The fabrication of self-aligned gate by ion implantation modulation doped (Al,Ga)As/GaAs field effect transistors (MODFETs) utilising a novel multilayer structure capable of withstanding the high-temperature furnace anneals required for Si implant activation is reported. Typical measured extrinsic transconductances of 175 mS/mm at 300 K and 290 mS/mm at 77 K were achieved on 1.1 μm-gate-length devices. Values of the two-dimensional electron gas saturation velocity of 1.9×10⁷ cm/s at 300 K and 2.7×10⁷ cm/s at 77 K were obtained from an analysis of the FET drain current/voltage characteristics using the charge-control model.