Thermal stability of Ti/Pt/Au ohmic contacts for cryogenically cooled InP-based HEMTs on (4 1 1)A-oriented substrates by MBE

Abstract

We investigated the contact resistance of a non-alloyed Ti/Pt/Au ohmic electrode to obtain thermally stable source–drain resistance of cryogenically cooled In 0.75 Ga 0.25 As / In 0.52 Al 0.48 As high electron mobility transistors (HEMTs) fabricated on (4 1 1)A-oriented InP substrates by molecular beam epitaxy. A contact resistance of 0.26 Ω mm , which strongly depended on InAlAs spacer and barrier layers, was achieved at 16 K (25% below the value at 300 K) for the non-alloyed Ti/Pt/Au ohmic electrode formed on the In 0.75 Ga 0.25 As / In 0.52 Al 0.48 As HEMT structure with 3-nm-thick InAlAs spacer and 10-nm-thick InAlAs barrier layers. For a 195-nm-gate HEMT, we achieved a maximum transconductance ( g m ) of 2.25 S / mm at 16 K (26% above the value at 300 K), 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 Ω mm at 16 K) 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.