The contact of metal and semiconductor can lead to the formation of a junction with either Schottky or Ohmic behavior.
Schottky Barriers
Schottky barriers arise when a metal with a work function (Φm) contacts a semiconductor with a different work function (Φs). Initially, electrons transfer until the Fermi levels of the metal and semiconductor align at equilibrium. For instance, if Φm > Φs, the semiconductor Fermi level is higher than the metal's before contact. The semiconductor's electrostatic potential must be raised to align the Fermi levels, resulting in a depletion region where positive charges from uncompensated donor ions balance the negative charge on the metal. The depletion width in the semiconductor can be calculated similarly to that in p-n junctions.
The equilibrium contact potential (Vo) prevents further electron diffusion from the semiconductor's conduction band into the metal. This potential is the difference in work function potentials (Φm – Φs). The potential barrier height (ΦB) for electron injection from the metal into the semiconductor conduction band is given by Φm – χ, where χ is the electron affinity.
Ohmic Contacts
In many applications, such as integrated circuits, it is crucial to have ohmic metal-semiconductor contacts with a linear I-V characteristic in both biasing directions. Ohmic contacts are formed when the charge induced in the semiconductor to align the Fermi levels is provided by majority carriers. For example, in an n-type semiconductor where Φm < Φs, electrons transfer from the metal to the semiconductor to align the Fermi levels, raising the semiconductor's electron energies. This results in a small barrier to electron flow, easily overcome by a small voltage. Similarly, for p-type semiconductors where Φm > Φs, hole flow across the junction is facilitated, ensuring minimal resistance and no signal rectification.
