Semiconductor basics Pdf for Upsc and other govt exams

All gadgets such as mobile phones, Tv, calculator, etc are Electronics, which has been part of our daily life.

Semiconductor basics Pdf

Evolution of Electronics

The story of electronics started with the invention of vacuum diodes by J.A. Fleming. It was succeeded by a vacuum triode executed by Lee De Forest to Amplify electrical signals. This started to form the foundation of tetrode and pentode tubes.

Consequently, the transistor age started with the invention of bipolar junction transistors by Bardeen, Brattain, and Shockley in 1948 they won the Nobel prize in 1956. The development of Germanium and Silicon semiconductor substances made transistors famous and used in various electronic circuits.

The invention of IC’s (Integrated Circuit) embedded different electronic components on a single chip that was small and less price. In 1969, Intel introduced Microprocessors which revolutionized computers from room size to the size of a palm.

Electronics is a subsidiary of physics with the technology of designing the circuit employing transistors and microchips. Electronics deal with active and passive components.

The active components are the ones that can generate power in the circuit. Examples are transistors, diodes, IC, and sensors. The passive electronic components are the ones that cannot generate power in a circuit. Examples are resistors, inductors, capacitors, and transformers. 

Energy Band Diagram

In a separated or an isolated atom, the electronic energy level is broadly separated and is distant apart. The energy of the electron is determined by the orbit in which it orbits around the nucleus.

But for solids, the atoms are closely aligned, thereby the electrons in the outermost energy levels of nearby atoms affect every other. This shapes the characteristics of the electron movement in solids.

The valence electrons in an atom are in charge of its binding nature. Consider an atom with one electron in the outermost orbit, which means there is only one valence electron.

When two such atoms are brought closer, the valence orbitals are split into two. Similarly, the empty orbitals of every atom will likewise divide into two.

The electrons have the option of preferring any one of the orbitals as the energy of both the orbitals is the same. Meanwhile, the third atom of the same element is induced to this system, the valence orbitals of all three atoms are divided into three.

The empty or unoccupied orbitals are also divided into three. In fact, a solid is built up of millions of atoms. When millions of atoms get close together, the valence orbitals and the unoccupied orbitals are divided according to the number of atoms.

When the energy levels are nearly aligned it will be hard to distinguish the orbitals of one atom from the other and look like a band.

This band of an extremely huge number of tightly separated energy levels in a very tiny energy range is called the energy band. The energy band developed by the valence orbitals is called the valence band.

The energy band formed by the unoccupied orbitals is called the conduction band. “The energy gap between the valence band and the conduction band is called the forbidden energy gap” and electrons never exist in the forbidden energy gap (Tamilnadu Textbook Corporation #).

From the above diagram, Ec represents the minimum energy of the conduction band. The energy gap between the conduction band and valence band, Eg = EC – Ev.

The kinetic energy of the electron rises from bottom to top and the potential energy drops symbolizing that the electrons in the orbitals near the nucleus are bound with enormous potential energy.

Hence, the electrons nearer to the nucleus need a lot of energy to be exited.


P-N Junction 

A p-n junction is formed by joining n-type and p-type semiconductor materials. The P side has a high hole concentration.

The N side has a high electron concentration. This causes diffusion current which exists because of the difference in concentration of atoms.

Junction Potential or barrier Potential

The power of the electric potential difference across the depletion region keeps building with the crossing of each electron until equilibrium is reached. At this point, the internal repulsion of the depletion layer stops further diffusion of free electrons across the junction.

This difference in potential across the depletion layer is called barrier potential. At 25° C, this barrier potential is nearly equal to 0.7 V for silicon and 0.3 V for Germanium.

P-N Junction Diode

A p-n junction diode is formed bt p-type and n-type semiconductors fused together.

Biasing a Diode

“Biasing means providing external energy to charge carriers to overcome the barrier potential and make them move in a particular direction”. (Tamilnadu Textbook Corporation #)

The charge carriers either move towards the junction or away from the junction. The external voltage applied to the p-n junction is called Bias Voltage. There are two types of biasing, one is forward bias and another one is reverse bias.

Forward Bias

When the positive terminal is connected to the p-side and the negative terminal to the n-side, is called Forward biased.

Reverse Bias

When the positive terminal is connected to the n-side and the negative terminal to the p-side is called Reverse Biased.

Breakdown Mechanism

When a reverse bias is applied to a p-n junction, at some point the junction breaks down and the reverse current rises sharply. The voltage at which this happens is called the breakdown voltage.

A normal p-n junction diode is damaged at this point and specially designed diodes like Zener diodes can operate at this point. Zener diode is used for rectification

Zener Diode

The Bipolar Junction Transistor

A bipolar junction transistor is a semiconductor device of two NPN and PNP. To operate the transistor in the active region, the emitter-base must be forward biased and the collector-base must be reverse biased.

A BJT can be operated in three different configurations: Common base, Common Emitter, Common Collector. The forward current gain in a common base configuration 𝞪 gives the ratio of the collector current to emitter current.

The forward current gain in common emitter configuration 𝜷 gives the ratio of the collector current to the base current. The BJT connected in common emitter configuration functions as a switch.

The BJT connected in a common emitter configuration can be used as an amplifier. There exists a phase reversal of 180° between the input signal and the amplified output signal.

Electronics Tnpsc Pdf

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* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
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