3.3: Contacts

There are three essential elements in a current-carrying device: a conductor, and at least two contacts to apply a potential across the conductor. By definition the contacts are large: each contact contains many more electrons and many more electron states than the conductor. For this reason a contact is often called a reservoir. We will assume that all electrons in a contact are in equilibrium. The energy required to promote an electron from the Fermi level in the contact to the vacuum energy is defined as the work function (\(\Phi\)).

Figure \(\PageIndex{1}\): An energy level model of a metallic contact. There are many states filled up with electrons to the Fermi energy. The minimum energy required to remove an electron from a metal is known as the work function.

Metals are often employed as contacts, since metals generally possess very large numbers of both filled and unfilled states, enabling good conduction properties. Although the assumption of equilibrium within the contact cannot be exactly correct if a current flows through it, the large population of mobile electrons in the contact ensures that any deviations from equilibrium are small and the potential in the contact is approximately uniform. For example, consider a large metal contact. Its resistance is very small, and consequently any voltage drop in the contact must be relatively small.