High-yield process for GaAs enhancement-mode MESFET integrated circuits

The paper describes a high-yield process for the fabrication of enhancement-mode MESFET integrated circuits. The process utilises light-stimulated anodic oxidation followed with oxide removal as a means of recessing the gate of an FET for normally-off operation. The oxidation technique has an important self-limiting property allowing different amounts of GaAs to be oxidised according to local doping and thickness variations. Yields of over 85% of 11-stage ring oscillators have been obtained by this method with a standard deviation in the pinch-off voltage of 75 mV, which represents a factor of four improvement over as-grown vapour-phase epitaxial material. The recessed-gate normally-off MESFET structure also has the highest transconductance reported to date due to the reduction in parasitic series resistance. Values of gm up to 70 mS/mm have been obtained from MESFETs having gate lengths of 1.5 ¿m.