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 870 $^{circ}hbox{C}$ 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: $hbox{W}/ hbox{HfO}_{2}$, $hbox{W}/hbox{Al}_{2}hbox{O}_{3}$ , and $hbox{W}/hbox{HfO}_{2}/hbox{Ga}_{2}hbox{O}_{3}$ . 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 $hbox{W}/hbox{HfO}_{2}/ hbox{Ga}_{2}hbox{O}_{3}$ 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.