We report on a sub-micron thick AlGaN/GaN high electron mobility transistor (HEMT) epilayers grown on silicon substrate with a state-of-the art vertical buffer breakdown field as high as 6 MV/cm enabling a high transistor breakdown voltage of 250 V for short gate to drain distances despite such a thin structure. HEMTs with a gate length of 100 nm exhibit good DC characteristics with a low drain-induced barrier lowering as low as 100 mV/V for a VDS of 30 V. Breakdown voltages of each epilayer from the decomposed heterostructure reveals that the outstanding breakdown strength is attributed to the insertion of Al-rich AlGaN in the buffer layers combined with an optimized AlN nucleation layer. As a result, large signal measurements at 10 GHz could be reliably achieved up to VDS = 35 V despite the use of a 100 nm gate length. These results demonstrate the potential of sub-micron thick buffer GaN-on-Si heterostructures for high frequency applications.