Hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunctions are promising candidates for high efficiency solar cells. The performance of such cells is strongly dependent on the interfacial properties namely defect states and band discontinuities. Capacitance spectroscopy of (p) a-Si:H/(n) c-Si heterojunctions has been used to study fundamental properties of such structures. Numerical calculations of a-Si:H/c-Si heterojunction capacitance (C) vs temperature (T) and frequency (f) show a capacitance step. We have studied the dependence of the step activation energy (E-a) on various heterojunction parameters such as the position of the Fermi level in a-Si:H, capture cross sections of gap states in a-Si:H, values of band offsets, carrier mobility. It is shown that E-a generally corresponds to the energy difference between the Fermi level and the bottom of the valence band in the bulk a-Si:H and is related to the transport in a-Si:H and to the response of gap states at the Fermi level through capture and emission of holes. However, if very large values of hole mobility in a-Si:H are introduced in the modeling, larger values of E-a, close to that of the valence band offset, are obtained, revealing a contribution from the modulation of free holes in the strong inversion layer at the c-Si surface.