Welcome to Rajasthan Polytechnic Physics (2002) Notes for 2nd Semester Students!
Hello, Polytechnic students! 🎓
Struggling with Physics for your 2nd semester? Don’t worry, I’ve got you covered! Here are detailed handwritten notes on essential Physics topics for your 2nd-semester exams. These notes are simplified and cover key concepts with examples to make studying easier and effective.
Download Your Notes
Additionally, you can join our WhatsApp and Telegram groups for easy access to PDF and Handwritten Notes as well as updates on new materials.
We hope this blog helps you in your academic journey.
📢 🔔 Important:
👉 Full PDF available in our WhatsApp Group | Telegram Channel
👉 Watch the full lecture on YouTube: BTER Polytechnic Classes
- Access Chapter-wise Video Tutorials:
5. SEMICONDUCTOR AND MODERN PHYSICS
This section covers important concepts like energy bands in solids, semiconductors, and the workings of diodes and lasers. It also touches on cutting-edge fields like nanotechnology.
5.1 Energy Bands in Solids
Energy Bands:
In solids, the energy levels of electrons form continuous ranges or bands, instead of discrete energy levels (as seen in atoms). These bands are formed due to the overlap of atomic orbitals of adjacent atoms.
Conduction Band (CB): The higher energy band where electrons are free to move and contribute to electrical conductivity.
Valence Band (VB): The band where electrons are bound to atoms and are not free to move.
Band Gap: The energy difference between the conduction band and the valence band. The size of this gap determines the electrical properties of the material:
- Insulators: Large band gap (e.g., > 3 eV).
- Semiconductors: Moderate band gap (e.g., ~1.1 eV for silicon).
- Conductors: Overlapping conduction and valence bands (no band gap).
5.2 Types of Materials
1. Insulators:
- Examples: Rubber, Glass, Wood, Plastic
- Band Gap: Large (typically > 3 eV).
- Explanation: The electrons in insulators are tightly bound to atoms, and there is a large energy gap between the valence band and the conduction band. This prevents electrons from moving freely, so no electrical current flows through them.
2. Semiconductors:
- Examples: Silicon, Germanium
- Band Gap: Small (around 1 eV).
- Explanation: In semiconductors, the band gap is small enough that electrons can be excited from the valence band to the conduction band if some energy is provided (such as heat or light). This makes semiconductors useful in electronic devices.
3. Conductors:
- Examples: Copper, Aluminum, Gold
- Band Gap: None (overlapping conduction and valence bands).
- Explanation: In conductors, the conduction band overlaps the valence band, which means electrons can move freely and easily conduct electricity. These materials have a high number of free electrons even at room temperature.
5.3 p-n Junction
A p-n junction is formed when p-type (positive) and n-type (negative) semiconductors are brought together.
- p-type Semiconductor: This type is "positive" because it has an excess of holes (missing electrons). It’s created by doping a semiconductor (like silicon) with an element that has fewer valence electrons than silicon (e.g., boron).
- n-type Semiconductor: This type is "negative" because it has an excess of free electrons. It’s created by doping silicon with an element that has more valence electrons than silicon (e.g., phosphorus).
At the p-n junction, electrons from the n-type region diffuse into the p-type region and recombine with holes, forming a depletion region. This region is electrically neutral because the electrons have combined with holes, leaving behind positively charged ions in the p-type region and negatively charged ions in the n-type region. This creates an electric field that prevents further movement of charge carriers.
5.3.1 Junction Diode and V-I Characteristics
A junction diode is made from a p-n junction and allows current to flow in one direction only. The V-I characteristics describe the relationship between voltage (V) and current (I) through the diode.
- Forward Bias: When the p-type is connected to the positive terminal and the n-type to the negative terminal, current flows through the diode. The voltage must exceed a threshold (typically 0.7V for silicon) for current to flow.
In forward bias, the p-type is connected to the positive terminal of the battery and the n-type to the negative terminal. This reduces the width of the depletion region, and once the forward voltage exceeds a threshold (around 0.7 V for silicon diodes), the diode allows current to flow easily.
V-I Curve: For small applied voltages, there is little to no current. Once the voltage exceeds the threshold, the current increases sharply.
- Reverse Bias: When the p-type is connected to the negative terminal and the n-type to the positive terminal, very little current flows (except for a small leakage current).
In reverse bias, the p-type is connected to the negative terminal and the n-type to the positive terminal. This increases the width of the depletion region, and the diode does not conduct current under normal conditions. Only a very small reverse leakage current exists, which is usually negligible.
V-I Curve: In reverse bias, current remains almost zero until a certain breakdown voltage is reached, after which the diode may conduct (in a Zener diode, this is a controlled breakdown).
5.3.2 Types of Junction Diodes
Zener Diode:
A Zener diode is a special type of diode designed to allow current to flow in the forward direction (like a regular diode), but more importantly, it allows current to flow in the reverse direction when the voltage exceeds a certain value known as the Zener breakdown voltage or Zener voltage.
Key Characteristics of Zener Diodes:
Reverse Breakdown: Unlike regular diodes, which break down and conduct heavily when the reverse voltage is high (and can be damaged), the Zener diode is designed to operate in the reverse breakdown region without getting damaged.
Zener Voltage: The voltage at which the Zener diode starts conducting in reverse is called the Zener voltage (denoted as V_Z). The diode remains in reverse conduction as long as the voltage is greater than the Zener voltage.
Forward Bias: Like a regular diode, a Zener diode allows current to flow in the forward direction when a voltage is applied above the threshold voltage (typically 0.7V for silicon Zener diodes).
Reverse Bias: The Zener diode does not conduct under reverse bias until the reverse voltage reaches the Zener voltage. After this point, it starts to conduct and maintains a stable reverse voltage at V_Z even if the current increases.
Zener Diode Working Principle:
Forward Bias: When a positive voltage is applied to the p-type material (the anode), and a negative voltage is applied to the n-type material (the cathode), the Zener diode behaves just like a regular diode. The current flows when the voltage exceeds the threshold (around 0.7V for silicon).
Reverse Bias: In reverse bias, the Zener diode behaves differently. Initially, there is no current. However, as the reverse voltage increases and reaches the Zener voltage (V_Z), the diode begins to conduct in the reverse direction without being damaged. This is due to a phenomenon called Zener breakdown.
The diode keeps the voltage across it at V_Z regardless of further increases in the reverse voltage, making it an excellent voltage regulator.
Photo Diode:
A photo diode generates a current when exposed to light. It works by converting light energy into electrical energy and is used in light sensors, optical communication, etc.
5.3.3 Diode as Rectifier – Half-Wave and Full-Wave Rectifier (Center Tapped)
A rectifier is a device that converts alternating current (AC) into direct current (DC).
Half-Wave Rectifier:
In a half-wave rectifier, the diode only allows current to pass through during one half of the AC cycle (positive or negative). The output is a pulsating DC current.Full-Wave Rectifier:
In a full-wave rectifier, both halves of the AC cycle are used. A center-tapped transformer is typically used to ensure that both halves are rectified. This results in smoother DC output than a half-wave rectifier.
5.4 Lasers
A laser is a device that emits light through the process of stimulated emission of radiation. Lasers are devices that produce coherent light through stimulated emission. The term LASER stands for Light Amplification by Stimulated Emission of Radiation. There are different types of lasers based on their active medium. Two common types are He-Ne (Helium-Neon) lasers and Ruby lasers. Below is a detailed explanation of both.
5.4.1 Energy Levels, Ionization and Excitation Potentials
In a laser:
- Energy Levels: Electrons in atoms or molecules can exist at discrete energy levels. Energy is absorbed or emitted when electrons transition between these levels.
- Ionization Potential: The energy required to remove an electron completely from an atom.
- Excitation Potential: The energy required to move an electron to a higher energy level (excited state).
5.4.2 Spontaneous and Stimulated Emission
- Spontaneous Emission: Occurs when an excited electron spontaneously drops to a lower energy level, emitting a photon (light) randomly in all directions.
- Stimulated Emission: Occurs when an incoming photon stimulates an excited electron to drop to a lower energy level, emitting a photon in the same direction, with the same phase, and frequency. This is the principle behind lasers.
5.4.3 Population Inversion
For stimulated emission to dominate over spontaneous emission, there must be more electrons in the excited state than in the ground state. This is called population inversion. In normal conditions, most electrons are in the ground state, but in lasers, energy is pumped into the system to achieve population inversion.
5.4.4 Pumping Methods
Pumping is the process of providing energy to the lasing medium to achieve population inversion. There are two main types of pumping:
- Optical Pumping: Using light (e.g., flash lamps or other lasers) to excite electrons.
- Electrical Pumping: Using electrical energy to excite the electrons.
5.4.5 Types of Lasers
Ruby Laser:
The ruby laser uses a ruby crystal (aluminum oxide with chromium impurities) as the lasing medium. The laser emits red light with a wavelength of 694 nm.The Ruby laser is a solid-state laser that uses a ruby crystal (a synthetic form of corundum or aluminum oxide, doped with chromium ions (Cr³⁺)) as its active medium. It produces red light with a wavelength of 694.3 nm and is considered the first successful solid-state laser ever developed.
Working Principle of Ruby Laser:
Optical Pumping:
The ruby crystal is optically pumped by an external light source, often a flashlamp. The flashlamp provides high-intensity light that excites the chromium ions (Cr³⁺) in the ruby crystal to a higher energy state.Excitation of Chromium Ions:
The chromium ions (Cr³⁺) absorb energy from the flashlamp and are excited to higher energy levels. This creates a population inversion, where more chromium ions are in the excited state than in the ground state.Spontaneous Emission:
After excitation, the chromium ions may spontaneously return to a lower energy state, emitting photons in the process. These photons stimulate other excited chromium ions to emit more photons of the same wavelength and phase, generating a stimulated emission.Optical Cavity:
The ruby crystal is placed inside a cylindrical cavity that has two mirrors at both ends. One of the mirrors is partially reflecting (so that the light can exit), and the other is highly reflective.Laser Output:
The light undergoes multiple reflections within the cavity, and once enough photons are generated, a coherent red laser beam at a wavelength of 694.3 nm is emitted from the partially reflecting mirror.
Key Characteristics of Ruby Laser:
- Wavelength: 694.3 nm (red light)
- Output Power: It can produce high peak power (on the order of 1000s of watts) for very short durations.
- Pulsed Operation: Ruby lasers are typically used in pulsed mode, meaning they emit light in brief bursts.
- Coherent Light: The laser output is coherent and monochromatic.
- High Efficiency: Ruby lasers can produce high-intensity light output due to the use of a flashlamp.
Applications of Ruby Lasers:
- Laser Surgery: Ruby lasers are used in medical surgeries, especially for eye surgeries (e.g., retinal surgery).
- LIDAR (Light Detection and Ranging): Used for distance measurement and topographical mapping.
- Material Processing: Ruby lasers are used in cutting, engraving, and surface treatment of materials.
- Laser Spectroscopy: Used in various scientific experiments and research.
- Art and Decoration: Used in laser light shows due to their powerful and visible red beam.
Comparison Between He-Ne Laser and Ruby Laser
Feature He-Ne Laser Ruby Laser Active Medium Helium-Neon gas mixture Ruby crystal (Al₂O₃ doped with Cr³⁺) Wavelength 632.8 nm (Red light) 694.3 nm (Red light) Type of Laser Gas laser Solid-state laser Operation Continuous wave (CW) Pulsed operation (usually flashlamp pumped) Power Output Low (1 mW to 10 mW) High peak power (1000s of watts in pulses) Beam Quality Excellent beam quality, collimated Good beam quality but may be slightly divergent Applications Holography, bar code scanners, medical imaging Laser surgery, LIDAR, material processing, scientific research Lifespan Long (tens of thousands of hours) Moderate lifespan, limited by flashlamp and ruby crystal Cost Relatively inexpensive Expensive due to the ruby crystal and flashlamp He-Ne Laser:
The Helium-Neon (He-Ne) laser uses a mixture of helium and neon gases as the lasing medium. It is commonly used for laser pointers and has a wavelength of 632.8 nm (red light).The He-Ne laser is one of the most widely used gas lasers. It consists of a mixture of helium (He) and neon (Ne) gases as its active medium. It produces red light of wavelength 632.8 nm (in the visible region) and is commonly used in laboratories, holography, and bar-code scanners.
Working Principle of He-Ne Laser:
Gas Discharge:
The He-Ne laser operates using a gas discharge tube containing a mixture of helium (He) and neon (Ne) gases. When an electrical current is passed through the tube, it excites the helium atoms first, which then transfer their energy to the neon atoms.Excitation of Neon Atoms:
The helium atoms in the gas mixture collide with neon atoms and excite them to a higher energy state. This process is known as collisional energy transfer.Stimulated Emission:
Neon atoms, after being excited to a higher energy level, can undergo spontaneous emission and release photons of specific wavelengths. These photons stimulate other excited neon atoms to emit more photons of the same energy, creating stimulated emission.Optical Cavity:
The neon atoms’ emissions are reflected back and forth between mirrors at both ends of the laser tube. One of these mirrors is partially transparent to allow the laser beam to exit the tube.Laser Output:
The emitted light is coherent, monochromatic, and collimated (parallel). The typical output is red light at a wavelength of 632.8 nm.
Key Characteristics of He-Ne Laser:
- Wavelength: 632.8 nm (red light)
- Output Power: Usually in the range of 1 mW to 10 mW
- Coherent Light: Produces coherent, monochromatic light.
- Long Lifespan: He-Ne lasers are known for their long operational life (tens of thousands of hours).
- Continuous Wave (CW) Operation: It typically operates in continuous wave mode, meaning it emits a steady, unbroken beam of light.
Applications of He-Ne Lasers:
- Holography: Used for creating and reconstructing holograms.
- Laser Scanners: Commonly used in bar code readers.
- Laser Pointers: Widely used in presentations as laser pointers.
- Optical Communication: Used in some fiber optic communication systems.
- Medical Applications: Used in optical coherence tomography (OCT) for medical imaging.
5.4.6 Laser Characteristics
- Monochromatic: Lasers emit light of a single wavelength.
- Coherent: The light waves emitted by a laser are in phase with each other.
- Directional: Laser light is emitted in a narrow beam, allowing it to travel long distances without significant divergence.
- High Intensity: The light emitted from a laser is highly concentrated.
5.4.7 Engineering and Medical Applications of Lasers
- Engineering:
- Laser Cutting: Used for precise cutting of materials like metals and plastics.
- Laser Welding: For joining materials in industries.
- Laser Printing: Used in laser printers for high-quality prints.
- Medical:
- Laser Surgery: Used for precise removal of tissue with minimal damage to surrounding areas.
- Laser Vision Correction: LASIK surgery to correct vision.
- Cancer Treatment: Lasers can be used to treat tumors by precisely targeting and destroying cancer cells.
5.5 Nanoscience and Nanotechnology
Nanoscience: The study of matter at the nanoscale, which is typically between 1 to 100 nanometers. At this scale, materials often have unique properties that differ from bulk materials.
Nanotechnology: The engineering and manipulation of materials at the nanoscale to create new devices, systems, and applications. This field has immense potential in areas like medicine, electronics, and energy.
- Applications:
- Drug delivery systems in medicine.
- Development of highly efficient solar cells.
- Miniaturization of electronics for faster and smaller devices.
Summary of Key Concepts
- Energy Bands: Conduction and valence bands determine whether a material is a conductor, semiconductor, or insulator.
- Diodes: p-n junctions, Zener diodes, and photo diodes have various applications in electronics.
- Lasers: Based on population inversion and stimulated emission, lasers have diverse applications in fields like medicine and industry.
- Nanotechnology: Deals with the manipulation of materials at the atomic or molecular scale to create new materials with novel properties.
0 Comments