Thermally stable Ge/Ag/Ni ohmic contact for InAlAs/InGaAs/InP HEMTs

Excellent annealed ohmic contacts based on Ge/Ag/Ni metallization have been realized in a temperature range between 385 and 500 °C, with a minimum contact resistance of 0.06 Ω·mm and a specific contact resistivity of 2.62 × 10^-7 Ω cm² obtained at an annealing temperature of 425 °C for 60 s in a rapid thermal annealing (RTA) system. Thermal storage tests at temperatures of 215 and 250°C in a nitrogen ambient showed that the Ge/Ag/Ni based ohmic contacts with an overlay of Ti/Pt/Au had far superior thermal stabilities than the conventional annealed AuGe/Ni ohmic contacts for InAlAs/InGaAs high electron mobility transistors (HEMTs). During the storage test at 215°C, the ohmic contacts showed no degradation after 200 h. At 250°C, the contact resistance value of the Ge/Ag/Ni ohmic contact increased only to a value of 0.1 Ω·mm over a 250-h period. Depletion-mode HEMTs (D-HEMTs) with a gate length of 0.2 μm fabricated using Ge/Ag/Ni ohmic contacts with an overlay of Ti/Pt/Au demonstrated excellent dc and RF characteristics.