Semiconductor Electronics: Materials, Devices and Simple Circuits

• Classification of Metals, Conductors, and Semiconductors

  • Energy Bands in Solids
    • Valence Band (VB): Lower energy band, completely or partially filled with electrons.
    • Conduction Band (CB): Higher energy band, empty or partially filled, responsible for conduction.
    • Band Gap ($\Delta E_g$): Energy gap between VB and CB.
  • Distinction based on Energy Bands
    • Metals: Overlapping bands or zero band gap; high conductivity.
    • Insulators: Large band gap ($\Delta E_g > 3\text{ eV}$); electrons cannot jump to CB.
    • Semiconductors: Small band gap ($\Delta E_g \approx 1\text{ eV}$); finite conductivity at room temperature.

• Intrinsic Semiconductor

  • Pure form of semiconductor (e.g., Silicon, Germanium).
  • Thermal Excitation: Breaks covalent bonds, creating electron-hole pairs.
  • Charge Carriers
    • Free Electrons in the conduction band.
    • Holes in the valence band.
  • Condition: Number of free electrons equals number of holes ($n_e = n_h = n_i$).

• Extrinsic Semiconductor

  • Formed by adding Impurities (Doping) to increase conductivity.
  • n-Type Semiconductor
    • Doped with Pentavalent impurity (e.g., Arsenic, Antimony).
    • Donors provide extra electrons.
    • Majority charge carriers: Electrons ($n_e \gg n_h$).
  • p-Type Semiconductor
    • Doped with Trivalent impurity (e.g., Indium, Boron, Gallium).
    • Acceptors create vacancies or holes.
    • Majority charge carriers: Holes ($n_h \gg n_e$).

• p-n Junction

  • Formation at the boundary of p-type and n-type semiconductors.
  • Depletion Region
    • Created by diffusion of electrons and holes across the junction.
    • Region devoid of mobile charge carriers.
  • Potential Barrier
    • Electric field due to stationary immobile ions stopping further diffusion.
  • Biasing of p-n Junction
    • Forward Bias
      • p-type connected to positive terminal, n-type to negative.
      • Reduces potential barrier and depletion layer width.
      • Low resistance, allows high current flow.
    • Reverse Bias
      • p-type connected to negative terminal, n-type to positive.
      • Increases potential barrier and depletion layer width.
      • High resistance, allows only negligible reverse saturation current.

• Semiconductor Diodes

  • p-n Junction as a Diode
    • Allows current to flow easily in forward bias, blocks in reverse bias.
  • Applications of Diode
    • Rectifier: Converts AC to DC.
      • Half-Wave Rectifier: Uses one diode, rectifies half cycle.
      • Full-Wave Rectifier: Uses two diodes (or bridge), rectifies full cycle.
    • Special Purpose Diodes: Zener Diode, Photodiode, LED, Solar Cell.

• Digital Electronics and Logic Gates

  • Analog vs Digital Signals
    • Analog: Continuous values.
    • Digital: Discrete values (binary 0 and 1).
  • Basic Logic Gates
    • OR Gate
      • Operation: $Y = A + B$
      • Output is 1 if either input is 1.
    • AND Gate
      • Operation: $Y = A \cdot B$
      • Output is 1 only if all inputs are 1.
    • NOT Gate (Inverter)
      • Operation: $Y = \bar{A}$
      • Inverts the input.
  • Universal Gates
    • NAND Gate (NOT + AND)
    • NOR Gate (NOT + OR)
    • Can implement any basic logic operation.