MetalOxideSemiconductor

A metal-oxide-semiconductor (MOS) structure refers to a semiconductor device geometry formed by a metal gate, an insulating oxide layer, and a semiconductor substrate. It is central to the metal-oxide-semiconductor field-effect transistor (MOSFET), which is the dominant transistor type in modern integrated circuits. The term MOS also covers MOS capacitors used to study and characterize the metal–oxide–semiconductor interface.

In a typical MOS stack, the gate electrode sits on a thin oxide dielectric that separates it from the semiconductor surface. Silicon dioxide has historically been used as the dielectric in silicon devices; modern devices employ high-k dielectrics such as hafnium oxide to reduce leakage current at scaled thicknesses. Oxide thickness is on the order of a few nanometers, and the interface with the substrate can introduce fixed charges and traps that affect device performance.

Operation relies on the gate voltage to modulate the carrier concentration in the semiconductor near the surface, forming a depletion region and eventually an inversion layer that conducts current between source and drain. The MOS capacitance and transistor characteristics are governed by the oxide capacitance, semiconductor doping, and interface quality. Reliability concerns include oxide breakdown, charge trapping, and bias-temperature instability.

Applications include digital logic and analog circuits in a wide range of devices. The MOS structure has evolved with scaling, giving rise to advanced geometries such as FinFETs and gate-all-around devices, and to alternative gate materials and dielectrics. The MOS paradigm remains central to microelectronics, though ongoing materials and process innovations address leakage, variability, and performance.