Typical Schottky Barriers
The discussion of ideal metal-semiconductor contacts does not include certain effects of the junction between the two dissimilar materials. Unlike a p-n junction, which occurs within a single crystal, a Schottky barrier junction includes a termination of the semiconductor crystal. The semiconductor surface contains surface states due to incomplete covalent bonds and other effects, which can lead to charges at the metal-semiconductor interface.
Typical Schottky Barriers:
- The contact is seldom an atomically sharp discontinuity between the semiconductor crystal and the metal.
- There is typically a thin interfacial layer, which is neither semiconductor nor metal.
- For example, silicon crystals are covered by a thin (10-20 A) oxide layer even after etching or cleaving in atmospheric conditions.
- Therefore, deposition of a metal on such a Si surface leaves a glassy interfacial layer at the junction.
- Although electrons can tunnel through this thin layer, it does affect the barrier to current transport through the junction.
Because of surface states, the interfacial layer, microscopic clusters of metal-semiconductor phases, and other effects, it is difficult to fabricate junctions
with barriers near the ideal values predicted from the work functions of the two isolated materials.
- Therefore, measured barrier heights are used in device design.
- In compound semiconductors the interfacial layer introduces states in the semiconductor band gap that pin the Fermi level at a fixed position, regardless of the metal used (Fig. given below).
- For example, a collection of interface states located 0.7 ~ 0.9 eV below the conduction band pins EF at the surface of n-type GaAs, and the Schottky barrier height is determined from this pinning effect rather than by the work function of the metal.
- An interesting case is n-type InAs (Fig. b), in which EF at the interface is pinned above the conduction band edge.
- As a result, ohmic contact to n-type InAs can be made by depositing virtually any metal on the surface.
- For Si, good Schottky barriers are formed by various metals, such as Au or Pt.
- In the case of Pt, heat treatment results in a platinum silicide layer, which provides a reliable Schottky barrier with ΦB = 0.85 V on n-type Si.
- A full treatment of Schottky barrier diodes results in a forward current equation of the form
- where B is a constant containing parameters of the junction properties and n is a number between 1 and 2.
- The mathematics of this derivation is similar to that of thermionic emission, and the factor B corresponds to an effective Richardson constant in the thermionic problem.