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O2-plasma-based digital etching of Al0.25Ga0.75N with a 0.8 nm AlN spacer on GaN was investigated. At 40 W RF bias power and 40 sccm oxygen flow, the etch depth of Al0.25Ga0.75N was 5.7 nm per cycle. The 0.8 nm AlN spacer layer acted as an etch-stop layer in 3 cycles. The surface roughness improved to 0.33 nm after 7 digital etch cycles.Compared to the dry etch only approach, this technique causes less damages. Compared to the selective thermal oxidation with a wet etch approach, this method is less demanding on the epitaxial growth and saves the oxidation process. It was shown to be effective in precisely controlling the AlGaN etch depth required for recessed-AlGaN HEMTs.

In this work, AlGaN/GaN HEMTs with dual-layer SiN x stressors (composed of a low-stress layer and a high-stress layer) were investigated. The low-stress padding layer solved the surface damage problem which was caused during the deposition of the high-stress SiN x and provided a good passivated interface. The HEMTs with the dual-layer stressors showed a 1 V increase in the threshold voltage (V th ) with comparable DC and RF amplification performance to the baseline devices. Moreover, the off-current (I off ) was shown to be reduced by one to three orders of magnitude in the strained devices. The reduction in the off-currents was a result of the lower electric field in AlGaN, which suppressed the gate injection current. These improvements using the dual-layer stressor scheme supports strain engineering as an effective approach in the pursuit of the normally-off operation of AlGaN/GaN HEMTs.