The fabrication of high transconductance AlGa /Ga high electron mobility transistors (HEMTs) grown on high-resistivity silicon substrate is reported with an AlGa barrier thickness of only 12.5 nm. A maximum DC current density of 655 mA/mm, a current gain cutoff frequency (F T ) of 75 GHz and a power-gain cutoff frequency (F MAX ) of 125 GHz are obtained for a 0.125 µm gate length transistor. The device provides a record peak extrinsic transconductance of 332 mS/mm and an intrinsic value of 509 mS/mm. To the authors' knowledge, the obtained transconductances are the highest reported values from AlGa /Ga devices grown on a Si(111) substrate. This performance demonstrates the potential of Ga transistors on silicon for low-cost microwave power applications.