Conductors

• Conductors allow current to flow easily.
• They have a low resistance to the flow of electricity.
• Examples of conductors include metals like copper, silver, and gold.
• In conductors, the valence band and the conduction band overlap, allowing free electron movement.
• Due to the availability of free electrons, conductors are good at conducting both heat and electricity.

Semiconductors

• Semiconductors are materials with properties between those of conductors and insulators.
• They have a moderate resistance to the flow of electricity.
• Silicon and germanium are commonly used as semiconductors.
• The band gap in semiconductors is small, but not negligible.
• When sufficient energy is applied, some electrons can move from the valence band to the conduction band.

Insulators

• Insulators do not allow current to flow easily.
• They have a high resistance to the flow of electricity.
• Examples of insulators include rubber, glass, and wood.
• The valence band in insulators is completely filled, and there is a large energy gap to the conduction band.
• Insulators are poor conductors of both heat and electricity.

Conductivity and Resistivity

• Conductivity is a measure of how easily a material can conduct electricity.
• It is the reciprocal of resistivity.
• Resistivity is a property that describes how strongly a material opposes the flow of electric current.
• Metals have high conductivity and low resistivity.
• Insulators have low conductivity and high resistivity.
• Semiconductors have moderate conductivity and resistivity.

Band Gap Energy

• The band gap energy is the energy difference between the valence band and the conduction band.
• It determines whether a material behaves as a conductor, a semiconductor, or an insulator.
• In conductors, the valence band and conduction band overlap, resulting in a small or negligible band gap energy.
• In semiconductors, the band gap energy is moderate, allowing some electrons to transition from the valence band to the conduction band.
• In insulators, the band gap energy is large, preventing electron movement.

Intrinsic and Extrinsic Semiconductors

• Intrinsic semiconductors are pure semiconducting materials.
• They are not intentionally doped with impurities.
• Examples of intrinsic semiconductors include silicon and germanium.
• Extrinsic semiconductors are semiconductors intentionally doped with impurities.
• Doping can be either n-type (with excess electrons) or p-type (with excess holes).

Charge Carriers in Semiconductors

• Intrinsic semiconductors have an equal number of electrons and holes, providing balance.
• When impurities are added, new charge carriers are introduced.
• In n-type semiconductors, the majority charge carriers are electrons.
• In p-type semiconductors, the majority charge carriers are holes.
• The presence of excess charge carriers facilitates electrical conduction in semiconductors.

Conductivity in Conductors, Semiconductors, and Insulators

• Conductors have high electrical conductivity due to freely moving electrons.
• Semiconductors have moderate electrical conductivity, which can be manipulated through doping.
• Insulators have low electrical conductivity due to complete electron valence bands and large band gaps.
• The conductivity of a material depends on its resistivity, which in turn depends on the availability of free charge carriers.

Summary

• Conductors are materials with low resistance and high conductivity.
• Semiconductors have moderate resistance and conductivity, with properties between conductors and insulators.
• Insulators have high resistance and low conductivity.
• The classification of materials into conductors, semiconductors, and insulators is determined by the band gap energy and the availability of charge carriers.

11. Specific Conductance and Resistivity

• Specific conductance is a measure of a material’s ability to conduct electricity and is given by the equation: Specific Conductance = Conductivity / Area

• It is denoted by the symbol σ (sigma) and has the unit siemens per meter (S/m).

• Resistivity (ρ) is the reciprocal of specific conductance and is defined as: Resistivity = 1 / Specific Conductance

• It is denoted by the symbol ρ (rho) and has the unit ohm-meter (Ω•m).

• Resistivity is a characteristic property of a material and depends on temperature and impurities present.

12. Temperature Dependence of Resistivity

• The resistivity of conductors generally increases with increasing temperature.

• This is because as temperature rises, the atoms in the conductor vibrate more vigorously, increasing their collision frequency and hindering electron flow.

• The increase in resistivity with temperature can be described by the equation: ρ = ρ0 (1 + α(T-T0))

  where ρ0 is the resistivity at a reference temperature T0, α is the temperature coefficient of resistivity, and T is the temperature in Kelvin.

13. Band Gap and Temperature

• In semiconductors, the band gap decreases with increasing temperature.

• As temperature rises, the vibration of atoms allows some electrons from the valence band to gain enough energy and move to the conduction band.

• This leads to an increase in electrical conductivity of the semiconductor with temperature.

• The relationship between band gap and temperature in semiconductors is described by the equation: E_g = E_{g0} - αT,

  where E_g is the band gap at a given temperature, E_{g0} is the band gap at 0 Kelvin, α is the temperature coefficient of the band gap, and T is the temperature in Kelvin.

14. Doping in Semiconductors

• Doping is the process of intentionally adding impurities to a semiconductor to alter its electrical properties.
• n-type doping involves adding impurities that introduce excess electrons, such as phosphorus or arsenic.
• p-type doping involves adding impurities that introduce excess holes, such as boron or gallium.
• Doping allows control over the conductivity and other electronic properties of the semiconductor.

15. pn Junctions

• A pn junction is formed when a p-type semiconductor is joined with an n-type semiconductor, forming a depletion region between them.
• The depletion region is a region with no free charges, as the electrons from the n-type side and the holes from the p-type side recombine, creating a barrier.
• When a forward bias voltage is applied, the depletion region becomes narrower, allowing current to flow.
• When a reverse bias voltage is applied, the depletion region becomes wider, preventing current flow.

16. Diodes

• A diode is a two-terminal electronic component that allows current to flow in only one direction.
• It is formed by a pn junction, which exhibits the characteristic behavior of allowing current in the forward bias and blocking current in the reverse bias.
• Diodes are commonly used in rectifiers, voltage clippers, and voltage limiters in electronic circuits.

17. Transistors

• Transistors are three-terminal electronic devices that amplify or switch electronic signals and power.
• They are made of semiconductors, usually silicon or germanium, and have three layers: emitter, base, and collector.
• NPN and PNP transistors are the two main types, characterized by the arrangement of the three layers.
• Transistors are the building blocks of modern electronic systems and are essential components in amplifiers, digital logic circuits, and switching applications.

18. Integrated Circuits

• Integrated circuits (ICs) are miniaturized electronic circuits consisting of multiple interconnected transistors, resistors, capacitors, and other electronic components.
• They are fabricated on a small semiconductor chip, typically made of silicon.
• ICs revolutionized the electronics industry by enabling the development of compact and high-performance electronic devices, such as computers, smartphones, and medical devices.

19. Superconductivity

• Superconductivity is a phenomenon exhibited by certain materials where they have zero electrical resistance below a critical temperature.
• Superconductors also display the Meissner effect, where they expel magnetic fields from their interior when cooled below the critical temperature.
• The critical temperature varies depending on the material, with some superconductors requiring extremely low temperatures.
• Superconductors have applications in a wide range of fields, including power transmission, medical imaging, and particle accelerators.

20. Applications of Conductors, Semiconductors, and Insulators

• Conductors are used in electrical wiring, transformers, and circuits due to their high conductivity and low resistance.
• Semiconductors are essential in electronic devices like computers, smartphones, and sensors.
• Insulators are used as electrical insulation in power cables, circuit boards, and safety equipment.
• The properties of conductors, semiconductors, and insulators allow for a wide range of technologies that have revolutionized our modern world.

21. Energy Bands in Solids

• In a solid, the energy levels of electrons are grouped into energy bands.
• The valence band is the highest energy band occupied by electrons in their ground state.
• The conduction band is the next higher energy band that electrons can move into when excited.
• The energy gap between the valence band and the conduction band determines the conductivity of the material.

22. Valence and Conduction Bands

• The valence band is filled with electrons in their lowest energy states.
• The conduction band is empty at absolute zero temperature.
• Electrons can move from the valence band to the conduction band by gaining energy, such as through thermal excitation or the absorption of photons.

23. Band Structure and Conductivity

• Conductors have overlapping valence and conduction bands, allowing electrons to move freely, contributing to high conductivity.
• Semiconductors have a small band gap between the valence and conduction bands, allowing for some electron movement at room temperature.
• Insulators have a large band gap, making it difficult for electrons to move between the valence and conduction bands.

24. Semiconductor Diodes

• Semiconductor diodes are commonly used in electronic circuits as rectifiers.
• They have a pn junction that allows current to flow in one direction.
• Under forward bias, the diode conducts current.
• Under reverse bias, the diode blocks current.

25. Light-Emitting Diodes (LEDs)

• LEDs are a type of semiconductor diode that emit light when current flows through them.
• Electrons and holes recombine in the semiconductor, releasing energy in the form of photons.
• LEDs are used in numerous applications, including lighting, displays, and indicators.

26. Field-Effect Transistors (FETs)

• FETs are three-terminal devices used for amplification and switching in electronic circuits.
• They are voltage-controlled devices that operate by controlling the flow of current between the source and drain terminals using an electric field.
• FETs have high input impedance and can be used in many applications, including amplifiers, analog switches, and oscillators.

27. Bipolar Junction Transistors (BJTs)

• BJTs are three-terminal devices used for amplification and switching in electronic circuits.
• They have two pn junctions and operate by controlling the flow of current between the collector and emitter terminals using the base current.
• BJTs have a low input impedance and high voltage gain, making them suitable for applications such as amplifiers, oscillators, and digital logic circuits.

28. Quantum Mechanics and Semiconductors

• The behavior of electrons in semiconductors is described by quantum mechanics.
• Quantum mechanics considers the wave-like nature of particles and the uncertainty principle.
• The energy levels and electronic properties of semiconductors are determined by the quantum mechanical properties of the electrons.

29. Density of States

• The density of states (DOS) describes the number of available energy states in a material.
• In a band structure diagram, the DOS represents the number of energy levels per unit volume.
• The DOS distribution determines the probability of finding an electron at a particular energy level.

30. Electron and Hole Transport

• In semiconductors, both electrons and holes can contribute to electrical conductivity.
• Electron transport occurs when electrons move from the valence band to the conduction band.
• Hole transport occurs when holes, which are vacancies in the valence band, move in the opposite direction.
• The mobility of electrons and holes determines the conductivity of a semiconductor material.