Gate stresses and threshold voltage instability in normally-OFF GaN HEMTs

Abstract

This paper presents a study of gate stress and threshold voltage instability in commercially available 600/650V GaN high electron mobility transistors (HEMTs). The technologies evaluated are an ohmic gate GaN HEMT and a Schottky gate GaN HEMT. The gate leakage currents have been evaluated for two different gate contact technologies and its temperature dependency is presented. It is shown that the gate leakage current could be a temperature indicator for both technologies evaluated, with a higher temperature sensitivity in the case of the Schottky gate HEMT (showing a sixtyfold increase from 22°C to 150°C). A novel characterization method based on the third quadrant operation of the device was applied to the two selected GaN HEMTs and the role of temperature, stress level and duration on the threshold voltage instability of GaN HEMTs has been evaluated. The method can capture both the peak shift and transient recovery. The results highlight the clear differences between both gate contact technologies with the Schottky gate HEMT exhibiting higher threshold voltage instability due to gate stress compared to the ohmic gate devices. The Schottky gate HEMT shows a positive threshold voltage shift for a gate stress voltage of 3 V, whereas at 5.5 V the shift is dependent of the stress time. For both HEMTs, the recovery transient after stress removal is accelerated with temperature.