In many cases we wish to have an ohmic metal-semiconductor contact, having a linear I-V characteristic in both biasing directions. For example, the surface of a typical integrated circuit is a maze of p and n regions, which must be contacted and interconnected. It is important that such contacts be ohmic, with minimal resistance and no tendency to rectify signals.
- Ideal metal-semiconductor contacts are ohmic when the charge induced in the semiconductor in aligning the Fermi levels is provided by majority carriers.
- For example, in the Φm < Φs (n-type) case of Fig.a, the Fermi levels are aligned at equilibrium by transferring electrons from the metal to the semiconductor.
- This raises the semiconductor electron energies (lowers the electrostatic potential) relative to the metal at equilibrium (Fig. b).
- In this case the barrier to electron flow between the metal and the semiconductor is small and easily overcome by a small voltage.
- Similarly, the case Φm > Φs (p-type) results in easy hole flow across the junction (Fig. d).
- Unlike the rectifying contacts discussed previously, no depletion region occurs in the semiconductor in these cases since the electrostatic potential difference required to align the Fermi levels at equilibrium calls for accumulation of majority carriers in the semiconductor.
- A practical method for forming ohmic contacts is by doping the semiconductor heavily in the contact region.
- Thus if a barrier exists at the interface, the depletion width is small enough to allow carriers to tunnel through the barrier.
For example, Au containing a small percentage of Sb can be alloyed to n-type Si, forming an n layer at the semiconductor surface and an excellent
- Similarly, p-type material requires a p surface layer in contact with the metal. In the case of Al on p-type Si, the metal contact also provides the acceptor dopant.
- Thus the required p surface layer is formed during a brief heat treatment of the contact after the Al is deposited.