We investigated the contact resistance of a non-alloyed Ti/Pt/Au ohmic electrode to obtain thermally stable source-drain resistance of cryogenically cooled In0.75Ga0.25As/In0.52Al0.48As high electron mobility transistors (HEMTs) fabricated on (4 1 I)A-oriented InP substrates by molecular beam epitaxy. A contact resistance of 0. 26 Q mm, which strongly depended on InAlAs spacer and barrier layers, was achieved at 16K (25% below the value at 300K) for the non-alloyed Ti/Pt/Au ohmic electrode formed on the In0.75Ga0.25As/In0.52Al0.48As HEMT structure with 3-nm-thick InAlAs spacer and 10-nm-thick InAlAs barrier layers. For a 195-nmgate HEMT, we achieved a maximum transconcluctance (g(m)) of 2.25S/mm at 16K (26% above the value at 300K), which, to our knowledge, is one of the highest values for HEMTs ever reported. This extremely high g(m) was attributed to not only 33% lower source-drain resistance (0.18 Omega mm at 16K) because of thermally stable and low contact resistance but also 19% lower transit time under the gate (0.39 ps at 16 K) because of phonon scattering suppression compared with the 300 K values. (c) 2006 Published by Elsevier B.V.