Why is Band Gap Called Forbidden Energy Gap?

The band gap is called the “forbidden energy gap” because it is the range of energy levels within a solid where no electron states can exist. Electrons cannot occupy energies within this gap, making it “forbidden” for them to transition or exist in this energy range. This is a key concept in understanding the electronic properties of materials, particularly semiconductors and insulators.

Here’s a more detailed explanation:

  1. Energy Bands in Solids: In a solid, atoms are closely packed, and their atomic orbitals overlap, forming continuous energy bands. The two most important bands are the valence band and the conduction band.
    • Valence Band: The energy band that contains the valence electrons (the outermost electrons involved in chemical bonding).
    • Conduction Band: The energy band where electrons are free to move and conduct electricity.
  2. Band Gap (Forbidden Energy Gap): The energy gap between the top of the valence band and the bottom of the conduction band is called the band gap or forbidden energy gap. It is “forbidden” because electrons cannot occupy energies within this gap; there are no available electronic states in this energy range for electrons to occupy.
    • Insulators: Have a large band gap, making it difficult for electrons to jump from the valence band to the conduction band, thus they do not conduct electricity well.
    • Semiconductors: A smaller band gap allows some electrons to be thermally excited from the valence band to the conduction band, enabling controlled conductivity.
    • Conductors: Either have overlapping valence and conduction bands or no band gap at all, allowing free movement of electrons and thus good electrical conductivity.
  3. Electronic Transitions: For an electron to move from the valence band to the conduction band, it must gain energy equal to or greater than the band gap. This can occur through thermal excitation, photon absorption, or other mechanisms.
  4. Physical Implications: The size of the band gap determines the electrical and optical properties of the material. For instance, semiconductors with a specific band gap are used in electronic devices like diodes and transistors, while materials with larger band gaps are used as insulators.

In summary, the band gap is termed “forbidden” because it represents an energy range where electron states are not allowed. This concept is fundamental in determining the electrical properties of materials.

  1. What is the forbidden energy gap in a pure conductor?
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  3. Energy Bands: Classification & Explanation
  4. What is n and p-type semiconductor?

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