A Metal-Oxide-Semiconductor (MOS) capacitor is a fundamental structure used extensively in semiconductor device technology, particularly in the fabrication of integrated circuits and MOSFETs (metal-oxide-semiconductor field-effect transistors). The MOS capacitor consists of three layers: a metal gate, a dielectric oxide, and a semiconductor substrate.

The metal gate is typically made from highly conductive materials such as aluminum or polysilicon. Beneath the metal gate lies a thin layer of insulating oxide, usually silicon dioxide (SiO₂), which is the dielectric. The semiconductor substrate is commonly silicon, which can be either p-type or n-type.

When a voltage is applied to the metal gate, it influences the distribution of electrical carriers in the semiconductor. The semiconductor's energy bands are flat at zero applied voltage, indicating no excess charge within the oxide or at the semiconductor surface. As the gate voltage is increased positively, it attracts electrons towards the interface of the oxide and the semiconductor. This creates an accumulation of electrons in n-type silicon and a depletion of holes in p-type silicon, forming the depletion layer.

A further increase in the voltage leads to strong inversion, where the semiconductor surface beneath the oxide changes its type; for example, a p-type becomes an n-type, as electrons become the majority carriers. This inversion layer is critical in the operation of MOSFETs.The capacitance of the MOS structure varies with the voltage applied to the gate.

The inversion layer is crucial for the function of MOS capacitors in DRAM. Writing data involves applying a voltage that creates this layer and storing the charge in the semiconductor. This stored charge represents binary data, enabling the storage and retrieval of information. Removing the voltage causes the charge to dissipate, deactivating the channel and preserving the data stored. This charge cycle is central to the functionality and reliability of DRAM in computing applications.