3.4 Non-ideal effects in Metal-Semiconductor junctions

Table of Contents - 1 2 3 4 5 6 7 8 9 R S ®

  1. Metal-Semiconductor junctions with an interfacial layer
  2. Surface states - Fermi energy pinning

The theory predicts that the barrier height, fB, between a metal and an n-type semiconductor equals the difference between the workfunction of the metal, FM, and the electron affinity, c. Experimental results on the other hand yield at times vastly different values for the barrier height, while the experimental value has been shown to depend on the preparation of the semiconductor before metal deposition.

This discrepancy is believed to be due to the presence of an interfacial layer, surface states or both. A chemical reaction between the metal and the semiconductor can further affect the metal-semiconductor contact.

Interfacial layers readily occur when a metal is deposited on a piece of semiconductor which has been exposed to air as most semiconductors oxidize in air yielding a thin (~3nm) native oxide. Such an interfacial layer increases the built-in potential as obtained from a capacitance-voltage measurement. It also increases the saturation current of a rectifying metal-semiconductor contact as well as the ideality factor. The effect is most significant for thick interfacial layer with low dielectric constant on a highly doped semiconductor.

Surface states are states caused by the incomplete bonds at the surface of a semiconductor. They result in electron energy levels within the energy bandgap. A large density of such energy levels causes Fermi energy "pinning" resulting in Schottky barrier heights which are almost independent of the workfunction of the metal.

A combination of an interfacial layer and surface states has been proposed by Cowley and Sze to explain the experimental barrier height as measured on a variety of structures with different metals and semiconductors.

3.3 ® 4.

© Bart Van Zeghbroeck 1997