Question

When an electron goes from the valence band to the conduction band in silicon, its energy is increased by 1.1 eV. The average energy exchanged in a thermal collision is of the order of kT which is only 0.026 eV at room temperature. How is a thermal collision able to take some to the electrons from the valence band to the conduction band?

Answer 1

Explanation:

• Fermi level: The highest energy electron occupies this energy level. For example, n-type semiconductor is an extrinsic semiconductor in which the fermi level stays close to the conduction bond.
• This requires a small quantity of energy, which is necessary for exciting the electron to the conduction band from the fermi level. To the thermal excitation energy, this energy can be compared.
• Thus, these semi-conductors can conduct even at the room temperature. Fermi level stays close to the valence bane for a p-type semiconductor.
• This is due to the conduction taking place in holes majorly. Thus, a bond is damaged by the thermal excitation and there will be a creation of electron hole pair.
• Due to this, for conduction, the hole comes to the valence band or equivalently, an electron moves to the conduction band. No impurity is doped in an intrinsic semiconductor, so at the centre of band gap, the fermi level lies.
• There are only few electrons getting required energy through the continuous thermal collisions from the fermi level to jump to the conduction band.
• So, the intrinsic semiconductor conductivity is less when compared to the extrinsic semiconductor.