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.
On this page, we will learn about the following:
Use of transistors
Applications of transistors
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 1947.
What is a 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.
These devices can both conduct and induct.
These devices are small-sized that a single IC can accommodate millions of these devices in itself.
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.
Basics of Bipolar Junction Transistors
Since the controlled current must go through two types of semiconductors materials which are P and N. The current consists of both electron and hole flow, in different parts of the transistor, and these are of two types:
n-p-n junction transistor:
p-n-p junction transistor
The base is responsible for activating the transistor
The emitter is the negative lead while Collector is the positive lead.
Uses of Transistor
A transistor can be used to amplify current.
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 neutralized, the collector current Ic will be increased. Therefore, a small change in current Ib in the base circuit would control a large amount of Ic current.
Example of 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.
Figure.2 shows a schematic representation of an oscillating circuit.
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. The frequency can be varied by varying L or C.
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.
The transistor as a switch used in making:
Memory cards in mobile phones.
Most of the parts of modern electronic devices like smartphones, smartwatches, etc are all made up with transistors where they act as switches.
Logic gates, flip-flops circuit.
Microprocessors and Microcontrollers.
Radio, medical instruments, control systems, cameras, autos, airplanes, radar, etc.
Applications of Transistors
Phototransistors: This is a type of transistor that operates depending upon the intensity of incoming light i.e. they are light sensitive. This transistor is mostly used in optoisolators and in light-dependent controlling units.
Heterojunction Bipolar Transistors (HBTs): are mainly used for microwave communications as they can provide much higher switching speed, and they can handle signals of varying frequencies up to several hundred GHz.
Darlington Transistors: These transistors are formed by a connection of two BJTs, connected in such a way that current amplified by first is re-amplified by the second. They are mostly used in current amplification circuits in the output end of the microcontrollers, and PLCs based systems that are required to draw high current.
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.
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 (〜 1 micrometer) as compared to the widths of the two layers at the sides.
1. What are the Advantages of BJT?
The advantage of BJT is that it has a large gain bandwidth, shows better performance at a high frequency, has a better voltage gain, and a high current density, can function in both low and high power applications. That’s why they are used as an amplifier.
2. Why Does Power Dissipation Occur Mainly in the Collector Junction?
Since the collector junction is reverse biased which means it offers more resistance to the flow of current due to which a huge voltage drop occurs at the collector junction. That’s why power dissipation occurs in the collector junction.
3. Which Type of Operation is Suitable for FETs?
Ans.One should use FETs in circuits with high power and current control applications. Digital and analogue circuits are mostly suitable for FET application.