10.8:
Metal-Semiconductor Junctions
Contact between a Metal and a semiconductor forms a junction with either Schottky or Ohmic behavior.
If the metal's work function exceeds that of the semiconductor, a Schottky junction is formed. Before contact, the energy band diagrams are aligned at the vacuum level, with the Fermi levels at different positions.
Upon contact, the Fermi levels align as electrons transfer from the semiconductor to the metal, reaching equilibrium.
Electron loss in the semiconductor decreases its potential, while their addition to the metal increases the potential, forming a Schottky barrier at the junction. The barrier height equals the difference between the metal's work function and the semiconductor's electron affinity.
The junction potential difference resists electron transfer from the semiconductor to the metal and is determined by the difference in their work functions.
If the metal's work function is smaller than that of the semiconductor, an Ohmic junction is formed. Here, the Fermi levels align as electrons transfer from the metal to the semiconductor.
This alignment raises the semiconductor electron energies relative to the metal, allowing electron flow from the semiconductor to the metal. So, the Ohmic junction conducts current in both directions.
10.8:
Metal-Semiconductor Junctions
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.