In this work, AlGaN/GaN high electron mobility transistors on (1 0 0) and (1 1 0) oriented silicon substrates are investigated in view of monolithic integration with silicon MOSFETs for making more compact microwave power electronics. Epilayers are grown by molecular beam epitaxy on highly resistive substrates. It was shown that a better crystal quality as well as higher low-field electron mobility are obtained on the (1 1 0) orientation. Sub-micron gate length devices are then processed to estimate the millimeter-wave and microwave power performances of this new generation of devices. Load-pull measurements are performed from 4 GHz up to 40 GHz. Optimizations for the best power-added efficiency or for maximum output power density show the great potential of the Si(1 1 0) substrate for GaN-based power devices. At 18 GHz, these two different optimizations lead to a saturated output power density and an associated power-added efficiency of 2.4 W mm-1-40% and 3.76 W mm-1-33%, respectively. At 40 GHz, a record saturated output power density of 3.3 W mm-1 is achieved with an associated power-added efficiency of 20.1% and a linear power gain of 10.6 dB. In comparison, devices on Si(1 0 0) show less attractive performance due to a lower material quality with an output power density of 2.9 W mm-1, an associated power-added efficiency of 20.4% and a linear power gain of 7.5 dB at 10 GHz.