Introduction to Solids - in ECET Physics (Chapter Electronics)

 a. Definition of Solids: 

  • Solids are one of the three states of matter, along with liquids and gases. 
  • In a solid, the atoms, molecules, or ions are arranged in a regular pattern and are held together by strong intermolecular forces, giving the solid a fixed shape and volume. 
  • Solids can be classified as crystalline or amorphous based on the arrangement of their atoms.


b. Energy Bands in Solids:

  •  In a solid, the electrons occupy discrete energy levels that are grouped together into energy bands. 
  • The valence band is the lowest energy band that contains electrons, while the conduction band is the highest energy band that is empty or partially filled with electrons. 
  • The energy gap between the valence and conduction bands is called the bandgap, and it determines whether the solid is a conductor, insulator, or semiconductor.


c. Valence Band, Conduction Band, and Forbidden Band:

  •  The valence band is the band of energy levels that contains the valence electrons, which are the electrons that participate in chemical bonding. 
  • The conduction band is the band of energy levels that are empty or partially filled with electrons that can move freely through the solid. 
  • The forbidden band, also known as the bandgap, is the range of energy levels that does not contain any allowed electronic states, so electrons cannot exist in this region.


d. Energy Band Diagram of Conductors, Insulators, and Semiconductors:

Energy Band Diagram of Conductors, Insulators, and Semiconductors:

 

  • Conductors have a small or no bandgap, which allows the valence and conduction bands to overlap. 
  • This makes it easy for electrons to move from the valence band to the conduction band, resulting in high electrical conductivity. Insulators have a large bandgap, which makes it difficult for electrons to move from the valence band to the conduction band.
  • This results in low electrical conductivity. Semiconductors have a moderate bandgap, which allows electrons to be excited from the valence band to the conduction band under certain conditions, resulting in intermediate electrical conductivity.


e. Concept of Fermi Level:

  • The Fermi level is the energy level at which there is a 50% probability of finding an electron. 
  • In a solid, the Fermi level separates the occupied and unoccupied energy levels. 
  • The energy difference between the Fermi level and the highest occupied energy level in the valence band is called the Fermi energy. 
  • The Fermi level determines the electrical and thermal properties of solids, such as their conductivity and heat capacity.