A special metal oxide semiconductor (MOS) diode is built with amorphous silicon (a-Si:H) as the semiconductor. The process is carried out so that the a-Si is not subject to high temperatures. This allows the characterization of solar grade a-Si:H by MOS diagnostic methods. Two types of structures (blanket a-Si layers as well as reactive ion etched a-Si mesas) are produced. Based on C-V and C(omega) measurements an equivalent network is established which includes the effects of the majority carrier dissipation time constant, of the space-charge punch-through in depletion and inversion, and of lateral current flows. The network is checked at selected bias and frequency conditions using given geometrical and material data. When applied to accumulation, it delivers the layer resistivity and net carrier density.