Impact of AlN layer sandwiched between the GaN and the Al2O3layers on the performance and reliability of recessed AlGaN/GaN MOS-HEMTs

This work compares the performance and the reliability of recessed-gate AlGaN/GaN MOS-HEMTs with two different gate dielectrics: a single layer of Al2O3 and a bilayer of AlN/Al2O3. Although the normally-OFF operation is well observed in the AlN/Al2O3 device, the other characteristics are remarkable poor and directly prejudice the performance of the device. PBTI phenomenon was studied by using a pulse characterization technique in order to capture the fast charge trapping when a single stress pulse is applied to the gate contact. The physical origin of PBTI is ascribed to electron trapping and de-trapping in defect sites located in the gate dielectrics. The results during the stress phase show that the positive threshold voltage shift (ΔVth) in both devices follows a saturating log-time dependence model. From this analysis, the double stack exhibits a higher saturation threshold voltage and a faster trap time constant compared with the single layer device. The universal relaxation function fits well the experimental data during the relaxation phase at different stress pulse widths and clearly shows a faster defect discharge in the Al2O3 device.