Semiconductor Physics

• Physics is a natural science that studies matter, its motion, and behavior through space and time, and that studies the related entities of energy and force.
• Semiconductor materials have electrical conductivity between conductors and insulators.
• Semiconductors are the foundation of modern electronics.

Semiconductor Properties

• Electrical Conductivity: Semiconductors can conduct electricity better than insulators but not as well as conductors.
• Energy Bands: The electronic band structure of a semiconductor determines its electrical properties.
• Band Gap: The energy difference between the valence band (where electrons reside) and the conduction band (where electrons can move freely).
  • Typical semiconductors have a band gap energy between 1 and 3 eV; this is the key property that allows semiconductors to act as switches.
• Doping: The process of adding impurities to a semiconductor to alter its electrical properties.
  • n-type: Doping with elements that donate extra electrons (e.g., phosphorus in silicon), increasing electron concentration.
  • p-type: Doping with elements that accept electrons (e.g., boron in silicon), increasing hole concentration.
• Carrier Mobility: How quickly electrons or holes can move through the material under an electric field.
• Temperature Dependence: Semiconductor conductivity generally increases with temperature due to increased thermal generation of charge carriers.

Semiconductor Materials

• Silicon (Si): The most widely used semiconductor due to its abundance, stable oxide, and well-established processing techniques.
• Germanium (Ge): Used in early transistors but less common now due to temperature sensitivity and higher leakage current.
• Gallium Arsenide (GaAs): Has higher electron mobility than silicon; used in high-frequency applications and optoelectronics.
• Indium Phosphide (InP): Used in high-speed electronics and optoelectronic devices, particularly in telecommunications.
• Silicon Carbide (SiC): A wide bandgap semiconductor used in high-power and high-temperature applications.
• Gallium Nitride (GaN): Another wide bandgap semiconductor used in high-power, high-frequency, and LED lighting applications.

Semiconductor Devices

• Diodes: Two-terminal devices that allow current to flow in one direction only.
  • p-n Junction Diode: Formed by joining p-type and n-type semiconductors, creating a depletion region at the junction.
  • Zener Diode: Designed to operate in the reverse breakdown region for voltage regulation.
  • Light-Emitting Diode (LED): Emits light when current passes through it.
  • Photodiode: Converts light into current.
• Transistors: Three-terminal devices that can amplify or switch electronic signals.
  • Bipolar Junction Transistor (BJT): A current-controlled device with three regions: emitter, base, and collector.
  • Field-Effect Transistor (FET): A voltage-controlled device with three terminals: source, gate, and drain.
    • MOSFET (Metal-Oxide-Semiconductor FET): The most common type of FET, using an electric field to control the channel conductivity.
    • JFET (Junction FET): Uses a reverse-biased p-n junction to control the channel width.
• Integrated Circuits (ICs): Complex circuits fabricated on a single semiconductor chip, containing many transistors, diodes, and other components.
• Thyristors: Four-layer semiconductor devices used for high-power switching applications.

Semiconductor Physics Concepts

• Energy Bands: Ranges of energy levels that electrons can possess within a solid.
  • Valence Band: The highest range of electron energies where electrons are normally present at low temperatures.
  • Conduction Band: The lowest range of empty electron energy levels where electrons can move freely through the solid.
• Fermi Level: The energy level at which the probability of finding an electron is 50%.
  • In intrinsic (undoped) semiconductors, the Fermi level is near the middle of the band gap.
  • In doped semiconductors, the Fermi level shifts closer to the conduction band (n-type) or valence band (p-type).
• Carrier Concentration: The number of electrons in the conduction band and holes in the valence band per unit volume.
  • Intrinsic Carrier Concentration (ni): The concentration of electrons and holes in an undoped semiconductor at a given temperature.
  • Doping Concentration (ND for donors, NA for acceptors): The concentration of impurity atoms added to the semiconductor.
• Drift Current: Current due to the movement of charge carriers under an electric field.
• Diffusion Current: Current due to the movement of charge carriers from regions of high concentration to regions of low concentration.
• Recombination: The process by which electrons in the conduction band lose energy and fall into the valence band, eliminating both an electron and a hole.
• Generation: The process by which electron-hole pairs are created due to thermal or optical excitation.

P-N Junction

• Depletion Region: A region formed at the junction between p-type and n-type semiconductors, depleted of free charge carriers due to diffusion.
• Built-in Potential (Vbi): The potential difference across the depletion region at equilibrium, caused by the diffusion of charge carriers.
• Forward Bias: Applying a positive voltage to the p-side and a negative voltage to the n-side, reducing the depletion region width and allowing current to flow.
• Reverse Bias: Applying a negative voltage to the p-side and a positive voltage to the n-side, widening the depletion region and blocking current flow (except for a small leakage current).
• Breakdown Voltage: The reverse voltage at which the diode starts conducting heavily due to avalanche breakdown or Zener breakdown.
• Diode Equation: Describes the current-voltage (I-V) characteristics of a diode.

Transistor Operation

• Bipolar Junction Transistor (BJT):
  • NPN and PNP types: Differ in the polarity of the voltage and current.
  • Active Region: The transistor operates as an amplifier.
  • Saturation Region: The transistor is fully on, acting as a closed switch.
  • Cut-off Region: The transistor is fully off, acting as an open switch.
• Field-Effect Transistor (FET):
  • MOSFET:
    • Enhancement Mode: Requires a gate voltage to create a conducting channel.
    • Depletion Mode: Has a channel at zero gate voltage and requires a gate voltage to deplete the channel.
  • Channel Length Modulation: The effective channel length decreases with increasing drain voltage, affecting the output current.
  • Subthreshold Conduction: A small current flows even when the gate voltage is below the threshold voltage.

Semiconductor Fabrication

• Crystal Growth: Growing high-quality single-crystal silicon ingots.
  • Czochralski (CZ) method: A seed crystal is dipped into molten silicon and slowly pulled out, forming a large single crystal.
  • Float Zone (FZ) method: A zone of molten silicon is passed through the ingot, purifying the material.
• Wafer Preparation: Slicing the silicon ingot into thin wafers and polishing them to a smooth surface.
• Photolithography: Transferring circuit patterns onto the wafer using light and a photoresist material.
• Etching: Removing unwanted material from the wafer using chemical or plasma etching processes.
• Doping: Introducing impurities into specific regions of the wafer using diffusion or ion implantation.
• Deposition: Growing thin films of various materials on the wafer using techniques like chemical vapor deposition (CVD) or sputtering.
• Metallization: Depositing metal layers to create electrical connections between different components.
• Testing and Packaging: Testing the fabricated chips and packaging them for use in electronic devices.

Applications of Semiconductors

• Microelectronics: Integrated circuits for computers, smartphones, and other electronic devices.
• Optoelectronics: LEDs, solar cells, and laser diodes.
• Power Electronics: High-power switches and rectifiers for power supplies and motor control.
• Sensors: Devices that convert physical quantities (e.g., temperature, pressure, light) into electrical signals.
• Telecommunications: High-speed transistors and diodes for wireless communication systems.