Gate-First AlGaN/GaN HEMT Technology for High-Frequency Applications

This letter describes a gate-first AlGaN/GaN high-electron mobility transistor (HEMT) with a W/high-k dielectric gate stack. In this new fabrication technology, the gate stack is deposited before the ohmic contacts, and it is optimized to stand the 870degC ohmic contact annealing. The deposition of the W/high-k dielectric protects the intrinsic transistor early in the fabrication process. Three different gate stacks were studied: W/ HfO₂, W/Al₂O₃, and W/HfO₂/Ga₂O₃. DC characterization showed transconductances of up to 215 mS/mm, maximum drain current densities of up to 960 mA/mm, and more than five orders of magnitude lower gate leakage current than in the conventional gate-last Ni/Au/Ni gate HEMTs. Capacitance-voltage measurements and pulsed-IV characterization show no hysteresis for the W/HfO₂/ Ga₂O₃ capacitors and low interface traps. These W/high-k dielectric gates are an enabling technology for self-aligned AlGaN/GaN HEMTs, where the gate contact acts as a hard mask to the ohmic deposition.