We report on the high breakdown performance of AlGaN/GaN high electron mobility transistors (HEMTs) grown on 4-inch silicon substrates.The HEMT structure including three Al-content step-graded AlGaN transition layers has a total thickness of 2.7μm.The HEMT with a gate width WG of 300μm acquires a maximum off-state breakdown voltage (BV) of 550 V and a maximum drain current of 527mA/mm at a gate voltage of 2 V.It is found that BV is improved with the increase of gate-drain distance LGD until it exceeds 8μm and then BV is tended to saturation.While the maximum drain current drops continuously with the increase of LGD.The HEMT with a WG of 3mm and a LGD of 8μm obtains an off-state BV of 500 V.Its maximum leakage current is just 13 μA when the drain voltage is below 400 V.The device exhibits a maximum output current of 1 A with a maximum transconductance of 242mS.