This document summarizes key concepts about MOS capacitors: 1. A MOS capacitor consists of a metal gate separated from a semiconductor substrate by an insulating layer. Applying different gate biases results in accumulation, depletion, or inversion of charge carriers near the semiconductor surface. 2. In depletion mode, a positive or negative gate bias depletes the semiconductor surface of holes or electrons. At the threshold inversion point, the surface is inverted from p-type to n-type or vice versa. 3. Key parameters that determine the depletion layer thickness and threshold voltage include acceptor doping concentration, permittivity of the semiconductor, and surface potential.