region where collector current and base current will be zero, no matter how much voltage is applied at the base terminal.
Three Basic Configurations of BJT
BJT is a current controlled device which is mainly used for amplification and switching purpose. There are three ways to connect this device with external electronic circuits called:
1. Common Base Configuration
2. Common Collector Configuration
3. Common Emitter Configuration
The nature of the current being controlled at the output is different for different configurations.
Common Base Configuration
- Common base configuration is a configuration where the common base is shared between input and output signal.
- Voltage is applied at the emitter-base junction and corresponding output signal is obtained at the output across the base-collector junction.
- The base voltage is connected to some reference voltage or can be grounded in some cases with the intention of making common base between input and output signals.
- Following figure shows the circuit diagram of common base configuration.
- Current at the emitter side is quite large, where electrons are diffused into the base terminal. These electrons make a pair with some holes present in the base, while most of them leave the base and are collected by the collector.
- This type of transistor comes with remarkable high voltage characteristics which don’t make it an ideal choice for many applications. In this configuration, an output and input voltage is in line with each other. The input characteristics of this transistor are quite identical to forward biased diode while output characteristics are similar to a regular diode and come with a high output to input resistance ratio.
- Common base current gain is a very important factor used in this configuration which is a ratio between collector current and emitter current. It is denoted by α alpha.
- α = Ic/Ie
- The alpha value ranges between 0.95 to 0.99, however, most of the time its value is taken as unity. High-frequency response of common base configuration makes it an ideal choice for single stage amplifier.
Common Collector Configuration
- This configuration is also known as voltage follower where the input is applied at the base terminal and output is taken from emitter terminal.
- This configuration is mainly used for impedance matching as the input impedance of this configuration is very high while output impedance is very low.
- Common collector configuration is termed as non-inverting amplifier where output signal and an input signal are in phase with each other.
- The current gain of this transistor is very large because the load resistance is at the receiving end of both collector current and base current, making it a suitable for amplification purpose.
- Hence very little voltage gain, around unity, can help in producing very large current gain.
- Following figure shows the circuit diagram of common collector configuration.
Common Emitter Configuration
- This configuration is widely used in transistor based amplifier, where an input signal is applied between emitter and base while the output is taken from emitter and collector.
- This configuration comes with highest current and power gain which makes it an ideal choice for amplification. Input impedance is connected to forward biased PN junction which shows low value while output impedance is connected to reverse biased PN junction which shows high value.
- Most of the transistors generally come with common emitter configuration because this exhibits the ideal power and current required for amplification purpose.
- Common emitter configuration is termed as inverting amplifier circuit where an input signal is out-of-phase with the output signal.
- Following figure shows the circuit diagram of common emitter configuration.
- The common emitter current gain of this transistor is very large as compared to a current gain of common base configuration which is a ratio between collector current and base current. It is denoted by β beta which is the measure of current being amplified.
- β = Ic/Ib
- Output current at the collector and emitter side is highly dependent on the current at the base side.
- Current at the emitter side is the sum of current at the base and collector side because emitter side is highly doped as compared to base and collector.
- Ie = Ib + Ic
- When the voltage is applied at the base terminal it triggers the electrons reaction which forces the electrons to move towards the collector side.
- Any small change at the voltage applied at the base terminal results in a very large change at the current obtained at the collector side.
Pros of BJTs
- Bipolar junction transistor comes with a large amplification factor.
- This type of transistor provides a better voltage gain.
- This transistor comes with a capability of operating in four regions i.e active region, reverse mode, saturation and cut-off region.
- BJT provides a better responese at higer frequiencies.
- BJTs also act as a switch.
Cons of BJTs
- BJT is very sensitive to heat and produces noise is some cases.
- The switching power of BJTs is very low as compared to unipolar transistors like FETs.
- BJTs come with two major applications called amplification and switching.
- They are the building blocks of most of the electronic circuits, especially where audio, current or voltage amplification is required.
- NPN transistors are preferred over PNP transistors for amplification purpose because conduction carried out through mobility of electrons is better than conduction through mobility of holes.
That’s all for today. I have tried my best to break down each and everything related to BJTs so you can digest the main concept easily. In case you are unsure or have any question you can ask me in the comment section below. I’d love to help you according to best of my expertise.
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