  In this work, AlGaN/GaN HEMTs with dual-layer SiNx stressors (composed of a
low-stress layer and a high-stress layer) were investigated. The low-stress
padding layer solved the surface damage problem caused during the deposition of
the high-stress SiNx, and provided a good passivated interface. The HEMTs with
dual-layer stressors showed a 1 V increase in the threshold voltage (Vth) with
comparable on-current and RF current gain to those without stressors. Moreover,
the off-current (I_off) was shown to be reduced by one to three orders of
magnitude in the strained devices as a result of the lower electric field in
AlGaN, which suppressed the gate injection current. The dual-layer stressor
scheme supports strain engineering as an effective approach in the pursuit of
the normally-off operation of AlGaN/GaN HEMTs.
