フェルミ準位

The Fermi level is the energy state with a 50 percent occupancy probability, positioned between the valence and conduction bands. It represents the boundary energy where electrons have an equal chance of being present or absent at a given temperature. The Fermi-Dirac function, an S-shaped curve, describes this probability distribution across all energy states in the material.

As temperature increases, electrons gain energy and transition from the valence band to the conduction band, causing the Fermi level to shift closer to the conduction band. At absolute zero, all states below the Fermi level are filled and those above are empty. Higher temperatures enable electrons to occupy previously vacant states above the Fermi level, altering the material's conductivity.

In n-type semiconductors, excess electrons shift the Fermi level close to the conduction band due to higher electron concentration. In p-type semiconductors, the Fermi level lies near the valence band because of higher hole concentration. In intrinsic semiconductors, equal electron and hole concentrations place the Fermi level in the middle of the band gap.

Electrons flow from the region with the higher Fermi level to the region with the lower Fermi level, establishing equilibrium at the junction. This electron movement aligns the Fermi levels across the interface, creating a balanced energy state. This process is fundamental to the operation of semiconductor junctions and electronic devices.

The Fermi-Dirac function is represented by a sigmoid curve that indicates the probability of an energy state being occupied by an electron at a given temperature. At the Fermi level, this probability equals 50 percent. Below the Fermi level, occupation probability approaches 100 percent; above it, probability decreases toward zero.

In intrinsic semiconductors, the Fermi level is positioned exactly in the middle of the band gap because electron and hole concentrations are equal. This central position reflects the balance between available electrons and holes. The band gap represents the energy difference between the valence and conduction bands, with the Fermi level bisecting this region.

Doping introduces impurities that alter carrier concentrations, shifting the Fermi level accordingly. Adding donor impurities creates n-type material with the Fermi level near the conduction band, while acceptor impurities create p-type material with the Fermi level near the valence band. This shift directly controls the semiconductor's electrical properties and conductivity.