Using the technique of in situ ion beam lithograph and molecular-beam epitaxial regrowth high-quality backgate high electron mobility transistor devices were fabricated. In situ gallium ion beam damage has enabled independent contacts to be made to two closely spaced conducting layers. In addition, the lower conducting layer has been divided up into three regions, separated by high energy ion implantation. The combination of these techniques has enabled us to demonstrate the operation of the split-backgate transistor.