A new method for avoiding air exposure of the mesa-floor during processing of shallow-mesa InAs/AlSb HEMTs is reported. The method is based on the in situ chemical vapor deposition (CVD) of a SiNx-film, right after the shallow-mesa dry-etch process. The in situ CVD method allows also growing a dielectric film up to five times thicker than in previous reports of similar early-protection approach. Devices featuring a 25 nm SiNx-film enabled by the in situ CVD method are compared to devices based on a previously developed process, where the mesa floor is protected by a 2 nm SiNx-film deposited by ex situ reactive sputtering (RS). Microscopy observations revealed that the new process is more robust, ensuring a long-term stability against oxidation. DC, RF and noise performance were measured for 110 nm gate-length HEMTs. Devices based on the CVD process demonstrated higher peak transconductance (+13%), elevated I-on/I-off ratio (factor 4.7) and one order of magnitude lower gate current leakage. The cut-off frequency f(T) and the maximum oscillation frequency f(max) at a drain-source voltage of 0.3 V increased up to 175 GHz (+20%) and 130 GHz (+18%), respectively. Moreover, the extracted minimum noise figure for the InAs/AlSb HEMT using the in situ CVD early-protection was 1.4 dB at 6 GHz, instead of 2.3 dB for the RS based devices. (c) 2013 Elsevier Ltd. All rights reserved.