CapacitanceVoltage

Capacitance-voltage (C-V) is a characterization technique used to study semiconductor structures, particularly metal-oxide-semiconductor (MOS) devices and p-n junctions. It involves applying a DC bias to a device while superimposing a small AC signal and measuring the resulting capacitance as a function of voltage. The C-V curve provides information about oxide thickness, doping profiles, and interface quality.

In a MOS capacitor, the measured capacitance results from the oxide layer in series with the depletion region of the semiconductor. As the bias is varied, the surface region can transition between accumulation, depletion, and inversion, causing the depletion width to change and the capacitance to vary. At accumulation and, to a degree, low-frequency inversion, the capacitance approaches Cox = εox / tox. In depletion, the series combination reduces the overall capacitance. At high frequency, the inversion layer cannot respond quickly, so the measured capacitance reflects mainly the depletion and oxide contributions. At low frequency, inversion can respond and the capacitance can rise toward Cox.

The C-V measurement yields several key parameters. Oxide thickness is inferred from Cox. The doping concentration in the near-surface region can be extracted from the slope of the C−2 versus voltage plot in the depletion region. The flat-band voltage, obtained from the C-V curve intercept, provides information about fixed oxide charges and interface charges. Frequency dispersion in the curve reveals interface trap densities.

Applications include semiconductor device fabrication and characterization, dopant profiling, oxide quality assessment, and process control. Limitations arise from leakage, series resistance, traps, and nonuniform doping, which can complicate interpretation.