Ohmic contacts in AlSb/InAs high electron mobility transistors for low-voltage operation

We report on the fabrication and characteristics of Pd/Pt/Au ohmic contacts that have been used in AlSb/InAs high electron mobility transistors (HEMTs) with low access resistance. The metalization exhibits minimal in-diffusion and a contact resistance of 0.08 Ω mm after a 175 °C hot-plate heat treatment. By comparison, AuGe/Ni/Pt/Au ohmic contact metalizations formed using a 300 °C rapid thermal anneal exhibit a contact resistance of 0.11 Ω mm, but with considerable Au in-diffusion. Using the Pd/Pt/Au contact, 0.6 μm gate-length AlSb/InAs HEMTs exhibit a low-field source-drain resistance of 0.47 Ω mm and a transconductance above 1 S/mm. After removal of the gate bonding pad capacitance from an equivalent circuit, an fTLg product of 38 GHz μm is obtained at VDS=0.4 V. HEMTs with a 60 nm gate length exhibit a low-field source-drain resistance of 0.35 Ω mm and a measured fT of 90 GHz at a drain voltage of only 100 mV. These fTLg and fT values are the highest reported for any field effect transistor at these drain voltages.