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Defects relating to Er-doping of hydrogenated amorphous silicon films in amorphous/ crystalline (a-Si:H(Er) / n-Si) heterostructures used as light emitters were studied. Electrical parameters of the defects were evaluated by thermally activated current (TAC) and capacitance (TACap) spectroscopy, and high-frequency (1 MHz) capacitance-voltage (C-V) characteristics measured at 77 K. For charging of the traps, visible light illumination at low temperatures and thermal-bias stress at various temperatures were used. The amplitude of TAC was found to increase monotonically with increase of Er concentration in the a-Si:H film. The traps, with activation energies in the range 0.10 - 0.35 eV and 0.5 - 0.6 eV, were charged under both light and thermal-bias stress, and may be associated with the deep levels in the band-gap of a-Si:H. High-temperature current peaks with activation energies of 0.8-0.9 eV emerged only under thermal-bias stress and were strongly dependent on the charging temperature. It is suggested that these high-temperature thermally activated processes are related to polarization of the Er-O bonds in the amorphous matrix of a-Si:H. C-V measurements show that charging of the structure by negative substrate bias leads to net electron trapping in the a-Si:H(Er) film, whereas charging by positive bias result in net hole trapping. The model of Er-related electroluminescence in a-Si:H(Er) / n-Si heterostructures involving electron-hole recombination via deep levels located in the vicinity of [Er-O] inclusions is discussed.