Threshold voltage modulation by interface charge engineering for high performance normally-off GaN MOSFETs with high faulty turn-on immunity

An efficient approach to engineering the Al<sub>2</sub>O<sub>3</sub>/GaN positive interface fixed charges (Q<sub>it</sub><sup>+</sup>) by post-dielectric annealing in nitrogen is demonstrated. The remarkable reduction of Q<sub>it</sub><sup>+</sup> from 1.44×10<sup>13</sup> to 3×10<sup>12</sup> cm-2 was observed, which leads to a record high threshold voltage (V<sub>TH</sub>) of +7.6 V obtained in the Al<sub>2</sub>O<sub>3</sub>/GaN MOSFETs. The positive interface charges were proposed originating from the N-vacancy and O-substitution at the Al<sub>2</sub>O<sub>3</sub>/GaN interface by Ab inito study. The significantly reduced Q<sub>it</sub><sup>+</sup> also effectively depresses the remote scattering effect that enables respectable improvement in the electron mobility, which results in a high drain current density of 355 mA/mm in the device with dimensions of L<sub>G</sub>/L<sub>GS</sub>/L<sub>GD</sub>/W<sub>G</sub>=2/1.5/5/50 μm. The device with L<sub>GD</sub>of 20 μm delivers a high breakdown voltage of 1054 V @ 1 μA/mm. Owing to the uniquely high V<sub>TH</sub> the fabricated normally-off device features substantially improved faulty turn-on immunity compared with the device with lower V<sub>TH</sub>. These competitive results reveal that the method reported in this work is promising in pushing the V<sub>TH</sub> more positive and simultaneously achieving good device performance of normally-off GaN power devices with improved fail-safe capability.

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