Thermal stability and small-signal characteristics of AlGaN/GaN HEMTs with gate insertion metal layer for millimeter-wave applications

The effect that insertion gate metals have on GaN millimeter-wave devices undergoing a postmetallization annealing (PMA) process was investigated. It was found that the PMA process increases the gate resistance (R_g), which is responsible for a decrease in the maximum oscillation frequency (f_max). The resistance was examined as a function of line patterns containing various gate metal stacks, including Ni/Au and Ni/Mo/Au, before and after annealing from a low temperature to 550 °C. The metal stack with an Mo insertion layer effectively suppressed Au diffusion into GaN and reduced the increase in the gate metal resistance. For the fabricated AlGaN/GaN-on-Si high-electron-mobility transistors with a Ni/Mo/Au gate, stable gate reliability, improved current collapse characteristics, and small-signal characteristics were also achieved compared to those of the Ni/Au gate.