Conductors, Semiconductors and Insulators

Lecture 1: An introduction

  • Introduction to conductors, semiconductors, and insulators
  • Differences in electrical conductivity between them
  • Basic properties and characteristics of each type
  • Role of electrons in determining conductivity
  • Examples of commonly encountered conductors, semiconductors, and insulators

Conductors

  • Definition: materials that allow the flow of electric current easily
  • Properties:
    • High electrical conductivity
    • Low resistivity
    • Availability of free electrons
    • Examples: metals like copper, silver, gold

Semiconductors

  • Definition: materials that have intermediate conductivity
  • Properties:
    • Moderate electrical conductivity
    • Resistivity in between conductors and insulators
    • Partially filled valence and conduction bands
    • Examples: silicon, germanium

Insulators

  • Definition: materials that do not allow the flow of electric current easily
  • Properties:
    • Low electrical conductivity
    • High resistivity
    • Almost completely filled valence band
    • Examples: plastics, ceramics, rubber

Electrical Conductivity

  • Definition: the measure of a material’s ability to conduct electric current
  • Factors influencing conductivity:
    • Number of free electrons in the material
    • Mobility of those free electrons
  • Conductivity equation:
    • σ = nqμ
      • σ: conductivity
      • n: number of free electrons per unit volume
      • q: charge of an electron
      • μ: electron’s mobility

Examples of Conductors

  • Copper wire: widely used for electrical wiring
  • Aluminum foil: used for wrapping food and in electrical applications
  • Silver: known for its exceptional conductivity
  • Gold: commonly used in electronics due to its reliability

Examples of Semiconductors

  • Silicon: extensively used in electronic devices like transistors
  • Germanium: early semiconductor material in electronic components
  • Gallium arsenide: used for high-frequency applications
  • Indium arsenide: utilized in infrared photodetectors

Examples of Insulators

  • Rubber: used to insulate electrical wires
  • Glass: a non-conductive material in most applications
  • PVC pipes: widely used for plumbing purposes
  • Wood: a natural insulator extensively used in construction

Electron Motion in Conductors

  • Valence electrons: electrons in the outermost shell
  • Conduction electrons: electrons that are responsible for electrical conduction
  • Electron motion in conductors:
    • Free electrons gaining energy
    • Colliding with lattice ions
    • Experiencing resistance
  1. Conductivity of Conductors
  • Conductivity is defined as the reciprocal of resistivity (σ = 1/ρ)
  • Units of conductivity: Siemens per meter (S/m)
  • Conductivity depends on temperature, impurities, and strain
  • Conductivity can be determined by measuring the current and voltage of a conductor
  • Example: A copper wire with a conductivity of 5.96 x 10^7 S/m
  1. Resistivity of Semiconductors
  • Resistivity is a measure of how strongly a material opposes the flow of electric current
  • Resistivity is given by the formula ρ = R × A / L
    • ρ: resistivity
    • R: resistance of the material
    • A: cross-sectional area of the material
    • L: length of the material
  • Semiconductors have higher resistivity compared to conductors
  • Example: Silicon with a resistivity of 2.3 x 10^-3 Ω·m
  1. Band Theory of Solids
  • Explains the electrical conductivity behavior of conductors, semiconductors, and insulators
  • Valence band: the band of energy levels filled with electrons
  • Conduction band: the band of energy levels available for electrons to move freely (free electrons)
  • Energy band gap: the energy difference between the valence and conduction bands
  • Conductors have no energy band gap, semiconductors have a small energy band gap, and insulators have a large energy band gap
  • Example: Semiconductor materials like silicon have a small energy band gap (around 1.1 eV)
  1. Intrinsic and Extrinsic Semiconductors
  • Intrinsic semiconductors: pure semiconductors with no intentional impurities
  • Extrinsic semiconductors: semiconductors with impurities intentionally added
  • N-type semiconductors: doped with impurities that provide extra electrons
  • P-type semiconductors: doped with impurities that create extra holes (electron vacancies)
  • Examples: Silicon doped with arsenic (n-type) and silicon doped with boron (p-type)
  1. PN Junctions
  • PN junctions are formed by joining a p-type semiconductor with an n-type semiconductor
  • Depletion region: the region near the junction where the excess electrons in the n-type side combine with the holes in the p-type side
  • Forward bias: applying a positive voltage to the p-side and a negative voltage to the n-side, allowing current flow across the junction
  • Reverse bias: applying a negative voltage to the p-side and a positive voltage to the n-side, preventing current flow across the junction
  1. Insulating Properties
  • Insulators have a large energy band gap
  • Valence band is completely filled, and conduction band is almost empty
  • Electrons are tightly bound to atoms and are not free to move
  • Insulating materials do not conduct electricity under normal conditions
  • Example: Glass is an insulator due to its high resistivity and inability to conduct electricity
  1. Superconductivity
  • Superconductors are materials that exhibit zero electrical resistance below a certain critical temperature
  • The critical temperature is unique to each material
  • Superconductors exhibit the Meissner effect, expelling magnetic fields from their interior
  • Applications of superconductors include magnetic resonance imaging (MRI) machines and particle accelerators
  1. Band Gap Engineering
  • The ability to modify the energy band gap in semiconductors through doping and alloying
  • Allows control over electrical conductivity, making it possible to design materials with specific properties
  • Widely used in the semiconductor industry to create different types of transistors and electronic devices
  • Example: Tuning the band gap of gallium nitride (GaN) to be suitable for use in LEDs
  1. Applications of Conductors
  • Electrical wiring in buildings and houses
  • Electric circuits and components in electronic devices
  • Conductive paints and coatings
  • Antennas and transmission lines for communication systems
  1. Applications of Semiconductors
  • Transistors and integrated circuits used in computers and electronic devices
  • Light-emitting diodes (LEDs) for lighting and display applications
  • Photovoltaic cells for solar energy conversion
  • Semiconductor lasers for optical communication and data storage

Energy Band Gap

  • The energy band gap determines the electrical conductivity of a material.
  • Conductors have no energy band gap, allowing electrons to move freely.
  • Semiconductors have a small energy band gap, requiring some energy to move electrons.
  • Insulators have a large energy band gap, making it difficult for electrons to move.

Fermi Level

  • The Fermi level is the highest energy level that is occupied by an electron at absolute zero temperature.
  • It represents the energy required for an electron to move from the valence band to the conduction band.
  • The position of the Fermi level determines the electrical conductivity of a material.

Doping in Semiconductors

  • Doping is the intentional introduction of impurities into a semiconductor material.
  • N-type doping involves adding impurities with extra electrons, creating more conduction electrons.
  • P-type doping involves adding impurities with electron deficiencies, creating more holes for electron movement.
  • Doping allows control over the electrical properties and conductivity of semiconductors.

Hall Effect

  • The Hall effect is the generation of voltage across a conductor or semiconductor when a magnetic field is applied perpendicular to the current flow.
  • It is used to determine the type (N or P) and concentration of charge carriers in a material.
  • The Hall coefficient represents the ratio of the voltage across the material to the product of the magnetic field and current density.

Thermistors

  • Thermistors are temperature-sensitive resistors made from semiconductor materials.
  • They have a negative temperature coefficient, meaning their resistance decreases as the temperature increases.
  • Thermistors are used in temperature sensors, thermostats, and temperature compensation circuits.

Photoconductivity

  • Photoconductivity is the increase in electrical conductivity of a material when exposed to light.
  • It occurs in certain semiconductors, such as cadmium sulfide (CdS) and lead sulfide (PbS).
  • The absorption of photons results in the generation of free electrons and holes, increasing the electrical conductivity.

Breakdown Voltage

  • Breakdown voltage is the voltage at which an insulator or a semiconductor suddenly allows a significant current to flow.
  • It is a result of the ionization of atoms or molecules in the material due to the high electric field.
  • Breakdown voltage is an important parameter for the design of electrical devices.

High-Temperature Superconductivity

  • High-temperature superconductivity refers to the phenomenon of zero electrical resistance at higher temperatures than traditional superconductors.
  • These materials exhibit superconducting properties at temperatures above the boiling point of liquid nitrogen (-196°C).
  • High-temperature superconductors have potential applications in energy transmission, magnet levitation, and medical imaging.

Quantum Confinement

  • Quantum confinement is the phenomenon in which the behavior of electrons is restricted due to their confinement in a nanoscale space.
  • Quantum dots, nanowires, and quantum wells are examples of structures that exhibit quantum confinement.
  • Quantum confinement affects the electronic properties of materials and can be tuned for specific applications.

Conclusion

  • Conductors, semiconductors, and insulators have different electrical conductivity properties.
  • Conductors allow easy flow of electric current, while insulators inhibit its flow.
  • Semiconductors have intermediate conductivity and can be controlled through doping.
  • Understanding the properties and behavior of these materials is crucial for various technological applications.