Key Concepts of Energy Band Diagrams to Know for Semiconductor Physics

Energy band diagrams illustrate how materials behave as insulators, conductors, or semiconductors based on their energy band gaps. Understanding these concepts is crucial for grasping how electronic devices function, from diodes to transistors.

1. Insulators

   • Have a large energy band gap (typically > 4 eV) preventing electron flow.
   • Electrons are tightly bound to their atoms, making them poor conductors of electricity.
   • Common materials include glass, rubber, and ceramics.

2. Conductors

   • Have overlapping energy bands or a very small band gap, allowing free movement of electrons.
   • Conduct electricity efficiently due to the presence of free charge carriers (electrons).
   • Common materials include metals like copper, silver, and gold.

3. Semiconductors

   • Have a moderate energy band gap (typically 1-3 eV), allowing controlled conductivity.
   • Conductivity can be altered by temperature, impurities, or external fields.
   • Essential for electronic devices, including diodes and transistors.

4. Intrinsic semiconductors

   • Pure semiconductors without any significant dopants, such as silicon or germanium.
   • Conductivity is solely due to thermally generated electron-hole pairs.
   • The number of charge carriers increases with temperature.

5. N-type semiconductors

   • Doped with elements that have more valence electrons (e.g., phosphorus in silicon).
   • Introduces extra electrons (negative charge carriers) into the conduction band.
   • Enhances conductivity by increasing the number of free electrons.

6. P-type semiconductors

   • Doped with elements that have fewer valence electrons (e.g., boron in silicon).
   • Creates "holes" (positive charge carriers) in the valence band.
   • Conductivity is primarily due to the movement of these holes.

7. P-N junction

   • Formed by joining P-type and N-type semiconductors.
   • Creates a depletion region where electrons and holes recombine, establishing an electric field.
   • Fundamental to the operation of diodes and other semiconductor devices.

8. Forward bias

   • Occurs when the P-side is connected to a positive voltage and the N-side to a negative voltage.
   • Reduces the width of the depletion region, allowing current to flow easily.
   • Essential for the operation of diodes in conducting mode.

9. Reverse bias

   • Occurs when the P-side is connected to a negative voltage and the N-side to a positive voltage.
   • Increases the width of the depletion region, preventing current flow.
   • Used in applications like rectifiers to block current in the reverse direction.

10. Metal-semiconductor junction (Schottky barrier)

    • Formed at the interface between a metal and a semiconductor.
    • Creates a potential barrier that affects the flow of charge carriers.
    • Important for high-speed electronic devices and solar cells due to low forward voltage drop.