CBSE Class 12 Physics Notes Chapter 14 Semiconductor Electronics - Materials, Devices and Simple Circuits

Semiconductor electronics are the building blocks of integrated circuits, which are the tiny chips that power everything from computers and smartphones to cars and appliances. 

• Semiconductors are materials that conduct electricity better than insulators but not as well as conductors. 
• This allows them to be used to create electronic devices such as transistors and diodes. 
• Transistors are the basic building blocks of integrated circuits. 
• They act as electronic switches, turning current on and off. 
• Diodes are a type of semiconductor device that allows current to flow in only one direction. 
• Semiconductor devices are small, consume low power, and have high reliability.
• These devices can be combined to create more complex circuits, such as logic gates and amplifiers. 

Semiconductor electronics have revolutionized the world to create smaller, faster, and more powerful electronic devices. 

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Class 12 Physics Chapter 14 Notes –  Semiconductor Electronics

Classification Of Metals, Conductors And Semiconductors

Based on electrical conductivity

• Metals: Low resistivity (high conductivity), ρ ~ 10^-2 - 10^-8 Ω m, σ ~ 10² - 10⁸ S m^-1.
• Semiconductors: Intermediate resistivity/conductivity between metals and insulators, ρ ~ 10^-5 - 10⁶ Ω m, σ ~ 10⁵ - 10^-6 S m^-1.
• Insulators: High resistivity (low conductivity), ρ ~ 10¹¹ - 10¹⁹ Ω m, σ ~ 10^-11 - 10^-19 S m^-1.

Based On Energy Bands

• Metals: The valence band overlaps with the conduction band, allowing electrons to move freely.
• Conductors: Have low energy gaps or no energy gap between the valence and conduction bands i.e. forbidden energy gap, E_g = 0 for conductors.
• Semiconductors: Possess a small energy gap between the valence and conduction bands. The forbidden energy gap, E_g ≅ 1 eV for semiconductor.
• Insulators: Have a large energy gap between the valence and conduction bands, restricting electron flow. The forbidden energy gap, E_g > 3 eV for insulators.
• Valence band: Occupied by electrons in their lowest energy states.
• Conduction band: Higher energy band where electrons can move freely in conducting materials.
• Energy gap (E_g): The energy difference between the valence and conduction bands.

Metals and conductors have overlapping or nearly touching energy bands, promoting electron mobility. Semiconductors have a small but distinct energy gap, allowing controlled electron movement. Insulators have a large energy gap, preventing electron conductivity even under applied voltage.

Forbidden Energy gap

Band Theory Of Solids

• Band theory explains the electronic structure of solids based on energy bands.
• In solids, electrons exist in continuous energy bands rather than discrete energy levels.
• Valence bands contain electrons at their lowest energy states, while conduction bands allow electron mobility.
• Energy gaps between bands determine the conductivity properties of solids: small gaps in conductors, larger gaps in semiconductors, and very large gaps in insulators.

Energy Band Diagram

Intrinsic Semiconductor

• In intrinsic semiconductors like Ge and Si, each atom shares one valence electron with each of its four nearest neighbors, forming covalent bonds.
• At low temperatures, electrons remain bound within covalent bonds, but thermal energy at higher temperatures can break some bonds, creating free electrons and holes.
• In intrinsic semiconductors, the number of free electrons (n_e) is equal to the number of holes (n_h), denoted by ni, the intrinsic carrier concentration.
• Holes, created by electrons breaking away from covalent bonds, move through the crystal lattice, effectively contributing to conductivity.
• Recombination occurs when electrons collide with holes, balancing the rate of generation and recombination of charge carriers at equilibrium.

Extrinsic Semiconductor

• Extrinsic semiconductors, also known as impurity semiconductors, are created by adding small amounts of suitable impurities to intrinsic semiconductors.
• The process of adding impurities to semiconductors is called doping, and the impurity atoms are referred to as dopants.
• Dopants must be carefully selected to ensure they do not distort the original crystal lattice of the semiconductor and occupy only a few original semiconductor atom sites.
• Two types of dopants are used: pentavalent (valency 5) dopants like Arsenic, Antimony, and Phosphorous, and trivalent (valency 3) dopants like Indium, Boron, and Aluminium.

N Type Semiconductor

• Pentavalent dopants, such as As, Sb, or P, are added to Si or Ge crystals to create n-type semiconductors.
• When a pentavalent atom occupies a position in the crystal lattice, its fifth electron is weakly bound to the parent atom, making it easily freed for conduction.
• The energy required to separate the extra electron from the dopant atom is much lower than the energy required to jump the forbidden band in intrinsic semiconductors.
• The pentavalent dopant acts as a donor impurity, contributing an extra electron for conduction.
• In n-type semiconductors, electrons become the majority carriers, while holes are the minority carriers, resulting in n_e >> n_h.

N-Type Semiconductor

P Type Semiconductor

• Trivalent impurities, such as Al, B, or In, are added to Si or Ge crystals to create p-type semiconductors.
• The trivalent dopant atom forms covalent bonds with three Si atoms but leaves a vacancy or hole in the fourth bond.
• The holes in p-type semiconductors are the majority carriers, while electrons are the minority carriers.
• In p-type semiconductors, the recombination process reduces the number of intrinsically generated electrons.
• Extrinsic semiconductors have additional energy states due to donor and acceptor impurities, affecting their energy band structure.

P-Type Semiconductor

P-N Junction

• A p-n junction is formed by adding a small quantity of pentavalent impurity to a thin p-type semiconductor wafer, creating n-type and p-type regions.
• During p-n junction formation, diffusion and drift processes occur due to concentration gradients across the junction.
• Diffusion current arises from the movement of charge carriers across the junction, while drift current results from the electric field within the junction.
• Initially, diffusion current is dominant, but as the process continues, drift current increases until equilibrium is reached.
• At equilibrium, there is no net current flow in the p-n junction.
• The region around the p-n junction that has no mobile charge carriers is known as the depletion region or depletion layer.
• The potential difference across the junction, known as the barrier potential, opposes further flow of carriers, maintaining equilibrium.

Semiconductor Diode

A semiconductor diode consists of a p-n junction with metallic contacts at its ends, forming a two-terminal device.

• Symbolically represented, the direction of the arrow in the diode symbol indicates the conventional current direction under forward bias.
• Under equilibrium (no bias), the diode maintains a barrier potential.

P-N Junction Diode Under Forward Bias

• A p-n junction is said to be forward-biased when the positive terminal of the battery is connected to the p-side, and the negative terminal to the n-side.
• The applied voltage mostly drops across the depletion region, reducing the effective barrier height.
• Minority carrier injection occurs as electrons from the n-side and holes from the p-side cross the junction.
• Due to the concentration gradient, injected carriers diffuse from the junction edge to the other end of each side, generating current.
• The forward current is the sum of hole diffusion current and conventional current due to electron diffusion, typically measured in mA.

P-N Junction Diode Under Reverse Bias

• Reverse bias occurs when the n-side of the diode is positive and the p-side is negative, causing the applied voltage to drop mostly across the depletion region.
• The effective barrier height under reverse bias is (V₀ + V), leading to a widening of the depletion region and a decrease in diffusion current.
• Drift current occurs due to the electric field sweeping carriers from the minority side to the majority side across the junction, typically in the order of a few µA.
• Reverse current is essentially voltage independent until the breakdown voltage (V_br) is reached, beyond which the current sharply increases, potentially damaging the diode.
• The circuit for studying the V-I characteristics of a diode involves varying the applied voltage and measuring the corresponding current using a milliammeter for forward bias and a micrometer for reverse bias.
• In forward bias, the diode current increases significantly after crossing a certain voltage threshold (~0.7V for silicon diode), while in reverse bias, the current remains small (~µA) and almost constant.
• The diode's dynamic resistance, defined as the ratio of a small change in the voltage (∆V) to a small change in current (ΔI), is used to characterize its behavior under changing conditions.

VI Characteristics of P-N Junction Diode

The VI characteristics of the P-N junction diode under forward-biased and reversed-biased conditions is shown below

VI characteristics of P-N Junction Diode

Application Of Junction Diode As A Rectifier

• A diode allows current flow only when forward-biased, making it useful for rectifying alternating voltages.
• A half-wave rectifier circuit passes current only during the positive half-cycle of the input AC voltage, producing an output voltage with only one direction.
• Full-wave rectifiers use two diodes to rectify both positive and negative half-cycles of the AC input, providing a more efficient rectification compared to half-wave rectifiers.
• The rectified output of both half-wave and full-wave rectifiers consists of pulses shaped like half-sinusoids.
• To obtain a steady DC output from the pulsating rectified voltage, a capacitor or inductor is connected across the output terminals to filter out the AC ripple, resulting in a pure DC voltage.
• Capacitors in filtering circuits get charged during rising voltage and discharge through the load during falling voltage, with the rate of voltage fall determined by the time constant, which depends on the capacitor's value and the effective resistance in the circuit.
• Capacitor input filters use large capacitors to achieve longer time constants, resulting in output voltages closer to the peak voltage of the rectified voltage.

Half and Full Wave Rectifier

There are Some important List Of Top Physics Questions On Semiconductor Electronics Asked In CBSE CLASS XII