Reduction of ohmic contact resistance of AlGaN/GaN HFETs by doping of thermally diffused Si

Low ohmic contact resistance is crucial to realization of good RF and power performances of AlGaN/GaN heterojunction field effect transistors (HFETs). However, with almost universal use of Ti/Al based electrode, usual specific contact resistance (pc) remains only at 1 × 10^-5 Ω·cm level which is insufficient for the ultimate goal of exceeding the performances of the GaAs based FETs. Therefore, in order to overcome the very high potential barrier of AlGaN, a more aggressive method is required. In this work, we report on a simple but very effective method to reduce the contact resistance by thermal diffusion of Si dopants selectively into the area underneath the contact electrodes. By diffusing Si from the AlGaN surface at 1000°C for 30 minutes, a record low ρ_c of 1.2 × 10^-6 Ω·cm² is realized.