Metal-Semiconductor Junctions

The metal's work function relative to the semiconductor's determines junction type. If the metal's work function exceeds the semiconductor's, a Schottky junction forms. If the metal's work function is smaller, an Ohmic junction forms. This difference controls whether electrons transfer from the semiconductor to the metal or vice versa upon contact.

When metal and semiconductor contact, electrons transfer until their Fermi levels align at equilibrium. Electron loss in the semiconductor decreases its potential, while their addition to the metal increases its potential. This charge redistribution creates a Schottky barrier at the junction. The barrier height equals the metal's work function minus the semiconductor's electron affinity.

The junction potential difference equals the difference in work functions between the metal and semiconductor. This potential resists electron transfer from the semiconductor to the metal in Schottky junctions. The equilibrium contact potential prevents further electron diffusion from the semiconductor's conduction band into the metal, establishing the barrier height.

In Ohmic junctions, the metal's work function is smaller than the semiconductor's, so electrons transfer from the metal to the semiconductor. This alignment raises the semiconductor's electron energies relative to the metal, allowing electrons to flow from the semiconductor back to the metal. The resulting low barrier is easily overcome, enabling bidirectional current flow.

Ohmic contacts form when the charge induced in the semiconductor to align Fermi levels is provided by majority carriers. In n-type semiconductors, electrons from the metal serve as majority carriers. In p-type semiconductors, holes facilitate current flow. This majority carrier mechanism ensures minimal resistance and linear I-V characteristics in both biasing directions.

Electron affinity is the energy required to remove an electron from the semiconductor's conduction band to vacuum. The Schottky barrier height is calculated as the metal's work function minus the semiconductor's electron affinity. This relationship determines how easily electrons can be injected from the metal into the semiconductor's conduction band.

Integrated circuits require linear I-V characteristics with minimal signal rectification and low resistance. Ohmic contacts achieve this by providing bidirectional current flow with easily overcome barriers. The majority carrier mechanism ensures consistent conductivity regardless of bias direction, making them ideal for interconnections and device contacts in modern electronics.