Updated: Dec 1, 2020
After a great struggle and repeated experiments, the electric current was invented in early 1600. William gilbert produced static electricity by rubbing amber. Now the biggest challenge is to control the flow of electric current.
During the initial stages, vacuum tubes are used in the process of controlling the flow of the electric current. But they were too bulky, and also require high operating voltage. They also consume more power and yield low efficiency.
All these aspects result in more heat production and the life of the tube is shortened. To sort out these problems John Bardeen, Walter Brattain, William Shockley invented a new component called transistors at Bell Labs in 1947. All those queries in the usage of vacuum tubes are rectified in the transistors. In this article, we are going to see in detail about the transistors, its types, and construction, and also the applications of it.
What is a transistor?
A transistor is nothing but an electrical component that is capable of switching or amplifying any electrical signal or electric power. The transistors are made up of semiconductor materials and possess at least three #terminals to establish a connection with the external circuit. The transistors perform the same function as that of vacuum tubes but using semiconductor junctions unlike heated electrodes in vacuum tubes.
The first transistor was made using the germanium. Nowadays, transistors are manufactured using silicon and other semiconductor materials. The basic concept of a transistor is nothing but it allows us to control the flow of current using one channel by varying the intensity of the smaller current flowing through the second channel.
The transistors can be broadly classified into bipolar junction transistors (BJT) and Field Effect Transistors (FET). Transistors played a major role in developing smaller and cheaper calculators, calculators, etc.
Bipolar Junction Transistors (BJTs):
A BJT consists of three terminals. They are emitter, base, and collector. The base terminal is lightly doped and responsible for activating the transistor. The collector terminal is a positive lead, reverse-biased with respect to the base terminal. The emitter terminal is a negative lead and forward-biased with respect to the base terminal.
BJT consists of both electrons and holes as their charge carriers. It allows a small current to flow through one of its terminals to control a larger current flowing through the other two terminals.
BJTs are manufactured by doping n-type and p-type semiconductors in a single crystal. BJTs can be operated in two different configurations. They are N-P-N (P-type semiconductor in between two slabs of N-Type), and P-N-P (N-type semiconductor in between two slabs of P-type)
In PNP transistor, the base current entering the collector is amplified as output. When PNP transistor is in ON state, its base is kept low with respect to the emitter.
In NPN transistor, by amplifying the base current we can obtain high collector and emitter current. Same as in PNP transistor, in NPN transistor also the base is pulled low with respect to the emitter when the transistor is in ON state. NPN transistor is the most employed BJT in this modern era.
The bipolar transistor is still used in switching applications, amplification of #signals, controlling heavy current. They are used in the manufacturing of radio-frequency amplifiers, and high voltage switches.
Field Effect Transistor (FET)
The FET transistors employ an electric field for controlling the flow of electric current. FETs are unipolar transistors (i.e) they use either holes or electrons as their charge carriers. FET transistors have three terminals. They are gate, source, and drain terminals. The charge carriers enter the channel through the source terminal and leave through the drain terminal. The gate terminal modulates the conductivity of the channel.
The FET transistors control the flow of current by the gate voltage, which controls the conductivity between source and drain. FET transistors are constructed with either n-type or p-type semiconductors but not both. N-channel FET and P-channel FET can be operated in both depletion and also in enhancement mode.
In an n-channel enhancement-mode device, positive gate-to-source voltage is necessary to create a conductive channel and it is not present as default. The applied positive voltage attracts the electrons within the body towards the gate, forming a conductive channel.
Whereas in an n-channel depletion mode, a negative gate-to-source voltage causes the depletion region to expand. This results in the narrowing of the channel. The FET will be turned OFF like a switch if the active region starts expanding. This occurs because of the increase in the resistance from source to drain.
In a p-channel enhancement-mode device, negative gate-to-source voltage is necessary to create a conductive channel. In a p-channel depletion mode, when a positive gate-source voltage is applied, the depletion layer starts to expand and repels the electrons towards the gate-semiconductor interface.
There are many kinds of FET transistors such as Metal Oxide Semiconductor Field Effect Transistor (MOSFET), Ion Sensitive Field Effect Transistor (ISFET), Junction Field Effect Transistor (JFET), and BioFET, FREDFET, etc.
MOSFET transistor and its types (N-MOS, P-MOS, C-MOS) transistors have been explained in a detailed manner in two separate articles and the links of those are given in the see also section below.
Advantages of transistors:
It provides a high degree of isolation between control and the flow since it has high gate-to-drain resistance.
Transistors play a major role in the manufacturing of digital integrated circuits.
They are very small in size and the cost of production is also less.
Transistors possess small mechanical sensitivity and extremely long service life when compared to conventional vacuum tubes.
Cathode heaters won't consume any power and also capable of doing fast switching.
Biasing can be done based on 5 different parameters. They are current, feedback, double base biasing, voltage dividing biasing, and double feedback biasing.
Disadvantages of transistors:
The reverse blocking capacity of the transistors is very low.
The second breakdown or thermal runaway can result in the damage of transistors.
The efficiency of Transistors is less when they are operated above the switching frequency of 15KHz.
Applications of transistors:
Schottky transistors are capable of controlling and diverting high input current and prevent the transistors from saturating.
They are mostly employed as switches in digital circuits.
Nowadays flexible transistors are employed in displays and switches.
In the military, transistor's high-power radiofrequency is used in radars and handheld two-way radios.
Extremely sensitive Darlington transistors are employed in the manufacturing of touch and light-sensing devices.
Invertors used in our houses, and car batteries employ high current transistors.
Transistors are also employed in the process of vehicle ignition, industrial machine control, and also in the manufacturing of medical devices such as ECG.