Accumulation-type GaN metal-oxide-semiconductor field-effect transistors (MOSFETs) with atomic-layerdeposited Al_2O_3 gate dielectrics are fabricated.The device,with atomic-layer-deposited Al_2O_3 as the gate dielectric,presents a drain current of 260 mA/mm and a broad maximum transconductance of 34 mS/mm,which are better than those reported previously with Al_2O_3 as the gate dielectric.Furthermore,the device shows negligible current collapse in a wide range of bias voltages,owing to the effective passivation of the GaN surface by the Al_2O_3 film.The gate drain breakdown voltage is found to be about 59.5 V,and in addition the channel mobility of the n-GaN layer is about 380 cm2 /Vs,which is consistent with the Hall result,and it is not degraded by atomic-layer-deposition Al_2O_3 growth and device fabrication. Accumulation-type GaN metal-oxide-semiconductor field-effect transistors (MOSFETs) with atomic-layer deposition of Al_2O_3 gate dielectrics are fabricated. The device, with atomic-layer-deposited Al_2O_3 as the gate dielectric, presents a drain current of 260 mA / mm and a broad maximum transconductance of 34 mS / mm, which are better than those previously reported with Al_2O_3 as the gate dielectric. Ferrthermore, the device shows negligible current collapse in a wide range of bias voltages, due to the effective passivation of the GaN surface by the Al 2 O 3 film. The gate drain breakdown voltage is found to be about 59.5 V, and in addition to the channel mobility of the n-GaN layer is about 380 cm 2 / Vs, which is consistent with the Hall result, and it is not degraded by atomic-layer-deposition Al 2 O 3 growth and device fabrication.