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Junction Transistor, also known as a Bipolar Junction Transistor is a semiconductor device that helps in transferring a weak signal from a low resistance circuit to a high resistance circuit. A Junction Transistor helps in regulating and amplifying electrical signals such as current or voltage. Transistors as a device are unique as they allow the control of how much current can flow through a circuit, which they achieve by controlling the voltage across two of the three transistor leads.
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Key Terms: Junction Transistors, n-p-n Transistors, p-n-p junction, p-Type Semiconductor, Circuit, Voltage, Current
What is Junction Transistor?
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Junction Transistors, also known as Bipolar Junction Transistor (BJT), can be defined as a three-terminal semiconductor device which contains two p-n junctions and helps in the amplification or magnification of a signal.
Junction Transistor
Frequently Asked Questions Related to Junction TransistorsQues. “Transistors are a current-controlled device.” Justify. (2 marks) Ans. Transistors are often also called current-controlled devices due to the fact that the collector current is usually controlled by the base current. Simply put, the changes in a collector-current is considered proportional to the corresponding changes in the base current. Ques. Demonstrate the Input and output characteristics of a transistor. (2 marks) Ans. The input characteristics, in common-emitter configuration, are the plot acquired by tracing the variation of input current, IB with respect to the input voltage, VBE. In a similar manner, the variation of output current, IC with respect to the Collector to emitter voltage VCE, is called output characteristics. |
The video below explains this:
Transistor Detailed Video Explanation:
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Construction of Junction Transistors
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Junction Transistors are a combination of two junction diodes and the three-layer structure of these can be of either of the combinations:
- A p-n-p type of semiconductor where an n-type of semiconductor is between two layers of p-type semiconductor, or
- An n-p-n type of semiconductor where a p-type of semiconductor is between two layers of n-type semiconductor.
Junction Transistor Construction
These types of transistors have two junctions between the p-type and n-type of semiconductors. Junction Transistors are mainly controlled by current, meaning that even a small amount of current flowing through its base will result in a large current flow from the emitter to the collector.
Structure of Junction Transistor
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Transistor is normally a three-layered semiconductor device where any one type of semiconductor is placed so that it is sandwiched between the other two same type of semiconductor.
In a Junction Transistor, the formation takes place by layering three semiconductor materials in p-n-p or n-p-n style, where in the p-n-p transistor we have two p regions and one n region and in the n-p-n transistor we have two n regions and one p region. These transistors have three terminals called an emitter, collector and base.
Structure of a Junction Transistor
These three materials have the following functions:
- Emitter: An emitter supplies a large section of charge carriers. With respect to the base, the emitter is always forward biased so that it can supply the majority charge carrier to the base. The emitter in a transistor is always heavily doped and is moderately sized.
- Collector: In a transistor, a collector is a section that collects the majority charge carrier supplied by the emitter and in this case, the collector-base junction is always reverse biased. In a transistor, the collector section is always moderately doped, but however, is large in size in order to collect most of the charge carrier supplied by the emitter.
- Base: The base is situated in the middle of the transistor and forms two circuits namely an input circuit with the emitter and an output circuit with the collector. The collector-base junction is reverse biased and gives high resistance to the circuit while the emitter-base junction is forward biased and offers low resistance to the circuit. The base is very thin and lightly doped which gives the majority of charge carriers to the base.
How Does an n-p-n Transistor Work?
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In an n-p-n transistor, there are two n-types of semiconductors between which a p-type of semiconductor is placed like so in a sandwich. Here, the holes are the minority charge carriers and the electrons are the majority charge carriers.
n-p-n Transistor
Even a small amount of current at the base terminal will cause a large amount of current flow from the emitter to the collector. As the emitter-base circuit in the transistor is forward biased, it repels the majority of the charge carriers towards the base.
As the electron-hole recombination is very small in the base region because of being lightly doped, most of the electrons then cross the collector region.
How Does a p-n-p Transistor Work?
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In a p-n-p transistor, there are two p-type of semiconductors between which an n-type of semiconductor is placed like so in a sandwich. In this, the holes are the majority charge carriers and the electrons are the minority charge carriers.
p-n-p Transistor
In this type of circuit, the emitter-base battery connects the forward biased p-type emitter, while the collector base battery connects the reverse biased p-type collector.
Things to Remember
- Junction Transistor is a semiconductor device that helps in transferring a weak signal from a low resistance circuit to a high resistance circuit.
- Junction transistor is also known as Bipolar Junction Transistor (BJT).
- Junction transistor is classified into three material parts, Emitter, Collector and Base.
- In Junction Transistor, the formation occurs by layering three semiconductor materials in p-n-p or n-p-n style, where in the p-n-p transistor we have two p regions and one n region and in the n-p-n transistor we have two n regions and one p region.
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Sample Questions
Ques. Why are n-p-n type transistors preferred over p-n-p type transistors? What is the meaning of the arrow in a transistor symbol? (1 mark)
Ans. n-p-n types are transistors preferred over p-n-p type transistors because of reasons like less production cost of n-p-n transistors, their carrier mobility and negative grounding.
The arrow in a transistor indicates the direction of the current flow.
Ques. What are the different applications of transistors? How does the depletion region of a p-n junction diode get affected under reverse bias? (Delhi 2011)
Ans. They are used for amplification, as phototransistors, for switching purposes and in microwave communication. Uye Depletion region widens under reverse bias.
Ques. Are all the regions of the transistor doped? (1 mark)
Ans. Yes all the regions of a transistor are doped but in different capacities. It indicates the direction of the current flow.
Ques. (a) Draw the circuit diagram of an n-p-n transistor with emitter-base junction forward biased and collector-base junction reverse biased. Describe briefly how the motion of charge carriers in the transistor constitutes the emitter current (IE), the base current (IB) and the collector current (IC). Hence deduce the relation IE = IB + IC
(b) Explain with the help of circuit diagram how a transistor works as an amplifier. (Comptt. All India 2012)
Ans.(a) In a p-n-p transitor, the heavily doped emitter which is p-type has a majority charge carrier of holes. These holes when move towards 21-type base get neutralized by e– in base. The majority carriers enter the base region in large numbers. As the base is thin and lightly doped, the majority carriers (holes) swamp the small number of electrons there and as the collector is reverse biased, these holes can easily cross the junction and enter the collector.
The function of emitter is to emit the majority carriers and collector is to collect the majority carriers. Base provides the proper interaction between the emitter and the collector.
Action of n-p-n transistor. The emitter-base junction of a transistor is forward biased while collector base junction is reverse biased as shown in adjoining Figure.
In case of n-p-n transistor, the negative terminal of VEB repels the electrons of the emitter towards the base and constitute emitter current IE. About 5% of the electrons combine with the holes of the base to give small base current IB.
The remaining 95% of the electrons enter the collector region under the reverse bias and constitute collector current IC.
According to Kirchhoff’s law, the emitter current is the sum of collector current and base current.
IE = IC + IB
(i) Common emitter configuration of n-p-n transistor
Ques. Describe briefly with the help of a circuit diagram, how the flow of current carriers in a p-n-p transistor is regulated with emitter-base junction forward biased and base-collector junction reverse biased. (All India 2012)
Ans. In a p-n-p transistor, the heavily doped emitter which is p-type has a majority charge carrier of holes. These holes when move towards 21-type base get neutralized by e– in base. The majority carriers enter the base region in large numbers. As the base is thin and lightly doped, the majority carriers (holes) swamp the small number of electrons there and as the collector is reverse biased, these holes can easily cross the junction and enter the collector.
Ques. What is a Transistor? (1 mark)
Ans. Transistor can be expressed as an electronic device which basically works by controlling the flow of the electric current.
Ques. How can you define Bipolar Junction Transistor or Junction Transistor? (1 mark)
Ans. Junction Transistors, also known as Bipolar Junction Transistor (BJT), can be defined as a three-terminal semiconductor device which contains two p-n junctions and helps in the amplification or magnification of a signal.
Ques. What is an Emitter in a Junction Transistor? (1 mark)
Ans. Emitter helps in the supply of a large section of charge carriers. With respect to the base, the emitter is always forward biased so that it can supply the majority charge carrier to the base. The emitter in a transistor is always heavily doped.
Ques. Assuming that a transistor is used as a switch, in which region of output characteristics it is seen to be operated? (1 mark)
Ans. Considering that a transistor is utilised as a switch, it is then generally operated alternately within the cut-off region and saturation region of output characteristics.
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