[Physics Class Notes] on Uses of Transistor Pdf for Exam

If we read the history of electronic devices, one of the most important components of these devices was a vacuum tube (electron tube). This tube was used to control the electric current. These tubes were larger, required higher operating voltage, high power consumption meant high heat generation which in turn affected the life of the tube due to its low efficiency.

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To resolve this problem, the three American physicists John Bardeen, Walter Brattain, and William Shockley invented a compact-sized and efficient semiconductor device called point-contact transistor at Bell Labs in December.

Transistor

A transistor is a kind of semiconductor that is used as a conductor and insulation of electric current or voltage. In simpler terms, a transistor is basically a regulator of the flow of electric signals. Read the following points to know more about transistor:

• Transistors are powerful devices because of their ability to control the current flowing through a circuit (current controlling device), which is generated by the flow of electrons and holes. There are two types: NPN (negative -positive-negative) and PNP (positive-negative-positive).

• The most widely used transistors are NPN transistors as the majority of charge carriers are electrons that are better mobile charge particles with less mass due to which they can easily accelerate.

• It is a semiconductor device that acts as a switch and an amplifier. Transistors can operate on a low-voltage supply for greater safety which means they yield higher efficiency and very long life.

• The transistors use semiconductor junctions instead of heating electrodes but perform the same function as a vacuum triode.

• The transistors can control the flow of current through one channel by changing the intensity of a small amount of current flowing via a second channel. That’s why they are called the current controlling device.

Parts of a Transistor

A transistor is a combination of three terminals made of semiconducting materials that help in making a connection to an external circuit and allow current to flow. The three terminals are:

1. Base: The base activates the transistor. It is thin and lightly doped. It is put in the centre of the transistor.

2. Emitter: The emitter is the negative terminal of the transistor. It is heavily doped and is moderately sized.

3. Collector: The collector is the negative terminal of the transistor. It is located on the right side of a transistor and is moderately doped. It is larger than the emitter.

How does a Transistor Work?

A Bipolar Junction Transistor or BJT consists of three terminals- base, emitter and collector. A p-n junction exists between base and emitter, and another junction exists between base and collector. Normally, in BJT when current flows through the base-emitter junction, a current will flow in the collector circuit. This is called bias and the base-emitter junction is forward biased whereas the base-collector junction is reverse biased.

Basics of Bipolar Junction Transistors

Since the controlled current must go through two types of semiconductors materials the current consists of both electron and hole flow, in different parts of the transistor, and these are of two types:

1. n-p-n junction transistor

2. p-n-p junction transistor

Isolated Gate Bipolar Transistor (IGBT): IGBT is a power semiconductor device used as an electronics switch in much high power and modern appliances such as electric cars, trains, variable speed refrigerators, air conditioning systems.

What are the Characteristics of a Transistor?

The characteristics of a transistor is the graph plotted for each type of configuration, which shows the relationship between the current and voltage of the transistor.

There are mainly two types of characteristics:

1. Input characteristics: This shows the change in input current with varying output current when the output voltage is constant.

2. Output characteristics: This graph shows a plot of changing output current with respect to a change in output voltage when the input current is constant.

Advantages of using Transistors

Transistor has been proven as a very important invention in science. It has many uses and advantages:

• It is small in size and is very cost-efficient.

• It needs very low voltage to function.

• It has a long life and requires no power to operate.

• A single integrated circuit can be developed using the transistor.

• Current switches fast in the terminals.

Limitations of using Transistors

Even though transistors are extremely efficient, there are some limitations to its uses:

• Transistors get damaged very easily due to changes in electrical and temperature conditions.

• They lack higher electron mobility.

• They can get affected by radiation.

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Uses of Transistor

Vbe biasing voltage produced in the base-emitter junction. Due to the forward biasing of the base-emitter junction, the electrons start flowing from emitter to recombine with holes in the base, the base becomes negatively charged. If the base current Ib is increased by a small amount, hole-electron recombination will get neutralised, the collector current Ic will be increased. Therefore, a small change in current Ib in the base.

• Microphone: The microphone is a transducer that converts our voice or sound wave to an electronic signal. As the sound wave doesn’t have a constant value, the magnitude of the sound wave varies with time according to our voice.

The electrical output of the microphone varies according to the sound waves as the base current Ib is varying because of the small alternating voltage produced by the microphone which means a small change in Ib can cause a large change in Ic.

When this output of the microphone is given to the transistor as an input. The varying collector current Ic flows into the loudspeaker, and we know that if there are changes in the input of the transistor there will be a large change in the output of the transistor. Thus, the transistor amplifies the electronic signal of the microphone.

The frequency remains constant but the amplitude of the sound wave from the loudspeaker is higher than sound waves fed into the microphone.

An electronic oscillator is a device that generates continuous electrical oscillations. In a simple oscillator circuit, a parallel LC circuit is used as a resonant circuit and an amplifier is used to feed energy to the resonant circuit.

The frequency gets resonantly amplified, and the output acts as a source of an alternating voltage of that frequency.

• Transistor Used as a Switch
  BJT Transistors can be used as a switching device to control DC power to a load. The switched (controlled) current goes between emitter and collector, and the controlling current goes between emitter and base.

Summary

• In normal operation of a transistor, the emitter-base junction is always forward-biased whereas the collector-base junction is reverse biased.

• In n-p-n junction transistors, there are a large number of electrons in the emitter and a large number of holes in the base.

• In the actual design of n-p-n transistors, the middle layer is very thin (micrometre) as compared to the widths of the two layers at the sides.