Abstract: A MODFET structure having a semi-insulating substrate overlayed with an undoped semiconductor buffer layer of a first composition. The buffer layer is overlayed with an undoped semiconductor layer having a second composition different from the composition of the buffer layer. An etch stop layer having a composition different from the composition of spacer layer is formed on the spacer layer, which in turn is overlayed with a doped semiconductor layer having the same composition as the spacer layer. A gate well is selectively etched through the doped semiconductor layer using a gate mask and is terminated at the top surface of the etch stop layer. In a first embodiment, a gate electrode is deposited on the surface of the stop layer at the bottom of the gate well. In an alternate embodiment the etch stop layer at the bottom of the gate well is removed and a thin dielectric layer is formed between the spacer layer and the gate electrode.