In this study, we report on the development of a fully normally-off high electron mobility transistor (HEMT) gallium nitride on silicon (GaN-on-Si) heterostructures designed for high voltage operation. The normally-off configuration was achieved by means of a p-gallium nitride (p-GaN) cap layer enabling a positive threshold voltage higher than +1 V. The high voltage operation was ensured with a buffer based on AlN/GaN superlattice (SL) delivering a vertical breakdown voltage about 1.1 kV for a leakage current density of 1mA/cm². With the substrate grounded, hard breakdown voltage transistors at VGS = 0V is about 1.45 kV, corresponding to an outstanding breakdown field higher than 2.4 MV/cm. High voltage characterizations revealed a state of the art combination of breakdown voltage at VGS = 0V together with low electron trapping effects up to 1.4 kV as assessed by means of substrate ramp measurements.