What is Transistor? Biasing, Operations, Advantages and Limitations of Transistors

Two PN junctions are joined together to form a new component called a transistor. A transistor is a three-terminal semiconductor device that controls the flow of current or voltage and acts as a switch or gate for a signal.

Transistors are one of the major components of most electronic devices. Developed in 1947 by three American physicists John Bardeen, Walter Bretton, and William Shockley, the transistor is considered one of the most important inventions in the history of science.

A transistor basically acts as a switch and an amplifier which is used to control or control the flow of electronic signals.

Constructional of Transistor

A transistor is a three-terminal solid-state device made by connecting two diodes back-to-back. So, it has got two PN junctions. Three terminals are drawn out of the three semiconductor materials present in it. This type of connection provides two types of transistors. They are PNP and NPN meaning the material of one N-type semiconductor between two P-type semiconductors and the other being the material of a P-type semiconductor between two N-type semiconductors respectively.

Transistor has three terminals which are represented by the emitter, base, and collector terminals. Their functionality varies.

Emitter

To the left of the structure shown above is the emitter which is denoted by E.
It is medium in size and heavily doped because its main function is to supply multiple majority carriers, i.e. either electrons or holes.
It emits electrons, so it is called an emitter.

Base

In the above figure, the middle semiconductor material is the base represented by B.
It is thin and lightly doped.
Its main function is to transport most of the carriers from the emitter to the collector.

Collector

In the above figure, the semiconductor material on the right is called the collector which is denoted by C.
Its name implies its function of gathering the carriers.
It is slightly larger in size as compared to the emitter and base. It is moderately doped.

In the above figures, the arrows point to the emitter of the transistor. Since the collector of the transistor has to dissipate a lot of power, it is made larger. Due to the specific functions of emitter and collector, they are not interchangeable. Hence terminals should always be kept in mind while using transistors.

A notch is present in the transistor to identify the emitter lead. PNP and NPN transistors can be differentiated using a multimeter.

Biasing of Transistor

The transistor is a combination of two diodes. One junction is between the emitter and the base, which is called the emitter-base junction and so is the other collector-base junction.

The biasing controls the operation of the circuit by providing the power supply. The function of both P-N junctions is controlled by providing bias to the circuit through some DC supply.

The N-type semiconductor material is connected with the negative supply and the P-type semiconductor material is connected with the positive supply to form the circuit forward bias.
The N-type semiconductor material is connected with the positive supply and the P-type semiconductor material is connected with the negative supply to form the circuit reverse bias.

By applying power, the emitter-base junction is always forward-biased because the emitter resistance is very small. The collector-base junction is reverse-biased and has a slightly higher resistance. A small forward bias is sufficient at the emitter junction while a high reverse bias is applied at the collector junction.

The direction of the indicated current in the above circuits, also called conventional current, is the speed of the hole current which is opposite to the electron current.

Operation PNP Transistor

In the operation of PNP transistors, the emitter-base junction is forward-biased and the collector-base junction is reverse-biased.

The emitter of Voltage V_EE provides a positive ability that retreats the holes in the P-type semiconductor material and these holes cross the emitter-base junction to reach the base area. The N-region has a very low percentage of rehabilitation holes with free electrons. It provides a very low current that makes the base current I_B. The remaining holes cross the collector-base junction to make the current I_C, which is the emitter current.

As soon as a hole reaches the collector terminal, the battery fills the location in an electron collector with a negative terminal. This flow gradually increases and the electron flows through the minority current emitter, where each electron entering the positive terminal of the V_EE is replaced by a hole by moving towards the emitter junction. It creates an emitter current I_E.

The PNP transistors occur through the conduction hole.
The collector is slightly less than the current emitter current.
The increase or decrease in emitter current affects the collector current.

Operation NPN Transistor

In the operation of NPN transistors, the emitter-base junction is forward-biased and the collector-base is reverse-biased.

The emitter of Voltage V_EE provides a negative ability that retreats the electrons in the N-type semiconductor material and these electrons cross the emitter-base junction to reach the base area. The P-region has a very low percentage of rehabilitation electrons with free holes. It provides a very low current that makes the base current IB. The remaining electrons cross the collector-base junction to make the current I_C, which is the collector current.

As an electron collector reacts out of the terminal and enters the positive terminal of the battery, the battery enters an electron emitter area from the negative terminal of V_EE. This flow gradually increases and the electron flows through the current transistor.

In an NPN transistor, the conduction occurs through electrons.
Collector current is more than emitter current.
The increase or decrease in emitter current affects the collector current.

Advantages of Transistors

High voltage benefits.
A low supply voltage is sufficient.
The most suitable for low-power applications.
Vacuum tubes are mechanically stronger than tubes.
No external heating is required like a vacuum tube.
It is very suitable to integrate with resistance and diode to produce ICS.
Low cost, small in size, light in weight, No electricity consumption, and Fast switching.
Small mechanical sensitivity.
Better efficiency circuits can be developed.
A single integrated circuit is used to develop.

There are some disadvantages such as low power wastage which cannot be used for high-strength applications. They have low input impedance and are dependent on temperature.

Limitations of Transistors

Transistors also have some limitations. They are as follows:

The transistor lacks high electron mobility.
Transistors can easily be damaged when electrical and thermal events arise. For example, electrostatic discharge in handling.
Transistors are affected by cosmic rays and radiation.