The electrical properties of AlGaN/GaN high electron mobility transistor structures grown on composite GaN(Fe)/GaN buffers by molecular beam epitaxy were reported. The concentration of Fe in the GaN(Fe) layer ranged from 8×1016 to 3×1017 cm-3 as established by secondary ion mass spectrometry. The thickness of the undoped GaN layer of the buffer was varied from 2.2 to 4.1 μm. For thinner buffers and higher Fe concentration, the buffer was semi-insulating, with the Fermi level pinned near Ec-0.57 eV. For thicker buffers and lower Fe concentration, the top part of the buffer was conducting. Admittance spectra measured in conducting buffers also showed a prominent contribution from Ec-(055–0.6) eV electron traps. Despite the universal prominence of these traps in all our films, the behavior of their concentration with Fe doping and with increased distance from the GaN (Fe)/GaN boundary is not compatible with the assumption that they are due to substitutional Fe acceptors. Possible compensation mechanisms in the studied structures were discussed.