We report on low RF losses at the interface between the epitaxial structure and the silicon substrate (less than 0.8 dB/mm up to 110GHz) of AlN/GaN high electron mobility transistors (HEMTs) grown on silicon substrate. This stateof-the-art performance makes GaN-on-Silicon HEMTs competitive with GaN-on-SiC in terms of parasitic RF losses. Furthermore, a maximum dc output current close to 1 A/mm together with low leakage current of 1 μA/mm and low trapping effects are achieved while using a short gate length of 0.2 μm. The large signal measurements confirmed the high quality of the epitaxy and the device processing as well as the low parasitic RF losses. This is reflected by a high output power density of 4.5 W/mm achieved at 18 GHz.