We have developed a back-side process on AlN/GaN-on-Si DHFETs in order to drastically enhance their electrical performances. With a buffer thickness of 1.8 μm, the Si substrate has been locally etched up to the buffer layers between the drain and the gate. After local silicon removal, the three-terminal breakdown voltage increased from 750 V to 1900 V for a gate-drain distance of 15 μm despite the very high 2DEG density (2.3×10^13cm-2). Extremely low specific on-resistance (1.6 mΩ.cm²) was maintained by keeping the trenches narrow in order to avoid a strong degradation of the thermal dissipation.