A special metal-oxide-semiconductor structure based on hydrogenated amorphous silicon has been fabricated. The quasi-static capacitance-voltage (CV) curves of this device are calculated for various trap densities of the amorphous silicon. Due to the occurrence of punch-through, Poisson＇s equation cannot be solved analytically. Thus, a finite elements approach has been used to compute the potential distribution and the charge density in the semiconductor. Differentiation of the charge with respect to the applied voltage delivers the low-frequency CV curve. In the last step, this CV curve is fitted to a measured one in order to determine the trap density We find a trap density of N-b0 = 9.10(16) V-1 cm(-3) at midgap.