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 degrees 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: 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.