Let ND be the concentration of donors in the material.
At 0ᵒK, the donor atoms are not ionized and are at the level ED, which is very near to EC.
When the temperature is raised above 0ºK, the donor atoms get ionized and free electrons appear in the conduction band. With increase in temperature more and more donor atoms get ionized and the electron concentration in the conduction band increases.
Electrons require an energy ED, for their transition to the conduction band from the donor levels.
