Let f(E) is the Fermi distribution function, characterized by some (unspecified) Fermi energy EF. Deduce formula for energy range E2-E1 between energies E2 and Ei if f(E1) = fi and f(E2) f2.

For T 300 K and T 77 K calculate the energy range (Ei E2) in ev between f(E1)=0.02 and f(E2)-0.9.