Meaning
Locking of the Fermi level at a specific energy position within the semiconductor bandgap occurs due to a high density of electronic states at the surface or interface. This condition of Fermi level pinning prevents the work function of the contacting metal from freely modulating the Schottky barrier height. The effect is bounded by the charge neutrality level of the interface states and the dielectric screening capability of the material, which limits the effectiveness of external gate modulation.
Interface Defect
Dangling bonds and chemical impurities at the semiconductor boundary create a large concentration of surface states. These states act as charge reservoirs that absorb or release electrons to resist any change in the surface potential. This makes it difficult to achieve low ohmic contact resistance on certain semiconductors.
Device Consequence
Transistor gate control is degraded when the Fermi level is locked at the dielectric boundary. This limits the modulation of the channel conductivity and increases the threshold voltage instability of the device. Advanced passivation techniques are required to reduce the interface trap density and restore gate control.
Surface Passivation
Chemical treatments such as sulfur passivation or atomic layer deposition of thin oxides are used to terminate dangling bonds. This reduces the density of localized states and relieves the locking of the Fermi level. Achieving an unpinned surface is crucial for the development of high-performance compound semiconductor devices.
