12.14:
MOS Capacitor
MOS capacitors have three layers: the bottom layer is a semiconductor substrate, the middle is an insulating layer, and the top metal layer is the gate electrode, which controls the capacitor's voltage.
At zero gate voltage, no current flows, and the capacitor is uncharged.
Applying a negative voltage at the gate attracts holes to the semiconductor's surface, forming an accumulation region. The device behaves as a conventional capacitor.
Applying a positive voltage, holes are repelled back into the substrate, establishing a space charge region called the depletion region, which acts as another capacitance in series.
With a high enough voltage, the concentration of electrons on the surface increases, forming an inversion layer and further decreasing the capacitance. This results in a sharp increase in current.
Removing the voltage dissipates the accumulated charge, making the channel disappear and switching off the device.
MOS capacitors are used in memory devices like DRAM, where data writing involves applying gate voltage to create an inversion layer, which temporarily stores charge. The presence or absence of this charge represents binary data, which needs periodic refreshing due to its transient nature.
12.14:
MOS Capacitor
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.