  Static metal-semiconductor contacts are classified into Ohmic contacts and
Schottky contacts. As for dynamic metal-semiconductor contacts, the in-depth
mechanism remains to be studied. We here define a "triboelectric junction"
model for analyzing dynamic metal-semiconductor contacts, where a space charge
region induced by the triboelectric effect dominates the electron-hole
separation process. Through theoretical analysis and experiments, we conclude
that the triboelectric junction affects the electric output in two aspects: 1)
the junction direction determines the output polarity; 2) the junction strength
determines the output amplitude. The junction direction and junction strength
are both related to the electron-affinity difference between the contact metal
and semiconductor. We find that the standard electrode potential in
electrochemistry best describes the electron affinity of a dynamic
metal-semiconductor contact.
