Comprehensive Notes on Semiconductor Physics and Semiconductors and Semiconductor and and Band Theory Physics
Introduction to Electronics and Atomic Structure
The Atom: An atom consists of a nucleus surrounded by electrons. These electrons occupy well-defined energy levels referred to as shells.
Definition of Electronics: Electronics is the branch of physics focused on the emission, effects, and precise control of electron flow within conductors, semi-conductors, and insulators.
Energy Bands in Solids
Formation of Energy Bands: In solid materials, atoms are packed closely together. This proximity causes many energy levels to overlap, forming Energy Bands. Every band contains a large quantity of closely packed energy levels.
Key Band Components:
Valence Band (VB): This is the lowest energy band, formed by the vibrations of outer electrons when atoms group together. Electrons in the valence band are firmly attached to individual atoms and represent the highest occupied energy band. These electrons have almost zero mobility and can be either partially or completely filled.
Conduction Band (CB): This is the highest energy band and it consists of free electrons responsible for electrical conduction. These electrons have the greatest mobility, are least tightly bound to the nucleus, and have higher energies compared to valence electrons.
Forbidden Gap (Band Gap): This is the region between the valence band and the conduction band. No energy or electrons exist within this band gap. It occurs in solids where the valence and conduction bands do not overlap.
Relationship and Mobility: The size of the forbidden gap and the quantity of electrons in the conduction band determines if a material is a conductor, insulator, or semi-conductor.
Comparison of Valence Band (VB) vs. Conduction Band (CB)
Position relative to Fermi-level: The valence band exists below the Fermi-level, whereas the conduction band exists above it.
Effect of External Excitation: Electrons move out of the valence band and reach the conduction band when excited.
Energy State: The valence band is at a lower energy state; the conduction band is at a comparatively higher state.
Electron Density: High in the valence band and low in the conduction band.
Nuclear Force: The force exerted by the nucleus is strong on valence band electrons and weak on conduction band electrons.
Occupancy: The valence band is partially or completely filled. The conduction band is empty or partially filled.
Classification of Solids
Solids are classified into three types based on the presence and arrangement of energy bands:
Conductors
High electrical conductivity.
Examples: Iron, silver, copper, gold, aluminum.
Band Structure: The conduction band and valence band overlap, meaning there is no forbidden gap. Both bands are completely filled with electrons.
Temperature Effects: Electrical conductivity is high at normal temperatures but decreases as temperature increases. Consequently, electrical resistivity increases with temperature (Positive Coefficient of Resistance).
Insulators
Materials that do not allow electric current to flow.
Examples: Glass, rubber, wood, plastic.
Band Structure: Completely filled valence band and a completely empty conduction band, separated by a wide forbidden gap.
Conductivity: Negligible at moderate temperatures as the wide gap prevents conduction. A perfect insulator has zero conductance and infinite resistance.
Temperature Effects: Electrical resistivity decreases and conductivity increases with increasing temperature. They have a Negative Coefficient of Resistance.
Semiconductors
Electrical conductivity lies between that of a conductor and an insulator.
Examples: Silicon, germanium, indium compounds.
Band Structure: Completely filled valence band but a partially filled conduction band with a narrow (small) forbidden gap.
Temperature Effects: Electrical conductivity increases with increasing temperature (Negative Coefficient of Resistance).
Purity Effects: Conductivity increases when foreign atoms (impurities) are added.
Detailed Semiconductor Characteristics and Conduction
Factors Affecting Conductivity:
Temperature: Increasing temperature breaks covalent bonds, freeing electrons and creating holes, which enhances conductivity.
Impurities: Adding specific impurities increases the number of free electrons or holes.
Charge Carriers:
Mobile particles carrying electric charge.
In solid conductors: Free electrons.
In fluids: Ions.
In semi-conductors: Both electrons and holes.
Density Correlation: A higher number of charge carriers results in better conductivity. A material with no charge carriers acts as an insulator.
Conduction Mechanism:
At : The conduction band is empty, and the valence band is full; the material behaves as an insulator.
At high temperatures: Valence electrons gain energy, break free, and cross the forbidden gap into the conduction band.
Holes: When electrons move to the conduction band, they leave behind "holes" in the valence band. Holes are considered positive charge carriers.
Current Flow: Applying an electric field causes electrons to move into the conduction band (electric current) and holes to move into the valence band (hole current).
Doping and Types of Semiconductors
Doping: The process of adding impurities to a semiconductor to improve conductivity. It reduces the effective energy gap by creating donor or acceptor levels.
Intrinsic Semi-conductors: Pure state materials with an equal number of free electrons and holes (e.g., Germanium, Silicon, gallium arsenide).
Extrinsic Semi-conductors: Impure materials with an unequal number of electrons and holes.
N-Type Semiconductor:
Doped with a pentavalent impurity (e.g., phosphorus, arsenic) from Group V.
These atoms have five valence electrons. Four form covalent bonds with the semiconductor; the fifth is an extra free electron.
Majority charge carriers: Electrons (negative).
Minority charge carriers: Holes.
A Donor level is created just below the conduction band.
P-Type Semiconductor:
Doped with a trivalent impurity (e.g., Boron, Aluminium) from Group III.
These atoms have three valence electrons. They form bonds with tetravalent semiconductor atoms (like Silicon), leaving an extra hole.
Hole creation: The trivalent atom attracts an electron from a nearby silicon atom to complete its bond, creating a mobile hole.
Majority charge carriers: Holes (positive).
Minority charge carriers: Electrons.
An Acceptor level is created just above the valence band.
The P-N Junction and Diode
P-N Junction: The plane where a p-type and n-type semiconductor meet, created through special manufacturing.
Formation Process:
Electrons from the N-type diffuse into the P-type, leaving immobile positive ions behind.
Holes from the P-type diffuse into the N-type, leaving immobile negative ions behind.
Depletion Layer: The region at the junction where electrons and holes combine and neutralize. It is depleted of mobile charge carriers and is approximately units wide on either side of the boundary. It acts as an insulator.
Potential Barrier: The opposing potential difference that prevents further diffusion of charge carriers across the junction.
Biasing procedures for P-N Junctions
Biasing: Connecting an external e.m.f. to the junction to change the opposition to charge flow.
Forward Biasing:
Positive terminal connected to P-type; negative terminal connected to N-type.
Effect: Decreases potential barrier and shrinks/collapses the depletion layer.
Conduction: The junction becomes conductive when the e.m.f. exceeds the potential barrier. Current increases with battery e.m.f. The V-I graph is non-ohmic and shows low resistance.
Reverse Biasing:
Positive terminal connected to N-type; negative terminal connected to P-type.
Effect: Holes and electrons are pulled away from the junction toward the battery terminals.
Conduction: The depletion layer widens, the potential barrier increases, and majority charge carriers cannot cross. Only a tiny "leakage current" (reverse bias current) flows due to minority carriers.
Diodes and Thermionic Emission
Diode: An electrical device allowing current to flow in one direction (forward bias) but blocking it in the other (reverse bias). This behavior is called rectification.
Thermionic Emission: The emission of electrons from a metal surface when heated. Absorbed energy allows electrons to break free from the metal.
Thermionic Diode (Vacuum Tube):
Consists of a cathode and an anode in an evacuated tube.
Mechanism: Heated cathode emits electrons attracted to the positive anode. If the anode is negative, electrons are repelled, and no current flows.
Uses: Rectification circuits, TVs (Cathode ray tubes), and old radio transmitters/receivers.
Comparison: P-N Junction Diode vs. Thermionic Diode:
Advantages of P-N Junction: Smaller size, no warm-up time, low operating voltage, less heat waste, cheap to manufacture, long life, and reliable.
Disadvantages of P-N Junction: Slight conduction in reverse bias and sensitivity to temperature.
Types of P-N Junction Diodes
Rectifier diode
Signal diodes
Light emitting diode (LED)
Zener diode
Varactor diode
Photodiodes
Specialized Diodes
Light Emitting Diode (LED):
Mechanism: When forward biased, electrons move from N-type to P-type and combine with holes, releasing energy as photons (electroluminescence).
Color: Determined by the energy gap of the semiconductor.
Advantages: Low energy consumption, long lifetime, robust, small size, fast switching.
Uses: Home/industrial bulbs, vehicle lights, mobile phone displays, traffic signals, and status indicators.
Zener Diode:
Designed to allow current to flow backward when a specific "Zener voltage" is reached.
Zener Effect: A sudden breakdown of the depletion layer at a specific reverse bias voltage, leading to a high current surge.
Uses: Voltage regulators for small circuits, signal clipping, and reference voltage generation for amplifiers.
Varactor Diode:
A voltage-dependent semiconductor where internal capacitance varies with reverse voltage.
Relationship: Lower bias voltage produces a narrower depletion zone and higher capacitance.
Uses: Voltage controlled capacitors, parametric amplifiers, and oscillators.
Rectification and Smoothing
Rectification: Converting alternating current (AC) to direct current (DC).
Half-wave Rectification:
Uses one diode.
Allows only one half-cycle of AC to pass, blocking the other.
Output: Intermittently pulsating DC.
Full-wave Rectification:
Converts both half-cycles of AC to DC.
Methods: Using two diodes with a center-tapped transformer or four diodes in a bridge circuit.
Smoothening (Filtering):
Rectified DC often contains a "hum" or pulsations.
A capacitor is connected in parallel with the load () to provide a smoothing effect.
Action: The capacitor charges as current builds and discharges through the load as current decreases, maintaining a constant high output.
Bipolar Junction Transistors (BJT)
Definition: A three-terminal device (Base, Emitter, Collector) used for current/voltage amplification or generation.
Types: NPN and PNP.
Operation:
One junction must be forward biased; the other must be reverse biased.
Emitter: The terminal for the forward-biased junction that provides majority charge carriers.
Collector: The terminal for the reverse-biased junction.
Base: Provides minority charge carriers.
Mnemonic for Symbols:
NPN: Never Points Nin (In).
PNP: Points Nin (In) Permanently.
Uses: Rectifiers, switches, oscillators, and current/voltage amplifiers.
Voltage Multipliers
Voltage Multiplier: A circuit using diodes and capacitors to produce a DC voltage several times larger than the AC input voltage.
Voltage Doubler: A specific multiplier circuit with a multiplication factor of two.
Questions & Discussion
Comparison Chart Completion: The comparison of conductors, semiconductors, and insulators includes:
Resistivity: Conductor (Low), Semiconductor (High), Insulator (Very High).
Conductivity: Conductor (Very High), Semiconductor (High), Insulator (Low).
Forbidden Gap: Conductor (None/Nil), Semiconductor (Narrow), Insulator (Wide).
Temperature on Conductivity: Conductor (Decreases), Semiconductor (Increases), Insulator (Increases).
Temperature on Resistivity: Conductor (Increases), Semiconductor (Decreases), Insulator (Decreases).
Charge Carriers: Conductor (Free electrons), Semiconductor (Free electrons and Holes), Insulator (None/Nil).