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Small‐signal capacitance and conductance of experimental samples of a‐Si n‐i‐n structures were measured in a wide frequency range under various bias conditions. The measured capacitance at low frequencies greatly exceeds the expected value derived from the ΔQ/ΔV ratio, where ΔQ is a change of the trapped charge corresponding to a change ΔV of the applied voltage. This capacitance increases with the steady‐state bias and decreases with the frequency of the measuring signal. The measured low‐frequency small‐signal conductance equals the differential conductance obtained from the steady‐state current–voltage characteristics, but it increases with the rising frequency of the measuring signal. A small‐signal analytical model of an a‐Si n‐i‐n structure is developed which agrees well with the experimental results. With this model, the high capacitive effect of the n‐i‐n device at low frequencies is explained on the basis of a phase shift which arises from the delayed capture–emission mechanism of carriers in the localized states. Using this model, it is shown that an increasing frequency of small‐signal excitation moves the energy region of gap states engaged in the delaying action toward the conduction band, resulting in a decreasing capacitance and an increasing conductance of the a‐Si n‐i‐n structure. © 1996 American Institute of Physics.