When two metals are separated by a very thin insulator, electrons from one metal can tunnel through the insulator to the other metal. Electrons flow until the Fermi levels of the two metals are equal. When a battery is connected between the two metals, as shown in Figure 14-15, the Fermi levels are displaced and a current flows if there are filled electron levels in one metal opposite empty levels in the other metal. Draw current voltage characteristics for the following junctions.
(a) Normal metal-normal metal.
(b) Normal metal-superconductor.
(c) Superconductor-superconductor. (Hint: The Fermi energy of a superconductor lies at the center of the energy gap.)
