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Contactless photo-electrochemical (CL-PEC) etching was used to fabricate recessed-gate AlGaN/GaN high-electron-mobility transistors (HEMTs). Self-termination of etching was observed during CL-PEC etching on an AlGaN barrier layer whose residual thickness had a uniform value of 6 nm overall on the same chip. After tetramethylammonium hydroxide post-treatment, the root-mean-square roughness of the etched surface was around 0.4 nm, which had smoothness comparable to that of the unetched surface. Recessed-Schottky HEMTs showed a positive shift in Vth, the suppression of drain leakage currents, and an improvement in the subthreshold-slope value as compared with planar-gate HEMTs. By applying a metal–insulator–semiconductor (MIS)-gate structure, the gate and drain leakage currents were significantly reduced, leading to an increased input dynamic range. Furthermore, the standard deviations (σ) of the Vth of CL-PEC-etched recessed-Schottky HEMTs and recessed-MIS HEMTs were very small, 5.5 and 16.7 mV, respectively. These results showed that the CL-PEC etching process is promising for the fabrication of recessed-gate AlGaN/GaN HEMTs having excellent uniformity for normally-off device operations. [ABSTRACT FROM AUTHOR]

Precise control of the Al content x in AlxGa1−xN/GaN epitaxial structures is a potentially important technical parameter for power/RF devices. We fabricated five 2 inch diameter high-electron-mobility transistor wafers by metal organic vapor phase epitaxy on free-standing (0001) Ga-face GaN as a substrate. The Al content x of the AlGaN layer was ∼0.2, and the substrate off-angle varied in the range 0.2°–1.0°. The Al content was measured as a function of the substrate off-angle using room-temperature optical reflectance. The values of the AlGaN bandgap energy derived from reflectance spectra were calibrated against the Al content determined by X-ray diffraction beforehand. The Al content in the AlGaN layers was found to be unaffected by the substrate off-angle, and a downward bowing was observed in the relation of the bandgap energy and the Al content, with a bowing parameter of 0.57 eV and a stress-corrected bowing parameter of 0.22 eV. [ABSTRACT FROM AUTHOR]