The vacuum level denotes the energy threshold required for an electron to escape from a material surface. It is usually positioned above the conduction band of a semiconductor and acts as a benchmark for comparing electron energies within various materials.
Electron affinity in semiconductors refers to the energy gap between the minimum of its conduction band and the vacuum level and it is a critical parameter in determining how easily a semiconductor can accept additional electrons.
The work function represents the least amount of energy required to move an electron from the Fermi level, which is the energy level where an electron has a 50% chance of being present, to the vacuum level. This value varies across materials, with metals typically showcasing high work functions ranging from 2 to 5 eV, attributed to their densely populated Fermi levels.
Semiconductors, on the other hand, display dynamic work functions due to the varying nature of their Fermi levels influenced by factors such as doping and temperature changes.
When a metal and a semiconductor come into contact, they seek equilibrium, leading to a uniform vacuum level across the junction through charge transfer, continuing until the Fermi levels of both materials align. This triggers energy band bending within the semiconductor, leading to the creation of a Schottky barrier, a potential energy barrier for electrons moving across the metal-semiconductor junction.
The Schottky barrier's height governs the conductivity of the junction and it is determined by the relationship between the metal's work function and the semiconductor's electron affinity.
