Effect of Impurity Concentration of Fermi Level:

• The position of Fermi level is also addition of impurity and also by variation in the concentration of impurity.
• If a donor impurity is added to an intrinsic semiconductor, it results in n-type of semiconductor and a donor level tomes into existence below the bottom of conduction band.
• At impurity concentrations, the impurity atoms ait so spaced that they do not interact with each other. Once the concentrations are increased, interaction among them starts.

Fig.1 Energy Band Diagram

• The donor levels starts splitting and form an energy band below the conduction band. The width of this band increases with increase in the impurity concentration.
• At one stage it overlaps on the conduction band. Due to broadening the donor levels into band reduces the width of forbidden energy gap and the Fermi level is found moved upwards.
• With increase in concentration of donor impurity, the Fermi level continues shifting towards conduction band and enters into conduction band.
• In the same way, for p-type semi conductors increase in the impurity concentration makes Fermi level shift into valence band.