Au-free AlGaN/GaN heterostructure field-effect transistor with recessed overhang ohmic contacts using a Ti/Al bilayer

We have developed a low-temperature ohmic contact process with a recessed overhang configuration for Au-free (complementary metal-oxide semiconductor) CMOS-compatible AlGaN/GaN heterostructure field effect transistors (HFETs). The recessed overhang configuration has a Ti/Al bilayer directly in contact with the AlGaN/GaN heterojunction interface at the recessed sidewall overlaid with the AlGaN barrier layer, which allows good and reproducible ohmic formation with low-temperature annealing. The optimum Ti/Al thickness was 40/200 nm, which resulted in an R_c of 0.76 Ω mm with excellent surface morphology when annealed at 550 °C for 1 min. The device fabricated with a Ni/Mo gate exhibited a maximum drain current density of ∼500 mA mm^-1, a specific on-resistance of 1.35 mΩ cm² and a breakdown voltage of >1 kV.