The word trans mean transfer property and istor means resistance property offered to the junctions. So, the transistor is a semiconductor device that transfers a weak signal from low resistance to high resistance circuit.
A transistor is a form of a semiconductor device that may be used to conduct and insulate electric current or voltage. A transistor basically acts as a switch and an amplifier. In simple words, we will say that a transistor is a miniature device that is used to control or regulate the flow of electronic signals.
The transistor is one of the basic fundamental building blocks of modern electronics. Transistor was developed in the year 1947 by 3 American physicists John Bardeen, Walter Brattain, and William Shockley, the transistor is taken into consideration as one of the most essential innovations in the records of science.
Characteristics of Transistor
Transistor characteristics represent the relationship b/w the electric current and voltage of a circuit. There are 3 kinds of Transistor characteristic curves based on the configuration of the circuit.
- Input Characteristic
- Output Characteristic
- Current Transfer Characteristic
Input Characteristics
The Input characteristics describe any changes that occur in the input current with the variation in the values of input voltage keeping the output voltage constant.
Output Characteristic
It is a plot of the output current on one axis and output voltage on another with the constant input current.
Current Transfer Characteristic
This plot shows the variation of output current in accordance with the input current, by keeping the voltage constant.
Construction Of Transistor
The transistor is composed PN diode connected back to back. Transistor has three terminals namely the emitter, base, and collector. The base is the center phase that is made up of thin layers. The right part of the diode is known as the emitter diode and the left part is known as the collector-base diode. These names are given according to the common terminal of the transistor. The emitter-based junction of the transistor is attached to the forward biased and the collector-base junction is attached to the reverse bias which gives an excessive resistance.
Terminals Of Transistor
The transistor has 3 terminals that are given below
- Emitter
- Collector
- Base
Emitter Terminal
The section that supplies the huge section of the majority charge carrier is known as the Emitter. The emitter is usually linked in forward biased with respect to the base so that it supplies the majority charge carrier to the base. The emitter-base junction injects a big amount of majority charge carrier into the base because it is heavily doped and slight in length.
Collector Terminal
The section which collects the major part of the majority charge carrier provided through the emitter is known as a Collector. The collector-base junction is usually in reverse bias. Its primary function is to eliminate the majority of charges from its junction with the base. The collector section of the transistor is fairly doped but large in size so that it is able to collect a maximum of the charge carrier provided through the emitter.
Base Terminal
The center section of the transistor is referred to as the Base. The base forms circuits, the input circuit with the emitter, and the output circuit with the collector. The emitter-base circuit is forward-biased and provided low resistance to the course. The collector-base junction is in reverse bias and gives better resistance to the rotation. The base of the transistor is lightly doped and very thin which gives the majority charge provider to the base.
Types Of Transistor
There are mainly two types of transistors used in a circuit which are given below
- Bipolar Junction Transistor (BJT)
- Field Effect Transistor (FET)
Bipolar Junction Transistor (BJT)
BJT are those types of transistors that are made of 3 regions specifically base, collector, and emitter. A very small current flowing between the base and the emitter can control a larger flow of current between the collector and emitter terminal. BJT are currently controlled devices. BJT is a bipolar transistor.
Field Effect Transistor (FET)
FETs are those types of transistors that are made up of three regions namely gate, source, and drain. The voltage at the gate terminal can control a current between the source and the drain. FETs are voltage-controlled bipolar transistors. FET is a unipolar transistor in which N-channel FET or P-channel FET is used for conduction.
Working Principle of Transistor
A transistor is based on the operating principle that it permits you to control the flow of current through one path by varying the intensity of the smallest quantity of current traveling via a second path.
Configurations Of Transistor
In every configuration, the base-emitter junction JE is always forward-biased and the collector-base junction JC is always reverse-biased. There are three types of Configuration that are given:
- Common Base
- Common Emitter
- Common Collector
Common Base Configuration
The input signal is applied between the emitter and base terminals while the corresponding output signal is taken across the collector and base terminals. Thus the base terminal of a transistor is common for both input and output terminals and hence it is named a common base configuration.
In this configuration, the base terminal is grounded so the common base configuration is also known as the grounded base configuration. Sometimes common base configuration is referred to as common base amplifier, CB amplifier, or CB configuration.
The supply voltage between the base and emitter is denoted by VBE while the supply voltage between the collector and base is denoted by VCB. In this configuration, the base-emitter junction JE is forward-biased and the collector-base junction JC is reverse-biased.
Common Emitter Configuration
The input signal is applied between the base and emitter terminals while the output signal is taken between the collector and emitter terminals. Thus, the emitter terminal of a transistor is common for both input and output and hence it is named a common emitter configuration.
In this configuration, the emitter terminal is grounded so the common emitter configuration is also known as the grounded emitter configuration. Sometimes common emitter configuration is also referred to as CE configuration, common emitter amplifier, or CE amplifier. The common emitter (CE) configuration is the most widely used transistor configuration.
The common emitter amplifier has medium input and output impedance levels. So the current gain and voltage gain of the common emitter amplifier is medium. However, the power gain is high.
Common Collector Configuration
In this configuration, the base terminal of the transistor serves as the input, the emitter terminal is the output and the collector terminal is common for both input and output. Hence, it is named a common collector configuration. The input is applied between the base and collector while the output is taken from the emitter and collector.
In this configuration, the collector terminal is grounded so the common collector configuration is also known as the grounded collector configuration. Sometimes common collector configuration is also referred to as emitter follower, voltage follower, common collector amplifier, CC amplifier, or CC configuration. This configuration is mostly used as a voltage buffer.
The input supply voltage between the base and collector is denoted by VBC while the output voltage between the emitter and collector is denoted by VEC. In this configuration, the input current or base current is denoted by IB, and the output current or emitter current is denoted by IE. The common collector amplifier has high input impedance and low output impedance. It has low voltage gain and high current gain. The power gain of the common collector amplifier is medium
Modes Of Transistor
The transistor can be operated in three modes which are given below
- Cut-off mode
- Saturation mode
- Active mode
In order to operate the transistor in one of these regions, we have to supply dc voltage to the NPN or PNP transistor. Based on the polarity of the applied dc voltage, the transistor operates in any one of these regions.
Cut-off mode
In the cutoff mode, both the junctions of the transistor (emitter to base and collector to base) are reverse-biased. In other words, if we assume two p-n junctions as two p-n junction diodes, both the diodes are reverse biased in cutoff mode. We know that in reverse bias conditions, no current flows through the device. Hence, no current flows through the transistor. Therefore, the transistor is in an off state and acts like an open switch.
The cutoff mode of the transistor is used in the switching operations for the switch OFF application.
Saturation mode
In the saturation mode, both the junctions of the transistor (emitter to base and collector to base) are forward-biased. In other words, if we assume two p-n junctions as two p-n junction diodes, both the diodes are forward-biased in saturation mode. We know that in forward bias conditions, current flows through the device. Hence, electric current flows through the transistor.
In saturation mode, free electrons (charge carriers) flow from emitter to base as well as from collector to base. As a result, a huge current will flow to the base of the transistor. Therefore, the transistor in saturation mode will be in the On state and acts like a closed switch.
The saturation mode of the transistor is used in the switching operation for the switch ON application.
Active mode
In the active mode, one junction (emitter to base) is forward biased and another junction (collector to base) is reverse biased. In other words, if we assume two p-n junctions as two p-n junction diodes, one diode will be forward biased and another diode will be reverse biased.
The active mode of operation is used for the amplification of current.
Limitations of Transistors
- Transistors lack higher electron mobility.
- Transistors can be easily damaged when electrical and thermal events arise.
- Transistors are affected by cosmic rays and radiation
Advantages
- Lower cost and smaller size
- Low operating voltage
- No power consumption
- Fast switching
- Better efficiency circuits can be developed
- Used to develop a single integrated circuit