Strain-engineering of SiNx layers for threshold voltage modulation of AlGaN/GaN high electron mobility transistors

This work investigates strain-engineering of silicon nitride passivation layers to modulate the threshold voltage of AlGaN/GaN high electron mobility transistors. Thermally stable compressive silicon nitride layers with stress up to −2.1 GPa were deposited using plasma-enhanced chemical vapor deposition with low-frequency plasma excitation at 100 kHz and 600 °C. Modification of the stress allows for controlled manipulation of the two-dimensional electron gas density at the AlGaN/GaN interface in the gate vicinity. A protection layer was introduced to prevent the AlGaN layer stack from damage caused by ion bombardment generated by the low-frequency excited plasma. Devices with the strained passivation show a positive threshold voltage shift from −1.7 to −0.4 V, confirming the feasibility of the method. In addition, the new passivation improves the extrinsic transconductance from 476 to 588 mS/mm. Current collapse measurements show no additional degradation. Both the strain-engineered and standard devices deliver 3.8 W/mm maximum output power at 10 GHz and 3.2 W/mm at 20 GHz. Power-added efficiencies of 34% at 20 GHz and 46% at 10 GHz for the strain-engineered devices, compared with 35% at 20 GHz and 48% at 10 GHz, respectively, for the reference transistors demonstrate stable high-frequency large-signal performance. Moreover, the maximum oscillation frequency increases from 145 to 171 GHz for the strained devices. This method offers an effective way of modulating the threshold voltage toward enhancement-mode operation of AlGaN/GaN high electron mobility transistors without causing degradation in electrical performance or high-frequency characteristics.