Electron Transport Mechanism for Ohmic Contact to GaN/AlGaN/GaN Heterostructure Field-Effect Transistors

The contact resistance of Mo-based ohmic contact was investigated for undoped GaN/AlGaN/GaN heterostructure field-effect transistors having different GaN cap layer thickness (t_cap) and AlN mole fraction (x_Al) values. Measured specific contact resistivity (ρ_c) exhibited a drastic increase by a factor of 10³ by increasing t_cap from 2 to 5 nm, and a minimum ρ_c value was 3.2×10^-7 Ωcm² at T=300 K. We have developed a model of the ohmic contact assuming an n-type interfacial layer (donor density: N_d, thickness: t_d). Theoretical calculations suggested that the drastic increase of ρ_c by increasing t_cap is due to td being between 2 and 5 nm. Also, the present model optimizing t_d and N_d values successfully explained measured trends of ρ_c with respect to t_cap, x_Al, and temperature.