Enhancement-Mode AlInN/GaN High-Electron-Mobility Transistors Enabled by Thermally Oxidized Gates

Enhancement mode AlInN/gallium nitride (GaN) high-electron-mobility transistors (HEMTs) are fabricated by thermally oxidizing the barrier region under the gate. The oxidation is performed at 850 $^{\circ}$ C in O $_{\text{2}}$ , and a SiN $_{\textit{x}}$ mask is used to achieve selective oxidization of the AlInN layer. For comparison, a standard Schottky gate and atomic layer deposition (ALD) Al $_{\text{2}}$ O $_{\text{3}}$ metal–insulator–semiconductor (MIS) HEMTs are fabricated from the same structure and show depletion mode behavior as expected. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) mappings are performed to characterize the gate of the oxidized HEMTs, showing complete oxidation of the AlInN barrier. All the devices are tested to determine their transfer and output characteristics. The results show that the thermally oxidized gate produces a positive shift in threshold voltage at $\sim$ 4 V and low currents ( $\sim$ 2 $\times$ 10 $^{-\text{7}}$ mA/mm) at zero gate voltage. The oxidized HEMTs are also subjected to postmetallization annealing (PMA) at 400 $^{\circ}$ C and 500 $^{\circ}$ C for 10 min flowing 1000 sccm of N $_{\text{2}}$ , retaining enhancement mode behavior and leading to a further positive shift in threshold voltage.