- MOS Capacitor Defined: An MOS capacitor is a structure that consists of a metal gate, a semiconductor body, and an insulating layer of silicon dioxide.
- Capacitance and Voltage: The capacitance of an MOS capacitor changes depending on the voltage applied to the gate, affecting how charges are distributed within the device.
- Flat Band Voltage: This critical voltage level signifies no net charge across the capacitor, establishing a baseline for measuring other phenomena in the device.
- Threshold Voltage: This is the point at which the depletion layer vanishes, and strong inversion begins, pivotal for the capacitor’s operation within transistors.
- C-V Curve Analysis: The capacitance-voltage curve helps identify the capacitor’s behavior in different charge accumulation states, crucial for understanding and designing circuits with MOS capacitors.
MOS stands for metal-oxide-Semiconductor. An MOS capacitor has a gate, an insulating layer and a semiconductor body or substrate. The gate can be metal or heavily doped polysilicon. Silicon dioxide is the traditional dielectric material, while modern gate stacks may use other high-permittivity dielectrics. The gate and body form the two electrical terminals of the capacitor.
The measured small-signal capacitance changes with DC gate-to-body voltage, test frequency, signal amplitude and sweep timing.
Flat-band voltage gives the gate-to-body bias that makes the semiconductor surface potential zero and its energy bands flat. Gate-semiconductor work-function difference and oxide or interface charge can shift this value away from zero. For the n-type body shown here, positive bias above flat band (Vgb > Vfb) attracts majority-carrier electrons to the oxide-semiconductor interface. This is accumulation, and measured capacitance approaches the oxide capacitance.
When gate voltage falls below flat-band voltage (Vgb < Vfb), negative gate charge repels electrons from the n-type surface. The exposed ionised donors form a positive space-charge region in the semiconductor.
This condition is surface depletion. More negative bias widens the depletion region until minority-carrier holes form an inversion layer at the surface; the depletion width then approaches its maximum value rather than vanishing.
The MOS capacitor is the gate-stack model behind MOS transistors and is also fabricated as a test structure for oxide thickness, doping, interface-trap and charge measurements.
The diagram below shows the bias regions of an ideal MOS capacitor with an n-type body.
On this capacitance-versus-gate-voltage curve, a MOS Capacitor moves from accumulation through flat band (Vfb) and depletion towards inversion. Threshold voltage (Vth) is the chosen strong-inversion condition. At high test frequency, inversion charge cannot follow the AC signal, so capacitance stays near the series combination of oxide capacitance and maximum depletion capacitance. A quasi-static or sufficiently low-frequency curve can rise towards oxide capacitance in inversion.
