ABSTRACT
When two different metallic materials come into contact, electrons in the metal having the higher Fermi level move into the other metal so as to equalize the Fermi levels of the two materials. Then the number of electrons in the material having a lower Fermi level increases, resulting in electrification to negative charge. As a matter of course, the other material is electrified to positive charge. Contact electrification of semiconductive materials is explained in a similar way. The contact region acts as an electrical capacitor. The amount of charge stored in the capacitor can be obtained by solving Poisson’s equation. Electrification of a nonconductive material is, however, more complicated. For metal-insulator contact, the charge density is estimated by the following equation1:
σ φ φ
ε ε
5 2
1 0 0eN N
ez+
(5.1)
where s 5 charge density (C/m2) fs 5 work function of nonconductive solid material (eV = 1.602 × 10219 J) fm 5 work function of metal (eV) Ns 5 density of surface states (l/(m2 eV)) Nb 5 density of traps of acceptor type (l/(m3 eV)) e 5 elementary charge (1.602 × 10219 C) e 5 dielectric constant of solid (F/m) e0 5 dielectric constant of vacuum (8.85 × 10212 F/m) z0 5 gap between solid and metal (4Å = 0.4 nm)
A similar equation is derived for insulator-insulator contact by assuming surface states for both materials.2
