The electrical properties of AlGaN/GaN high electron mobility transistor structures with composite Fe-doped GaN/undoped GaN buffers grown on (1 1 0) Si substrates by ammonia molecular beam epitaxy were reported. Fe concentration in the range of 10(17) cm(-3) is sufficient to compensate the residual donors (< 10(16) cm(-3) ) of undoped GaN and demonstrated a highly resistive GaN on (1 1 0) Si substrates. For buffers with Fe concentration of 1 x 10(17) similar to 1 x 10(18) cm(-3), the buffer was semi-insulating, with the Fermi level pinned near 0.5-0.7 eV which is determined by temperature-dependent I-T measurements. From the PICTS measurements, the Fe-related peaks of 0.69 eV (B) and 0.86 eV (C) are observed. The peak B is most likely due to the center pinning the Fermi level which is close to the activation energy observed in the temperature-dependent current measurement of Fe-doped sample, tentatively attributed to complexes between substitutional Fe ions and native defects such as nitrogen vacancies. The peak C is similar to the hole trap of 0.85 eV previously reported. In spite of similar transfer device characteristics, the Fe-doped buffer shows a noticeable difference in the off-state leakage behaviors. The sample grown with the optimized Fe concentration of 1 x 10(17) cm(-3) had a buffer resistivity of approximately 3.84 x 10(11)Omega cm, increased by almost four orders of magnitude with respect to that (4.5 x 10(7) Omega cm) of the reference sample with undoped GaN buffer.