Fabrication of self-aligned T-gate AlGaN/GaN high electron mobility transistors

Self-aligned AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated and the direct current and radio frequency small signal performance of self-aligned devices is characterized in comparison with non-self-aligned devices. An ultra-thin Ti/Al/Ti/Au ohmic metal scheme is used for gate to source and drain self-alignment. To suppress the gate leakage current, the ohmic contact annealing of self-aligned devices is performed in a furnace. The self-aligned devices with 0.25 μm gate-length and 100 μm gate-width exhibit good pinch-off characteristics. The maximum drain current at a gate bias of 1 V is 620 mA/mm for self-aligned HEMTs, and 400 mA/mm for non-self-aligned devices, respectively. A maximum extrinsic transconductance of 146 mS/mm is measured in self-aligned devices, while non-self-aligned HEMTs show only a peak gm of 92 mS/mm. The self-aligned devices exhibit an extrinsic f_T of 39 GHz and an f_MAX of 130 GHz, whereas non-self-aligned HEMTs show an f_T of 15 GHz and an f_MAX of 35 GHz.