Comparison of shallow-mesa InAs/AlSb HEMTs with and without early-protection for long-term stability against Al(Ga)Sb oxidation

The fabrication process of InAs/AlSb high electron mobility transistors (HEMTs) has been improved by depositing, after the shallow-mesa isolation of the active area, a thin SiNx-film on the exposed AlGaSb mesa floor. Devices with and without this early-protection against Al(Ga)Sb oxidation have been fabricated simultaneously on the same chip for fair comparison. Optical observations and electrical measurements over four and half years demonstrated the physical stability provided by this extra-coverage. The electrical measurements also revealed that the induced deposition of the probing pads and of the extrinsic part of the gates on SiNx slightly reduced the maximum drain current ID (-9%) and the transconductance gm (-12%) to, respectively, 700 mA mm-1 and 1220 mS mm-1 for 2 × 20 m2 InAs/AlSb HEMTs with a 140 nm recessed gate. On the other hand, the gate-leakage current IG was lowered by more than one order of magnitude, leading to a better pinch-off behavior and increased values of cut-off frequency fT (+4%) and maximum frequency of oscillation fmax (+36%) to, respectively, 230 GHz and 190 GHz at a drain voltage VDS of 0.5 V.