NCERT Class 12 Physics Chapter 14 Notes Semiconductor Electronics Materials Devices and Simple Circuits - Download PDF

NCERT Notes for Class 12 Physics Chapter 14: Download PDF

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NCERT Notes for Class 12 Physics Chapter 14

The NCERT Notes for class 12 Physics Semiconductor Electronics: Materials, Devices and Simple Circuits are a comprehensive collection of all concepts in the NCERT textbook. These notes are easy to understand and explain the complex topics in detail.

History of Electronic Devices

Earlier electronic devices were made up of vacuum tubes or valves.

• The evolution of vacuum tubes started with diodes and proceeded to date with triode, tetrode and pentode.

• Valves are responsible for controlling the flow of electrons. In diodes there used to be two electrodes i.e. cathode and anode.

• Similarly, triode used to have three electrodes i.e. cathode, grid and anode and tetrode used to have four electrodes i.e. anode, two grids and cathode and pentode used to have five electrodes i.e. anode, three grids and cathode.

• In vacuum tubes, the electrons are produced by heating the cathode using a low tension battery. The vacuum helps the electron not to lose its energy by collision with air molecules in the way.

• The disadvantages of vacuum tube devices are that they used to be

1. Huge
2. Operate at high voltages
3. Consume a lot of power
4. Have restricted life
5. Low responsibility

Classification of Metals, Semiconductors and Insulators

According to the physical phenomenon, we will simply tell whether or not it is classified as metals, semiconductors or insulators.

The electrical physical phenomenon is mostly diagrammatic by σ whereas the reciprocal of physical phenomenon is electric resistance and is diagrammatic by ρ = 1/σ

Metals

Metals have high physical phenomenon and low electric resistance

The order of ρ is 10^-2 to 10^-8 Ω m

The order of σ is 10² to 10⁸ S/m

Semiconductors

They have conductivity and resistivity in between metals and insulators.

The order of ρ is 10^-5 to 10^-6 Ω m

The order of σ 10⁵ to 10⁶ S/m

Insulators

They have high resistivity and hence low conductivity.

The order of ρ is 10^-11 to 10^-19 Ω m

The order of σ 10² to 10¹⁹ S/m

Classification of semiconductors

The semiconductors can be classified into two types i.e. Elementary kind semiconductor and compound kind semiconductor.

Elementary type semiconductor

These types of semiconductors are available in natural form for example Silicon (Si) and germanium (Ge)

Compound-type semiconductor

When semiconductors are made by compounding the metals, compound type semiconductor is obtained. Further, they can be classified into

1. Inorganic semiconductors like CdS, GaAs, etc.

2. Organic semiconductors like Anthracene, doped phthalocyanines

3. Organic polymers – Polypyrrole, polyaniline, polythiophene

Band Theory of Solids

The energy levels of the atom form a nonstop band, wherever the electrons move This is known as the band theory of solids.

• In an atom, the protons and the neutrons constitute the central part called the nucleus.

• The electrons revolve around the nucleus in definite orbits.

• The orbits are named as 1s, 2s, 2p, 3s, 3p, 3d etc. each of which features totally different energy.

• All electrons of the same orbit will have the same energy.

• The electrons within the innermost orbits that are fully crammed are valence electrons whereas the electrons within the outermost orbit that don't fully fill that shell are known as conduction electrons.

Classification on the Basis of Energy Bands

Depending upon the relative position of the valence band and the conduction band, the solids are classified into three categories i.e. conductors, insulators and semiconductors.

Conductors

• The conduction band and also the valence band part overlap one another and there's no impermissible energy bandgap in between.

• The electrons from the valence band can easily move into the conduction band.

• A large number of electrons are available for conduction.

• The resistance of such materials is low but the conductivity is high.

Insulators

• In insulators, there is a large energy gap between the valence band and the conduction band.

• The energy gap is very high such that the electrons from the valence band cannot move to the conduction band by thermal excitation.

• As there are no electrons in the conduction band, electrical conduction is not possible.

Semiconductors

• A finite but small energy gap exists between the valence band and the conduction band.

• At room temperature, some of the electrons from the valence band acquire energy and move into the conduction band.

• At high temperatures semiconductors have conductivity. The resistance is not as high as in insulators.

• Semiconductors are of two types: Intrinsic And Extrinsic Semiconductor

Intrinsic Semiconductor

It is a pure semiconductor. Silicon and germanium are the most common examples of intrinsic semiconductors. Both these semiconductors are most frequently used in the manufacturing of transistors, diodes and other electronic components.

Both Si and Ge have four valence electrons. In its crystalline structure, every Si or Ge atom tends to share one of its four valence electrons with each of its four nearest neighbour atoms, and also to take a share of one electron from each such neighbour as shown in the below figure. This shared pair of the electrons is called a Covalent bond or a Valence bond.

The above figure shows the structure with all bonds intact (i.e. no bonds are broken). This is possible only at low temperatures. As the temperature increases, more thermal energy becomes available to these electrons and some of these electrons may break–away from the conduction band becoming the free electron and creating a vacancy in the bond. This vacancy with an effective positive electronic charge is called a hole.

In intrinsic semiconductors, the number of free electrons ( n_e ) is equal to the number of holes ( n_h )

i.e $n_e=n_h=n_i$ where $n_i$ is called intrinsic carrier concentration.

Semiconductors possess the unique property in which, apart from electrons, the holes also move. The free-electron moves completely independently as a conduction electron and gives rise to an electron current, I_e under an applied electric field. while Under an electric field, these holes move towards the negative potential generating hole current (I_h).

Hence, the total current (I) is given as $I=I_e+I_h$

Apart from the process of generation of conduction electrons and holes, a simultaneous process of recombination occurs in which the electrons recombine with the holes. At equilibrium, the rate of generation is equal to the rate of recombination of charge carriers.

The conductivity of an intrinsic semiconductor at room temperature is very low. As such, no important electronic devices can be developed using these semiconductors. Hence there is a necessity of improving their conductivity. This can be done by making use of impurities.

Extrinsic Semiconductor

An extrinsic semiconductor is a semiconductor doped by a specific impurity which is able to deeply modify its electrical properties, making it suitable for electronic applications. The deliberate addition of a desirable impurity is called doping and the impurity atoms are called dopants.

n-type Semiconductor

When a pentavalent impurity is added to an intrinsic or pure semiconductor, then it is said to be an n-type semiconductor. Pentavalent impurities such as phosphorus, arsenic, antimony, etc are called donor impurities.

The four valence electrons of each phosphorus atom form 4 covalent bonds with the 4 neighbouring silicon atoms. The free-electron (fifth valence electron) of the phosphorus atom does not involved in the formation of covalent bonds. This shows that each phosphorus atom donates one free electron. Therefore, all the pentavalent impurities are called donors.

So, there is a donor energy level between the valence band and the conduction band. Just below the conduction band.

The number of free electrons depends on the amount of impurity (phosphorus) added to the silicon.

In an n-type semiconductor, conduction is mainly because of the motion of free electrons.

In an n-type semiconductor, the population of free electrons is more whereas the population of holes is less (i.e. n_e >>n_h). In an n-type semiconductor, free electrons are called majority carriers and holes are called minority carriers.

p-type semiconductor

When the trivalent impurity is added to an intrinsic semiconductor, then it is said to be a p-type semiconductor. Trivalent impurities such as Boron (B), Gallium (G), Indium(In), Aluminium(Al), etc are called acceptor impurities.

The three valence electrons of each boron atom form 3 covalent bonds with the 3 neighbouring silicon atoms. For the fourth covalent bond, only the silicon atom contributes one valence electron. Thus, the fourth covalent bond is incomplete with the shortage of one electron. This missing electron is called a hole.

This shows each boron atom accepts one electron to fill the hole. Therefore, all the trivalent impurities are called acceptors. So there is an acceptor energy level just above the valence band. A small addition of impurity (boron) provides millions of holes.

In a p-type semiconductor, conduction is mainly because of the motion of holes in the valence band.

In a p-type semiconductor, the population of free electrons is less whereas the population of holes is more (i.e n_h >>n_e)

In a p-type semiconductor, holes are called majority carriers and free electrons are called minority carriers.

p-n Junction Formation

• A p-n junction is the basic building block of many semiconductor devices like diodes, transistors, etc.

• The holes are the majority carriers in the p-type semiconductor whereas the electrons are the majority carriers in the n-type semiconductor.

• In n-type semiconductors, the concentration of electrons is more as compared to the concentration of holes. Similarly, in p-type semiconductors, the concentration of holes is more as compared to the concentration of electrons.

• Diffusion is the first process that occurs in the p-n semiconductor.

• In the formation of the p-n junction, due to the concentration gradient across the p and the n sides, there is the diffusion of electrons from n region and therefore the holes are diffused from the p region to the n region.

Diffusion Current

• It is due to the motion of charge carriers due to the difference in concentration in two regions of the p-n junction, across the junction.

• The diffusion leaves behind a positive charge on the n-side close to the junction. This positive charge known as an ionised donor is immobile due to bonding with the surrounding atoms.

• Similarly, in the p-region near the junction, there is a negative charge or acceptor ions that are immobile.

Depletion region formation

• The space charge region on both sides of the p-n junction is devoid of any charge carriers and has immobile ions.

• Due to this, it results in the formation of a depletion region near the junction.

Field setup

• As the depletion region is formed, it results in setting up a field at the junction.

• The field along the junction acts like a fictitious battery that is connected across the junction with a positive terminal connected to the n-region.

Barrier creation

• The electric field at the junction sets a barrier opposing further diffusion of majority charge carriers through the junction.

• Thus, the barrier that is created at the junction prevents further diffusion.

• Width of the barrier:- The distance from one side of the barrier to the other is called the width of the barrier.

• Height of the barrier:- The potential difference from one side of the barrier to the other side is known as the height of the barrier.

Drift current

• Since the electric field is developed at the junction, the electrons from the p-region move to the n-region. Similarly, the hole from the n-region moves to the p-region. Due to this drift current is produced.

Drift current Vs Diffusion current

• The drift current is in the opposite direction of the diffusion current.

• At a particular stage, the drift current becomes equal to the diffusion current.

• This stage is set to be in an equilibrium state when no current flows across the p-n junction.

• The Potential barrier is maximum and is equal to V_B.

• Thus, a p-n junction is formed and under equilibrium, there is no net current.

• The electrons or holes move a large distance before the collision with another electron or hole when doping concentration is small.

• As a result, the width of the p-n junction becomes large.

• On the contrary, the electric field becomes small as the width of the p-n junction increases.

Forward Biasing

• When we connect the positive terminal of the external battery to the p-side and the negative terminal of the external battery to the n-side, the p-n junction is said to be forward-biased.

• The direction of the applied voltage V is in the opposite direction of the potential barrier setup at the junction.

• The depletion layer width decreases and the barrier height is reduced. The effective barrier height beneath forward bias is V_B-V.

Reverse Biasing

• When we connect the positive terminal of the external battery to the n-side and the negative terminal of the external battery to the p-side, then the p-n junction is said to be reverse biased.

• The applied voltage is in the same direction of the barrier potential.

• The barrier height increases and the depletion region widens due to change in electric field.

• The effective barrier height is V_B+V

Application of Junction Diode as a Rectifier

The rectifier is a device used for converting alternating current or voltage into direct current or voltage.

• Half-wave rectifier: The figure below represents the half-wave circuit:

Input and output waveform of half-wave rectifier is as follows:

• Full-wave rectifier: The figure below represents the half-wave circuit-

• A p-n junction diode is used both as a half-wave and full-wave rectifier.

• The resistance of a p-n junction diode is low when forward biased and is high when reverse biased.

Semiconductor Devices Previous Year Question and Answer

Q.1 Draw the energy band diagrams for p-type and n-type semiconductors. Depict the donor/acceptor energy levels in these diagrams and write their significance.

Answer:

The energy band diagram of n-type, Semiconductor at temperature (left figure)
The energy band diagram of p-type, Semiconductor at temperature (right figure)

Q.2 Explain the two processes which occur during the formation of a p-n junction diode. Hence, define the terms (i) depletion region and (ii) potential barrier.

Answer:

The two processes are

1. Diffusion
2. Drift

Due to the concentration gradient of electrons, diffusion takes from n to p and holes diffuse from p to n.

Due to the diffusion, an electric field is developed across the junction. Due to this electric field electron moves from p to n side and holes from n to p side. The current due to this flow of charge is known as drift current.

After some time the diffusion current equals the drift current and further flow of charge is stopped.

(i) Depletion Region -
It is the space charge region on either side of the junction, which got depleted of free charges,

(ii) Potential barrier-
The potential difference developed across the junction and opposes the diffusion of charge and brings equilibrium condition is known as the potential barrier.

Q.3 In Fig. Vo is the potential barrier across a p-n junction, when no battery is connected across the junction.

(1) 1 and 3 both correspond to forward bias of junction.
(2) 3 corresponds to forward bias of junction and 1 corresponds to reverse bias of junction.
(3) 1 corresponds to forward bias and 3 corresponds to reverse bias of junction.
(4) 3 and 1 both correspond to reverse bias of junction.

Answer:

The height of a potential barrier increases when the p-n junction is biased forward, and it opposes the potential barrier junction. But if p-n is reversed biased, the potential barrier junction is supported, which increases the potential barrier.

Hence the answer is the option (2).

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