High-performance GaN HEMTs with GaON under-gate cap layer via barrier-friendly selective plasma oxidation

Schottky-gated terahertz high-electron mobility transistors with ultrathin barriers suffer from metal-induced gap states (MIGSs), leading to gate leakage and electron scattering that degrade transport properties. While oxygen plasma oxidation of the InAlN barrier can mitigate MIGS, it introduces defects and impurity scattering, further deteriorating channel performance. This study resolves this paradox by exploiting the thermodynamic oxidation selectivity between gallium nitride (GaN) and InAlN to selectively convert a GaN cap under the gate region into a wide-bandgap gallium oxynitride (GaON) layer, leaving the InAlN barrier intact. This barrier-friendly approach suppresses MIGS without sacrificing channel quality, achieving a near-theoretical intrinsic electron effective velocity (veff. i = 2.2 × 107 cm/s). The enhanced transport enables record RF performance: (fT/fmax = 240/530) GHz, a noise figure below 1 dB at Ka-band, attributed to suppressed interfacial scattering, and 8 dB linear gain at 94 GHz. With excellent uniformity, this GaON cap technology provides a scalable, reliable pathway to high-frequency GaN HEMTs that harmonize noise and power performance.