We report on the barrier thickness dependence of the electrical properties and DC device characteristics of Al_0.4Ga_0.6N/GaN heterostructure field-effect transistors (HFETs). The HFET structures with 8–25-nm-thick AlGaN barrier layers were grown on sapphire substrates by plasma-assisted molecular-beam epitaxy. All of the fabricated HFET devices with a gate length of 1 µm showed a good DC performance and an excellent pinch-off characteristic. The extrinsic transconductance of the HFET devices increased from 185 to 360 mS/mm with decreasing AlGaN barrier thickness from 25 to 8 nm, while the maximum drain current for a gate bias of +1.0 V decreased from 1.1 to 0.68 A/mm.