In this paper, we present the operation principle of an organic metal-insulator-semiconductor (MIS) capacitor where the organic semiconductor is undoped. In spite of a low charge concentration within the semiconductor, this device exhibits a capacitance variation with respect to the applied gate voltage yielding the capacitance-voltage characteristics similar to that of a traditional MIS capacitor based on the doped semiconductor. A physics-based model is developed to derive the charge concentration, surface potential, and the capacitance of the organic MIS capacitor. The model is validated with TCAD simulation results as well as with experimental data obtained from the fabricated organic MIS capacitor consisting of poly(4-vinylphenol) and poly(3-hexylthiophene-2, 5-diyl) as an insulator and a semiconductor, respectively. © 1963-2012 IEEE.